J Roger P Angel

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Scholarly Contributions

Chapters

• Hill, J., Martin, H., & Angel, R. (2013). Honeycomb Mirrors for Large Telescopes. In Planets, Stars and Stellar Systems.~Volume 1: Telescopes and Instrumentation(p. 137).

Journals/Publications

• Angel, R. (2024). A 600 m 2 array of 6.5 m telescopes at the lunar pole. Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, 382(2271). doi:10.1098/rsta.2023.0076
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  The proposed lunar telescope for optical and infrared astronomy aims at very large aperture, 600 m 2, at a fundable cost. It comprises an array of 18 separate telescopes, each of 6.5 m aperture. The 200 m diameter array will be located within 1/2° (15 km) of a lunar pole on approximately level ground, with a perimeter screen deployed to provide shade and cooling to cryogenic temperature. The 500 m diameter screen will allow unobscured access down to 8° elevation. All 18 telescopes will reflect light into a central beam combiner to form a single image covering wavelengths from 0.4 µm to 10 µm. The initial instrument complement will include high-resolution and multi-object spectrographs to exploit the single combined field of view of two arcminute diameter, with the diffraction limited resolution of 6.5 m aperture. Scientific applications include the search for molecular biosignatures in transiting exoplanets, and the study of galaxy evolution using red-shifted spectra to beyond z = 10. The array cost, including delivery to the Moon by SpaceX Starship for installation using lunar base infrastructure, is around $10 billion, similar to that of the 25 m 2 JWST. To test the concept, first a single prototype 6.5 m unit would be operated at the lunar south pole. This article is part of a discussion meeting issue 'Astronomy from the Moon: the next decades (part 2)'.
• Berkson, J., Angel, R., Bender, C., Monson, A., Gray, P., Schwab, C., Foster, W., & Kim, D. (2024). Large Fiber Array Spectroscopic Telescope: Optical Design for a Scalable Unit Telescope. Nanomanufacturing and Metrology, 7(Issue 1). doi:10.1007/s41871-024-00235-8
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  The Large Fiber Array Spectroscopic Telescope (LFAST) concept involves the utilization of 2640 individual, 0.76 m “unit telescopes” to collect light from a target object. Through optical fibers, the lights from all telescope foci are directed to a high-resolution spectrograph, with fibers subtending a 1.47 arcsec diameter on the sky. The total light-collecting area of all telescopes combined is 1200 m2. Given the telescope’s application for spectroscopy of seeing-limited images, the need for coherence is eliminated. The primary goal of the LFAST prototype is to demonstrate a telescope design that can be rapidly and cost-effectively replicated to achieve a 1200 m2 collecting area and match the capabilities of extremely large telescopes (ELTs) presently under construction. Engineering, manufacturing, and design challenges unique to LFAST differ significantly from those encountered in traditional ELTs. Considering that the unit telescope is to be replicated thousands of times, reducing the manufacturing cost per telescope is essential. The aim of the described design is to achieve this goal, with strong considerations for manufacturability, alignment, few moving parts, and high yield. Experience from the first prototype unit telescope built in 2022 will be leveraged to build the first 20-unit tracking module in 2024, with a collecting area of a 3.4-m-diameter mirror. The design is minimalistic and features commercial tolerances, thereby ensuring cost-effectiveness in manufacturing, assembly, and alignment processes. Moreover, the telescope is designed to operate at angles as low as 70° from the zenith by laterally translating a single lens to correct for atmospheric dispersion. We present a combined average of over 80% encircled energy across 2640 Monte Carlo trials. Simulations were conducted using commercially specified tolerances and capabilities to model the overall performance of the array.
• Woolf, N., & Angel, R. (2021). Pantheon habitat made from regolith, with a focusing solar reflector: Pantheon Habitat. Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, 379(Issue 2188). doi:10.1098/rsta.2020.0142
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  We describe a polar Moon base habitat using direct solar energy for construction, food production and atmospheric revitalization. With a growing area as large as 2000 m 2, it could provide for 40 or more people. The habitat is built like the ancient Roman Pantheon, a stone structure with a top circular oculus, bringing in focused sunlight that is spread out to crops below. The conical, corbelled structure is built from cast regolith blocks, held in compression despite the large internal atmospheric pressure by a regolith overlayer 20-30 m thick. It is sealed on the inside against leaks with thin plastic. A solar mirror concentrator used initially to cast the building blocks is later used to illuminate the habitat through a small pressure window at the oculus. Three years of robotic preparation of the building blocks does not seem excessive for a habitat which can be expected to last for millennia, as has the Treasury of Atreus made by similar dry-stone construction. One goal of returning to the Moon is to demonstrate the practicality of long-term human habitation off the Earth. The off-axis, paraboloidal reflecting mirror is rotated about the vertical polar axis in order to direct horizontal sunlight downward to a focus. In this way, the heavy materials needed from Earth to build and power the habitat are largely limited to the solar concentrator and regolith moving and moulding equipment. By illuminating with a reflector rather than with electricity, the solar collection area is 20 times smaller than would be needed for PV cells. This article is part of a discussion meeting issue 'Astronomy from the Moon: The next decades'.
• Angel, J. R., & Eads, R. W. (2020). A 20 m wide-field diffraction-limited telescope. Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, 379(2188), 20200141. doi:10.1098/rsta.2020.0141
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  A 20 m space telescope is described with an unvignetted 1° field of view- A hundred times larger in area than fields of existing space telescopes. Its diffraction-limited images are a hundred times sharper than from wide-field ground-based telescopes and extend over much if not all the field, 40 arcmin diameter at 500 nm wavelength, for example. The optical system yielding a 1°, 1.36 m diameter image at f/3.9 has relatively small central obscuration, 9% by area on axis, and is fully baffled. Several carousel-mounted instruments can each access directly the full image. The initial instrument complement includes a 400 gigapixel silicon imager with 2 µm pixels (0.005 arcsec), and a 60 gigapixel HgCdTe imager with 5 µm pixels (0.012 arcsec). A multi-object spectrograph with 10 000 fibres will allow spectroscopy with 0.02 arcsec resolution. Direct imaging and spectroscopy of exoplanets can take advantage of the un-aberrated, on-axis image (5 nm RMS wavefront error). While this telescope could be built for operation in free space, a site accessible to a human outpost at the Moon's south pole would be advantageous, for assembly and repairs. The lunar site would allow also for the installation of new instruments to keep up with evolving scientific priorities and advancing technology. Cooling to less than 100E K would be achieved with a surrounding cylindrical thermal shield. This article is part of a discussion meeting issue 'Astronomy from the Moon: The next decades'.
• Eads, R. W., & Angel, J. R. (2021). A 20 m wide-field diffraction-limited telescope. Philosophical transactions. Series A, Mathematical, physical, and engineering sciences, 379(2188), 20200141.
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  A 20 m space telescope is described with an unvignetted 1° field of view-a hundred times larger in area than fields of existing space telescopes. Its diffraction-limited images are a hundred times sharper than from wide-field ground-based telescopes and extend over much if not all the field, 40 arcmin diameter at 500 nm wavelength, for example. The optical system yielding a 1°, 1.36 m diameter image at f/3.9 has relatively small central obscuration, 9% by area on axis, and is fully baffled. Several carousel-mounted instruments can each access directly the full image. The initial instrument complement includes a 400 gigapixel silicon imager with 2 µm pixels (0.005 arcsec), and a 60 gigapixel HgCdTe imager with 5 µm pixels (0.012 arcsec). A multi-object spectrograph with 10 000 fibres will allow spectroscopy with 0.02 arcsec resolution. Direct imaging and spectroscopy of exoplanets can take advantage of the un-aberrated, on-axis image (5 nm RMS wavefront error). While this telescope could be built for operation in free space, a site accessible to a human outpost at the Moon's south pole would be advantageous, for assembly and repairs. The lunar site would allow also for the installation of new instruments to keep up with evolving scientific priorities and advancing technology. Cooling to less than 100E K would be achieved with a surrounding cylindrical thermal shield. This article is part of a discussion meeting issue 'Astronomy from the Moon: the next decades'.
• Eads, R., & Angel, R. (2020). 6.5-m telescope for simultaneous spectroscopy of > 10,000 objects. Appl. Opt., 59(22), G154--G159.
• Woolf, N., & Angel, R. (2021). Pantheon habitat made from regolith, with a focusing solar reflector. Philosophical transactions. Series A, Mathematical, physical, and engineering sciences, 379(2188), 20200142.
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  We describe a polar Moon base habitat using direct solar energy for construction, food production and atmospheric revitalization. With a growing area as large as 2000 m, it could provide for 40 or more people. The habitat is built like the ancient Roman Pantheon, a stone structure with a top circular oculus, bringing in focused sunlight that is spread out to crops below. The conical, corbelled structure is built from cast regolith blocks, held in compression despite the large internal atmospheric pressure by a regolith overlayer 20-30 m thick. It is sealed on the inside against leaks with thin plastic. A solar mirror concentrator used initially to cast the building blocks is later used to illuminate the habitat through a small pressure window at the oculus. Three years of robotic preparation of the building blocks does not seem excessive for a habitat which can be expected to last for millennia, as has the Treasury of Atreus made by similar dry-stone construction. One goal of returning to the Moon is to demonstrate the practicality of long-term human habitation off the Earth. The off-axis, paraboloidal reflecting mirror is rotated about the vertical polar axis in order to direct horizontal sunlight downward to a focus. In this way, the heavy materials needed from Earth to build and power the habitat are largely limited to the solar concentrator and regolith moving and moulding equipment. By illuminating with a reflector rather than with electricity, the solar collection area is 20 times smaller than would be needed for PV cells. This article is part of a discussion meeting issue 'Astronomy from the Moon: the next decades'.
• , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , ., , , ., et al. (2019). LSST: From Science Drivers to Reference Design and Anticipated Data Products. Astrophysical Journal, 873(Issue 2). doi:10.3847/1538-4357/ab042c
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  We describe here the most ambitious survey currently planned in the optical, the Large Synoptic Survey Telescope (LSST). The LSST design is driven by four main science themes: probing dark energy and dark matter, taking an inventory of the solar system, exploring the transient optical sky, and mapping the Milky Way. LSST will be a large, wide-field ground-based system designed to obtain repeated images covering the sky visible from Cerro Pachón in northern Chile. The telescope will have an 8.4 m (6.5 m effective) primary mirror, a 9.6 deg 2 field of view, a 3.2-gigapixel camera, and six filters (ugrizy) covering the wavelength range 320-1050 nm. The project is in the construction phase and will begin regular survey operations by 2022. About 90% of the observing time will be devoted to a deep-wide-fast survey mode that will uniformly observe a 18,000 deg 2 region about 800 times (summed over all six bands) during the anticipated 10 yr of operations and will yield a co-added map to r ∼27.5. These data will result in databases including about 32 trillion observations of 20 billion galaxies and a similar number of stars, and they will serve the majority of the primary science programs. The remaining 10% of the observing time will be allocated to special projects such as Very Deep and Very Fast time domain surveys, whose details are currently under discussion. We illustrate how the LSST science drivers led to these choices of system parameters, and we describe the expected data products and their characteristics.
• Rabien, S., Angel, R., Barl, L., Beckmann, U., Busoni, L., Belli, S., Bonaglia, M., Borelli, J., Brynnel, J., Buschkamp, P., Cardwell, A., Contursi, A., Connot, C., Davies, R., Deysenroth, M., Durney, O., Eisenhauer, F., Elberich, M., Esposito, S., , Frye, B., et al. (2019). ARGOS at the LBT: Binocular laser guided ground-layer adaptive optics. Astronomy and Astrophysics, 621(Issue). doi:10.1051/0004-6361/201833716
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  Having completed its commissioning phase, the Advanced Rayleigh guided Ground-layer adaptive Optics System (ARGOS) facility is coming online for scientific observations at the Large Binocular Telescope (LBT). With six Rayleigh laser guide stars in two constellations and the corresponding wavefront sensing, ARGOS corrects the ground-layer distortions for both LBT 8.4 m eyes with their adaptive secondary mirrors. Under regular observing conditions, this set-up delivers a point spread function (PSF) size reduction by a factor of 2-3 compared to a seeing-limited operation. With the two LUCI infrared imaging and multi-object spectroscopy instruments receiving the corrected images, observations in the near-infrared can be performed at high spatial and spectral resolution. We discuss the final ARGOS technical set-up and the adaptive optics performance. We show that imaging cases with ground-layer adaptive optics (GLAO) are enhancing several scientific programmes, from cluster colour magnitude diagrams and Milky Way embedded star formation, to nuclei of nearby galaxies or extragalactic lensing fields. In the unique combination of ARGOS with the multi-object near-infrared spectroscopy available in LUCI over a 4â×â4 arcmin field of view, the first scientific observations have been performed on local and high-z objects. Those high spatial and spectral resolution observations demonstrate the capabilities now at hand with ARGOS at the LBT.
• Yu, Z. J., Yu, Z. J., Fisher, K. C., Fisher, K. C., Meng, X., Meng, X., Hyatt, J. J., Hyatt, J. J., Angel, R. P., Angel, R. P., Holman, Z. C., & Holman, Z. C. (2019). GaAs/silicon PVMirror tandem photovoltaic mini-module with 29.6% efficiency with respect to the outdoor global irradiance. Progress in Photovoltaics: Research and Applications, 27(Issue 5). doi:10.1002/pip.3095
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  Balance-of-system costs now dominate the installed cost of photovoltaic systems, causing the annually averaged module efficiency to become a primary system cost driver. The resulting continued push towards higher module efficiencies, coupled with the dominance of single-axis tracking in the utility-scale PV market, may create an opportunity for a low-concentration tandem module technology. Here, we demonstrate such a tandem, using the “PVMirror” concept, on the mini-module scale. The tandem couples a (concentrating) silicon PVMirror having an aperture area of 156.25 cm 2 with a gallium arsenide receiver to achieve 29.6% efficiency with respect to the outdoor global irradiance. Unlike most concentrating technologies, the silicon PVMirror collects some of the diffuse light, but the tandem would nevertheless achieve 31% efficiency in the absence of diffuse light, as in a laboratory measurement. The same tandem technology can be implemented with a wide-bandgap thin-film PVMirror and silicon receiver—a potentially cost-competitive combination—when efficient wide-bandgap cells have been developed.
• Vorndran, S. D., Chrysler, B., Wheelwright, B., Angel, R., Holman, Z., & Kostuk, R. (2016). Off-axis holographic lens spectrum-splitting photovoltaic system for direct and diffuse solar energy conversion. Applied Optics, 55(Issue 27). doi:10.1364/ao.55.007522
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  This paper describes a high-efficiency, spectrum-splitting photovoltaic module that uses an off-axis volume holographic lens to focus and disperse incident solar illumination to a rectangular shaped high-bandgap indium gallium phosphide cell surrounded by strips of silicon cells. The holographic lens design allows efficient collection of both direct and diffuse illumination to maximize energy yield. We modeled the volume diffraction characteristics using rigorous coupled-wave analysis, and simulated system performance using nonsequential ray tracing and PV cell data from the literature. Under AM 1.5 illumination conditions the simulated module obtained a 30.6% conversion efficiency. This efficiency is a 19.7% relative improvement compared to the more efficient cell in the system (silicon). The module was also simulated under a typical meteorological year of direct and diffuse irradiance in Tucson, Arizona, and Seattle, Washington. Compared to a flat panel silicon module, the holographic spectrum splitting module obtained a relative improvement in energy yield of 17.1% in Tucson and 14.0% in Seattle. An experimental proof-of-concept volume holographic lens was also fabricated in dichromated gelatin to verify the main characteristics of the system. The lens obtained an average first-order diffraction efficiency of 85.4% across the aperture at 532 nm.
• Vorndran, S., Chrysler, B., Wheelwright, B., Angel, R., Holman, Z., & Kostuk, R. (2016). Off-axis holographic lens spectrum-splitting photovoltaic system for direct and diffuse solar energy conversion. \ao, 55, 7522.
• Vdovin, G., Soloviev, O., Loktev, M., Angel, R., Angel, J. R., & Sarro, P. M. (2015). Structured film for compensation of anthropogenic radiative forcing.. Optics letters, 40(8), 1702-4. doi:10.1364/ol.40.001702
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  A possible solution for the global warming problem consists of scattering the solar radiation by a cloud of screens, placed near the inner Lagrange point (L1), between the sun and the earth. The thin film material for such a screen should have a high longevity in space environment, low areal density, high on-axis scattering, very low integral reflectivity over the whole solar spectrum, and good handling properties such as foldability and robustness. We report on experimental fabrication of free-standing robust nitride films satisfying to most optical and mechanical requirements for such a screen.
• Yu, Z. J., Fisher, K. C., Wheelwright, B. M., Angel, R. P., & Holman, Z. C. (2015). PVMirror: A New Concept for Tandem Solar Cells and Hybrid Solar Converters. IEEE JOURNAL OF PHOTOVOLTAICS, 5(6), 1791-1799.
• Yu, Z. J., Fisher, K. C., Wheelwright, B. M., Angel, R. P., & Holman, Z. C. (2015). PVMirror: A New Concept for Tandem Solar Cells and Hybrid Solar Converters. IEEE Journal of Photovoltaics, 5(Issue 6). doi:10.1109/jphotov.2015.2458571
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  As the solar electricity market has matured, energy conversion efficiency and storage have joined installed system cost as significant market drivers. In response, manufacturers of flat-plate silicon photovoltaic (PV) cells have pushed cell efficiencies above 25% - nearing the 29.4% detailed-balance efficiency limit - and both solar thermal and battery storage technologies have been deployed at utility scale. This paper introduces a new tandem solar collector employing a 'PVMirror' that has the potential to both increase energy conversion efficiency and provide thermal storage. A PVMirror is a concentrating mirror, spectrum splitter, and light-to-electricity converter all in one: It consists of a curved arrangement of PV cells that absorb part of the solar spectrum and reflect the remainder to their shared focus, at which a second solar converter is placed. A strength of the design is that the solar converter at the focus can be of a radically different technology than the PV cells in the PVMirror; another is that the PVMirror converts a portion of the diffuse light to electricity in addition to the direct light. We consider two case studies - a PV cell located at the focus of the PVMirror to form a four-terminal PV-PV tandem, and a thermal receiver located at the focus to form a PV-CSP (concentrating solar thermal power) tandem - and compare the outdoor energy outputs to those of competing technologies. PVMirrors can outperform (idealized) monolithic PV-PV tandems that are under concentration, and they can also generate nearly as much energy as silicon flat-plate PV while simultaneously providing the full energy storage benefit of CSP.
• Angel, R., & Angel, J. R. (2014). Big optics for astronomy and solar energy. Proceedings of SPIE, 9186. doi:10.1117/12.2064830
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  Aden Meinel came from the University of Arizona’s Steward Observatory and the Department of Astronomy to found the Optical Sciences Center (OSC). Aden conceived and made at the Center of the optics for the revolutionary Multiple Mirror Telescope (MMT), which greatly influenced the design of future large research telescopes and the technology needed to make them. The Steward Observatory Mirror Lab was built to make honeycomb mirrors up to 8.4 m diameter, and with much faster focal ratio. In use in the current Large Binocular Telescope and future Giant Magellan Telescope, these mirrors provide powerful astronomical research capabilities with unique sensitivity for exoplanet observations in the infrared. The solar energy field can also benefit from Aden’s legacy, by using multiple large solar mirrors configured like the MMT to power very high efficiency photovoltaic cells at each focus.
• Coughenour, B. M., Stalcup, T., Wheelwright, B., Geary, A., Hammer, K., & Angel, R. (2014). Dish-based high concentration PV system with Kohler optics. OPTICS EXPRESS, 22(5), A211-A224.
• Coughenour, B. M., Stalcup, T., Wheelwright, B., Geary, A., Hammer, K., & Angel, R. (2014). Dish-based high concentration PV system with Köhler optics. Optics Express, 22(5), A211-A224.
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  Abstract: We present work at the Steward Observatory Solar Lab on a high concentration photovoltaic system in which sunlight focused by a single large paraboloidal mirror powers many small triple-junction cells. The optical system is of the XRX-Köhler type, comprising the primary reflector (X) and a ball lens (R) at the focus that reimages the primary reflector onto an array of small reflectors (X) that apportion the light to the cells. We present a design methodology that provides generous tolerance to mispointing, uniform illumination across individual cells, minimal optical loss and even distribution between cells, for efficient series connection. An operational prototype has been constructed with a 3.3m × 3.3m square primary reflector of 2m focal length powering 36 actively cooled triplejunction cells at 1200x concentration (geometric). The measured end-to-end system conversion efficiency is 28%, including the parasitic loss of the active cooling system. Efficiency ∼32% is projected for the next system. © 2014 Optical Society of America.
• Angel, R., Coughenour, B., Stalcup, T., & Wheelwright, B. (2013). Concentrating optics with big glass mirrors for inexpensive CPV generation. Optics InfoBase Conference Papers.
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  Abstract: Large, inexpensive, back-silvered glass mirrors are widely used at solar thermal generation plants. The electrical output per mirror may be doubled by optical coupling to highly efficient, triple junction photovoltaic cells. OCIS codes: (350.6050) Solar Energy; (220.1770) Concentrators. Renewable Energy and the Environment Congress. © 2013.
• Coughenour, B., Angel, R., Stalcup, T., Wheelwright, B., Hammer, K., & Geary, A. (2013). Development of a commercial dish-based HCPV system. Optics InfoBase Conference Papers.
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  Abstract: We present work on our third generation HCPV system. This system will use paraboloidal dish mirrors with our patented optical system redesigned for commercial production to convert sunlight into electricity with 32% end-to-end system efficiency. Renewable Energy and the Environment Congress. © 2013.
• Guyon, O., Eisner, J. A., Angel, R., Woolf, N. J., Bendek, E. A., Milster, T. D., Ammons, S. M., Shao, M., Shaklan, S., Levine, M., Nemati, B., Martinache, F., Pitman, J., Woodruff, R. A., & Belikov, R. (2013). Simultaneous exoplanet characterization and deep wide-field imaging with a diffractive pupil telescope. Astrophysical Journal, 767(1).
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  Abstract: High-precision astrometry can identify exoplanets and measure their orbits and masses while coronagraphic imaging enables detailed characterization of their physical properties and atmospheric compositions through spectroscopy. In a previous paper, we showed that a diffractive pupil telescope (DPT) in space can enable sub-μas accuracy astrometric measurements from wide-field images by creating faint but sharp diffraction spikes around the bright target star. The DPT allows simultaneous astrometric measurement and coronagraphic imaging, and we discuss and quantify in this paper the scientific benefits of this combination for exoplanet science investigations: identification of exoplanets with increased sensitivity and robustness, and ability to measure planetary masses to high accuracy. We show how using both measurements to identify planets and measure their masses offers greater sensitivity and provides more reliable measurements than possible with separate missions, and therefore results in a large gain in mission efficiency. The combined measurements reliably identify potentially habitable planets in multiple systems with a few observations, while astrometry or imaging alone would require many measurements over a long time baseline. In addition, the combined measurement allows direct determination of stellar masses to percent-level accuracy, using planets as test particles. We also show that the DPT maintains the full sensitivity of the telescope for deep wide-field imaging, and is therefore compatible with simultaneous scientific observations unrelated to exoplanets. We conclude that astrometry, coronagraphy, and deep wide-field imaging can be performed simultaneously on a single telescope without significant negative impact on the performance of any of the three techniques. © 2013. The American Astronomical Society. All rights reserved.
• Guyon, O., Eisner, J. A., Angel, R., Woolf, N. J., Bendek, E. A., Milster, T. D., Ammons, S. M., Shao, M., Shaklan, S., Levine, M., Nemati, B., Martinache, F., Pitman, J., Woodruff, R. A., & Belikov, R. (2013). Simultaneous exoplanet characterization and deep wide-field imaging with a diffractive pupil telescope. Astrophysical Journal, 767(Issue 1). doi:10.1088/0004-637x/767/1/11
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  High-precision astrometry can identify exoplanets and measure their orbits and masses while coronagraphic imaging enables detailed characterization of their physical properties and atmospheric compositions through spectroscopy. In a previous paper, we showed that a diffractive pupil telescope (DPT) in space can enable sub-μas accuracy astrometric measurements from wide-field images by creating faint but sharp diffraction spikes around the bright target star. The DPT allows simultaneous astrometric measurement and coronagraphic imaging, and we discuss and quantify in this paper the scientific benefits of this combination for exoplanet science investigations: identification of exoplanets with increased sensitivity and robustness, and ability to measure planetary masses to high accuracy. We show how using both measurements to identify planets and measure their masses offers greater sensitivity and provides more reliable measurements than possible with separate missions, and therefore results in a large gain in mission efficiency. The combined measurements reliably identify potentially habitable planets in multiple systems with a few observations, while astrometry or imaging alone would require many measurements over a long time baseline. In addition, the combined measurement allows direct determination of stellar masses to percent-level accuracy, using planets as test particles. We also show that the DPT maintains the full sensitivity of the telescope for deep wide-field imaging, and is therefore compatible with simultaneous scientific observations unrelated to exoplanets. We conclude that astrometry, coronagraphy, and deep wide-field imaging can be performed simultaneously on a single telescope without significant negative impact on the performance of any of the three techniques. © 2013. The American Astronomical Society. All rights reserved.
• Stalcup, T., Hammer, K., Lesser, D., Olbert, B., Warner, S., Wheelwright, B., Angel, R., & Villaneuva, J. (2013). Initial results of advanced glass slumping for commercial CSP systems. Optics InfoBase Conference Papers.
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  Abstract: We present the initial results of advanced glass slumping for commercial CSP systems. It is shown that the fast-slumping process is repeatable and accurate using a furnace and curved steel mold. Renewable Energy and the Environment Congress. © 2013.
• Wheelwright, B., Angel, R., Weiser, J., Stalcup, T., & Coughenour, B. (2013). A line-scanning laser hartmann test for parabolic trough mirrors. Optics InfoBase Conference Papers.
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  Abstract: We present a simple method to determine slope errors in large parabolic trough mirrors. While most slope-measurement schemes determine slope errors via indirect computation, the method presented here is direct, and has inherently high absolute accuracy. Renewable Energy and the Environment Congress. © 2013.
• Butel, G. P., Coughenour, B. M., Macleod, H. A., Kennedy, C. E., & Angel, J. R. (2012). Reflectance optimization of second-surface silvered glass mirrors for concentrating solar power and concentrating photovoltaics application. Journal of Photonics for Energy, 2(Issue 1). doi:10.1117/1.jpe.2.021808
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  Methods developed to maximize the overall reflectance of the second-surface silvered glass used in concentrating solar power (CSP) and concentrating photovoltaics (CPV) solar systems are reported. The reflectance at shorter wavelengths is increased with the aid of a dielectric enhancing layer between the silver and the glass, while at longer wavelengths it is enhanced by use of glass with negligible iron content. The calculated enhancement of reflectance, compared to unenhanced silver on standard low-iron float glass, corresponds to a 4.5% increase in reflectance averaged across the full solar spectrum, appropriate for CSP, and 3.5% for CPV systems using triple junction cells. An experimental reflector incorporating these improvements, of drawn crown glass and a silvered second-surface with dielectric enhancement, was measured at National Renewable Energy Laboratory to have 95.4% solar weighted reflectance. For comparison, nonenhanced, wet-silvered reflectors of the same 4-mm thickness show reflectance ranging from 91.6% to 94.6%, depending on iron content. A potential drawback of using iron-free drawn glass is reduced concentration in high concentration systems because of the inherent surface errors. This effect is largely mitigated for glass shaped by slumping into a concave mold, rather than by bending. Finally, an experiment capable of determining which junction limits the triple junction cell is demonstrrated. © 2012 Society of Photo-Optical Instrumentation Engineers (SPIE).
• Butel, G. P., Coughenour, B. M., Macleod, H. A., Kennedy, C. E., & Roger, J. (2012). Reflectance optimization of second-surface silvered glass mirrors for concentrating solar power and concentrating photovoltaics application. Journal of Photonics for Energy, 2(1).
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  Abstract: Methods developed to maximize the overall reflectance of the second-surface silvered glass used in concentrating solar power (CSP) and concentrating photovoltaics (CPV) solar systems are reported. The reflectance at shorter wavelengths is increased with the aid of a dielectric enhancing layer between the silver and the glass, while at longer wavelengths it is enhanced by use of glass with negligible iron content. The calculated enhancement of reflectance, compared to unenhanced silver on standard low-iron float glass, corresponds to a 4.5% increase in reflectance averaged across the full solar spectrum, appropriate for CSP, and 3.5% for CPV systems using triple junction cells. An experimental reflector incorporating these improvements, of drawn crown glass and a silvered second-surface with dielectric enhancement, was measured at National Renewable Energy Laboratory to have 95.4% solar weighted reflectance. For comparison, nonenhanced, wet-silvered reflectors of the same 4-mm thickness show reflectance ranging from 91.6% to 94.6%, depending on iron content. A potential drawback of using iron-free drawn glass is reduced concentration in high concentration systems because of the inherent surface errors. This effect is largely mitigated for glass shaped by slumping into a concave mold, rather than by bending. Finally, an experiment capable of determining which junction limits the triple junction cell is demonstrrated. © 2012 Society of Photo-Optical Instrumentation Engineers (SPIE).
• Coughenour, B., Wheelwright, B., & Roger, J. (2012). Illumination characterization of glass and metal secondary concentrator elements in a photovoltaic solar concentration system. Proceedings of SPIE - The International Society for Optical Engineering, 8468.
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  Abstract: Using illumination modeling, we provide a comparison of glass Total Internal Reflection Concentrators (TIRC) and metal Hollow Reflective Concentrators (HRC) used as secondary concentrator elements in a dish-based highconcentration photovoltaic (CPV) system. Comparisons of optical efficiency, flux uniformity, off-axis acceptance angle, and cost are vital to choosing an ideal secondary concentrator element for a CPV system employing multi-junction (MJ) cells. In many CPV systems, a free-form optic or sharp-cornered rectangular TIRC composed of glass is used to increase the geometrical flux concentration at the surface of the MJ cell, and may also serve as homogenizers to mix the light to increase flux uniformity. We have demonstrated in on-sun testing that an electroformed metal HRC can be used in place of a glass TIRC of the same geometry, eliminating the need for polymeric bonding to the MJ cell surface, and providing a side-contact surface pathway for active cooling. Although geometrically equivalent, we show that glass TIRC's achieve superior off-axis performance (higher etendue from surface refraction) and are generally acknowledged to have less degradation than optics with over-coated silver, yet metal HRC's employing over-coated silver are superior in spectral absorption characteristics under high solar flux (no losses from glass absorption or Fresnel surface reflections) and don't require accurate glass pressing into many shapes. To better understand the trade-offs between optical efficiency, off-axis performance, mechanical tolerances, cost and reliability, metal (HRC) and glass (TIRC) tapered funnels are analyzed at the surface of equal irradiance in a Kohler-Illumination concentrator system, and a trade study is presented. © 2012 SPIE.
• Guyon, O., Bendek, E. A., Eisner, J. A., Angel, R., Woolf, N. J., Milster, T. D., Ammons, S. M., Shao, M., Shaklan, S., Levine, M., Nemati, B., Pitman, J., Woodruff, R. A., & Belikov, R. (2012). High-precision astrometry with a diffractive pupil telescope. Astrophysical Journal, Supplement Series, 200(2).
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  Abstract: Astrometric detection and mass determination of Earth-mass exoplanets require sub-μas accuracy, which is theoretically possible with an imaging space telescope using field stars as an astrometric reference. The measurement must, however, overcome astrometric distortions, which are much larger than the photon noise limit. To address this issue, we propose to generate faint stellar diffraction spikes using a two-dimensional grid of regularly spaced small dark spots added to the surface of the primary mirror (PM). Accurate astrometric motion of the host star is obtained by comparing the position of the spikes to the background field stars. The spikes do not contribute to scattered light in the central part of the field and therefore allow unperturbed coronagraphic observation of the star's immediate surroundings. Because the diffraction spikes are created on the PM and imaged on the same focal plane detector as the background stars, astrometric distortions affect equally the diffraction spikes and the background stars and are therefore calibrated. We describe the technique, detail how the data collected by the wide-field camera are used to derive astrometric motion, and identify the main sources of astrometric error using numerical simulations and analytical derivations. We find that the 1.4 m diameter telescope, 0.3 deg 2 field we adopt as a baseline design achieves 0.2 μas single measurement astrometric accuracy. The diffractive pupil concept thus enables sub-μas astrometry without relying on the accurate pointing, external metrology, or high-stability hardware required with previously proposed high-precision astrometry concepts. © 2012. The American Astronomical Society. All rights reserved..
• Guyon, O., Bendek, E. A., Eisner, J. A., Angel, R., Woolf, N. J., Milster, T. D., Mark Ammons, S., Shao, M., Shaklan, S., Levine, M., Nemati, B., Pitman, J., Woodruff, R. A., & Belikov, R. (2012). High-precision astrometry with a diffractive pupil telescope. Astrophysical Journal, Supplement Series, 200(Issue 2). doi:10.1088/0067-0049/200/2/11
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  Astrometric detection and mass determination of Earth-mass exoplanets require sub-μas accuracy, which is theoretically possible with an imaging space telescope using field stars as an astrometric reference. The measurement must, however, overcome astrometric distortions, which are much larger than the photon noise limit. To address this issue, we propose to generate faint stellar diffraction spikes using a two-dimensional grid of regularly spaced small dark spots added to the surface of the primary mirror (PM). Accurate astrometric motion of the host star is obtained by comparing the position of the spikes to the background field stars. The spikes do not contribute to scattered light in the central part of the field and therefore allow unperturbed coronagraphic observation of the star's immediate surroundings. Because the diffraction spikes are created on the PM and imaged on the same focal plane detector as the background stars, astrometric distortions affect equally the diffraction spikes and the background stars and are therefore calibrated. We describe the technique, detail how the data collected by the wide-field camera are used to derive astrometric motion, and identify the main sources of astrometric error using numerical simulations and analytical derivations. We find that the 1.4 m diameter telescope, 0.3 deg2 field we adopt as a baseline design achieves 0.2 μas single measurement astrometric accuracy. The diffractive pupil concept thus enables sub-μas astrometry without relying on the accurate pointing, external metrology, or high-stability hardware required with previously proposed high-precision astrometry concepts. © 2012. The American Astronomical Society. All rights reserved..
• Stalcup, T., Roger, J., Coughenour, B., Wheelwright, B., Connors, T., Davison, W., Lesser, D., Elliott, J., & Schaefer, J. (2012). On-sun performance of an improved dish-based HCPV system. Proceedings of SPIE - The International Society for Optical Engineering, 8468.
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  Abstract: The University of Arizona has developed a new dish-based High Concentration Photovoltaic (HCPV) system which is in the process of being commercialized by REhnu, Inc. The basic unit uses a paraboloidal glass reflector 3.1 m x 3.1 m square to bring sunlight to a high power point focus at a concentration of ~20,000x. A unique optical system at the focus reformats the concentrated sunlight so as to uniformly illuminate 36 triple junction cells at 1200x geometric concentration. The relay optics and cells are integrated with an active cooling system in a self-contained Power Conversion Unit (PCU) suspended above the dish reflector. Only electrical connections are made to the PCU as the active cooling system within is completely sealed. Eight of these reflector/PCU units can be mounted on a single two axis tracking structure2. Our 1 st generation prototype reflector/PCU unit consistently generated 2.2 kW of power normalized to 1kW/m2 DNI in over 200 hours of on-sun testing in 20113. Here, we present on-sun performance results for our 2 nd generation prototype reflector/PCU unit, which has been in operation since June 2012. This improved system consistently generates 2.7 kW of power normalized to 1kW/m2 DNI and has logged over 100 hours of on-sun testing. This system is currently operating at 28% DC net system efficiency with an operating cell temperature of only 20°C above ambient. Having proven this system concept, work on our 3rd generation prototype is underway with a focus on manufacturability, lower cost, and DC efficiency target of 32% or better. © 2012 SPIE.
• Stephens, K., & Roger, J. (2012). Comparison of collection and land use efficiency for various solar concentrating field geometries. Proceedings of SPIE - The International Society for Optical Engineering, 8468.
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  Abstract: In this paper we analyze the effects of neighbor shadowing of tracking solar photovoltaic arrays when they are set out in solar farms for large scale generation. Closer tracker spacing yields more power per unit area of land, but less power per tracking unit because of shadowing. A model has been developed to quantify and compare efficiencies for different tracker aspect ratios and field layouts, on an hourly, daily and annual basis. The model accounts for atmospheric absorption as well as neighbor shadowing at low solar altitude angles. We have focused on the case of CPV arrays which are oriented normal to the rays from the sun. The field layout is best characterized by the ratio of total array area to land area (the ground cover ratio or GCR). We explore as a function of GCR both the fraction of all the direct sunlight energy that is intercepted by the arrays (the irradiance collection efficiency) and the energy lost by each array because of shadowing. Examples are worked out for rectangular arrays on dual axis trackers at 33° latitude. We find that for a ground cover ratio of 30% the annual irradiance collection efficiency is 50%, almost independent of the layout pattern or the array aspect ratio. For a ground cover ratio of 40%, the irradiance collection efficiency rises to 65%. The corresponding shadowing losses do depend on aspect ratio, thus for 30% GCR the annual average of shadowing loss is 7.2% for 3:1 aspect ratio, rising to 7.8% for 2:1 aspect ratio. High GCR is not realizable for higher aspect ratios, which lead to large swing radius, but for 2:1 aspect ratio 40% GCR results in shadowing loss of 11.5%. One conclusion is that a solar farm with arrays of 2:1 aspect ratio set out with 40% GCR is good compromise when land is scarce: 64% of all the direct sunlight energy incident on the land is harvested by the arrays, with only 11.5% shadowing loss. We have compared these efficiencies with those for trough CSP systems, which also harvest direct sunlight but with reflectors turning about a single, horizontal N-S axis. For given GCR, the shadowing loss is slightly less (0.5%) than for the above dual-axis arrays, however the irradiance collection efficiency is worse in winter, leading to a lower annual average for a given GCR. For example, at 40% GCR, a single-axis system realizes a 56% irradiance collection efficiency compared to 64% for the dual axis systems. © 2012 SPIE.
• Angel, R. (2011). Concentrating optics: From giant astronomical telescopes to low-cost HCPV. AIP Conference Proceedings, 1407, 61-65.
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  Abstract: Triple-junction PV cells used at 1000x concentration are both highly efficient and inexpensive, per watt of electricity produced. A power system based on telescope design principles uses these cells to make utility-scale solar electricity at cost parity with fossil fuel. First, sunlight is concentrated by an array of large square dish reflectors, co-aligned in a mechanically-efficient, open spaceframe structure with built-in elevation tracking axis and drive. Second, the concentrated sunlight at each focus is converted into electricity by many cells packaged in a small receiver, with a ball lens and optical funnels to ensure even distribution between cells. This architecture is optimized for minimum cost in high volume production. The large steel and glass elements and the small integrated receiver and radiator are separately manufactured and shipped for assembly in a facility near the solar plant. © 2011 American Institute of Physics.
• Angel, R., Connors, T., Davison, W., Rademacher, M., Coughenour, B., Butel, G., & Lesser, D. (2011). Development and on-sun performance of dish-based HCPV. AIP Conference Proceedings, 1407, 34-37.
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  Abstract: The paper describes a new system architecture optimized for utility-scale generation with concentrating photovoltaic cells (CPV). The system concept is optimized to use predominantly low-cost materials manufactured by methods proven for high volume production. Triple-junction cells are used to convert 1000x concentrated sunlight into electricity. Compared to silicon panels, these commercially available cells convert at least twice as much of the incident sunlight energy into electricity, and at 1000x optical concentration, they cost one-tenth as much per watt of power output. The architecture combines three novel elements: large (3.1mx3.1m square) paraboloidal glass dish reflectors to collect and concentrate the sunlight; compact receivers at each dish focus, each one incorporating multiple, actively cooled cells; and a lightweight steel spaceframe structure to hold multiple dish/receiver units in co-alignment and oriented to the sun. A manufacturing process for replicating the reflector dishes is well advanced in development at the Steward Observatory Mirror Lab. A lightweight steel spaceframe structure to hold and track eight dish/receiver units to generate 20 kW has been completed. Tests over several months showed that for 99% of the time the tracking error was less than 0.1 degree. A test receiver populated with 8 out of 36 cells and attached to the tracker was operated for two months, yielding consistently over 500 W for much of each day. The receiver also maintained 95% of full power for mispointing of 1/2°. The receiver is now being completed with its full 36 cell complement, for >2kW output. This technology is being commercialized by REhnu, LLC, under an exclusive license from the University of Arizona. © 2011 American Institute of Physics.
• Butel, G. P., Coughenour, B. M., MacLeod, H. A., Kennedy, C. E., Olbert, B. H., & Roger, J. (2011). Second-surface silvered glass solar mirrors of very high reflectance. Proceedings of SPIE - The International Society for Optical Engineering, 8108.
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  Abstract: This paper reports methods developed to maximize the overall reflectance second-surface silvered glass. The reflectance at shorter wavelengths is increased with the aid of a dielectric enhancing layer between the silver and the glass, while at longer wavelengths it is enhanced by use of glass with negligible iron content. The calculated enhancement of reflectance, compared to unenhanced silver on standard low-iron float glass, corresponds to a 4.4% increase in reflectance averaged across the full solar spectrum, appropriate for CSP, and 2.7% for CPV systems using triple junction cells. An experimental reflector incorporating these improvements, of drawn crown glass and a silvered second-surface with dielectric boost, was measured at NREL to have 95.4% solar weighted reflectance. For comparison, non-enhanced, wetsilvered reflectors of the same 4 mm thickness show reflectance ranging from 91.6 - 94.6%, depending on iron content. A potential drawback of using iron-free drawn glass is reduced concentration in high concentration systems because of the inherent surface errors. This effect is largely mitigated for glass shaped by slumping into a concave mold, rather than by bending. © 2011 SPIE.
• Butel, G., Connors, T., Coughenour, B., & Angel, R. (2011). Design, optimization and characterization of secondary optics for a dish-based 1000x HCPV system. Optics InfoBase Conference Papers.
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  Abstract: This paper presents a novel design of a solar secondary optics used in a dish-based HCPV system at 1000x. Different optimizations were conducted as well as experiments to determine its optimum configuration. © 2011 OSA.
• Coughenour, B., Butel, G., & Angel, R. (2011). Defining system conversion efficiency for dish-based solar concentrator PV. Optics InfoBase Conference Papers.
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  Abstract: Concentrator system conversion efficiency may be characterized by using measurements of the optical components. Techniques of characterizing radiometric throughput via geometric ray shadowing, optical efficiency, and cell efficiency are presented. © 2011 OSA.
• Angel, R. (2010). New optical concept for concentrator photovoltaics. Optics InfoBase Conference Papers.
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  Abstract: Sunlight is collected by 3.1 x 3.1 m square dish reflectors. At each focus, light is apportioned by secondary optics onto many cells in a concave array, with each cell receiving the same 1000x concentration. © OSA/SOLAR 2010.
• Angel, R., Connors, T., Davison, W., Olbert, B., & Sivanandam, S. (2010). New architecture for utility-scale electricity from concentrator photovoltaics. Proceedings of SPIE - The International Society for Optical Engineering, 7769.
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  Abstract: The paper describes a new system architecture optimized for utility-scale generation with concentrating photovoltaic cells (CPV) at fossil fuel price. We report on-sun tests of the architecture and development at the University of Arizona of the manufacturing processes adapted for high volume production. The new system takes advantage of triple-junction cells to convert concentrated sunlight into electricity. These commercially available cells have twice the conversion efficiency of silicon panels (40%) and one-tenth the cost per watt, when used at 1000x concentration. Telescope technology is adapted to deliver concentrated light to the cells at minimum cost. The architecture combines three novel elements: large (3.1 m × 3.1 m square) dish reflectors made as back-silvered glass monoliths; 2.5 kW receivers at each dish focus, each one incorporating a spherical field lens to deliver uniform illumination to multiple cells; and a lightweight steel spaceframe structure to hold multiple dish/receiver units in coalignment and oriented to the sun. Development of the process for replicating single-piece reflector dishes is well advanced at the Steward Observatory Mirror Lab. End-to-end system tests have been completed with single cells. A lightweight steel spaceframe to hold and track eight dish/receiver units to generate 20 kW has been completed. A single 2.5 kW receiver is presently under construction and is expected to be operated in an end-to-end on-sun test with a monolithic dish before the end of 2010. The University of Arizona has granted an exclusive license to REhnu, LLC to commercialize this technology. © 2010 SPIE.
• Peng, S. u., Parks, R. E., Angel, R., Wang, L., & Burge, J. H. (2010). Scots, an alternative to surface topographic interferometry. Proceedings - ASPE Summer Topical Meeting on Precision Interferometric Metrology, ASPE 2010, 49, 52-57.
• Peng, S. u., Parks, R. E., Wang, L., Angel, R. P., & Burge, J. H. (2010). SCOTS: A quantitative slope measuring method for optical shop use. Optics InfoBase Conference Papers.
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  Abstract: Software Configurable Optical Test System (SCOTS)[1], a computerized "reverse Hartmann test", can rapidly quantitatively measure highly aspherical shapes such as solar collectors, Giant Magellan Telescope (GMT) primary segment with accuracy to micron rms or better without complex calibration. Implementation of SCOTS needs only hardware like a laptop computer. It illuminates test surface/system with patterns from a LCD screen and use reflected/refracted image to determine surface/system wavefront slope variations. © 2010 Optical Society of America.
• Peng, S. u., Parks, R. E., Wang, L., Angel, R. P., & Burge, J. H. (2010). Software configurable optical test system: A computerized reverse Hartmann test. Applied Optics, 49(23), 4404-4412.
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  PMID: 20697443;Abstract: A software configurable optical test system (SCOTS) based on the geometry of the fringe reflection or phase measuring deflectometry method was developed for rapidly, robustly, and accurately measuring large, highly aspherical shapes such as solar collectors and primary mirrors for astronomical telescopes. In addition to using phase shifting methods for data collection and reduction, we explore the test from the point view of performing traditional optical testing methods, such as Hartmann or Hartmann-Shack tests, in a reverse way. Using this concept, the slope data calculation and unwrapping in the test can also be done with centroiding and line-scanning methods. These concepts expand the test to work in more general situations where fringe illumination is not practical. Experimental results show that the test can be implemented without complex calibration for many applications by taking the geometric advantage of working near the center curvature of the test part. The results also show that the test has a large dynamic range, can achieve measurement accuracy comparable with interferometric methods, and can provide a good complement to interferometric tests in certain circumstances. Avariation of this method is also useful for measuring refractive optics and optical systems. As such, SCOTS provides optical manufacturers with a new tool for performing quantitative full field system evaluation. © 2010 Optical Society of America.
• Tenerelli, D., Angel, R., Burge, J., Guyon, O., Zabludoff, A., Belikov, R., Pluzhnik, E., & Egerman, R. (2010). A 4-meter, Wide Field Coronagraph space Telescope for general astrophysics and exoplanet observations. Proceedings of SPIE - The International Society for Optical Engineering, 7731.
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  Abstract: The Wide Field Coronagraph Telescope (WFCT) is a 4-meter space telescope for general astrophysics and exoplanet observations that meets the 2000 Decadal Committee requirements. This paper presents a design for a 4-m diameter, off-axis space telescope that offers high performance in both wide field and coronagraphic imaging modes. A 3.8 × 3.3-m unobstructed elliptical pupil is provided for direct coronagraphic imaging of exoplanets and a 4-m diameter pupil for wide-field imaging from far-ultraviolet (UV) to near-infrared (IR). The off-axis wide-field optics are all reflective and designed to deliver an average of 12 nm wavefront aberrations over a 6 × 24 arcminute field of view (FOV), therefore providing diffraction-limited images down to 300 nm wavelength and 15 mas images down to a wavelength limit set only by the mirror coatings. The coronagraph with phase-induced amplitude apodization (PIAA) provides diffraction suppression around a 360-degree field with high Strehl and sensitivity at the 1e-10 level to an inner working angle of 2 λ/D (or 50 mas at 500 nm wavelength). This paper focuses on the optical design that allows the above imaging features to be combined in single telescope, and gives a preliminary spacecraft design and costing, assuming a distant trailing orbit. © 2010 SPIE.
• Wang, L., Peng, S. u., Parks, R., Angel, R., Sasian, J., & Burge, J. (2010). A low-cost, flexible, high dynamic range test for free-form illumination optics. Proceedings of SPIE - The International Society for Optical Engineering, 7652.
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  Abstract: Software Configurable Optical Test System (SCOTS)[1], a computerized "reverse Hartmann test", can rapidly, robustly and quantitatively measure complex 3-dimensional specular surfaces, for instance large segmented solar collectors, faceted automotive headlight reflector or other free-form surfaces. © 2010 Copyright SPIE - The International Society for Optical Engineering.
• Guyon, O., R., J., Belikov, R., Egerman, R., Gavel, D., Giveon, A., Greene, T., Cahoy, K., Kern, B., Levine, M., Ridgway, S., Shaklan, S., Tenerelli, D., Vanderbei, R., & Woodruff, R. A. (2009). Detecting and characterizing exoplanets with a 1.4-m space telescope: The Pupil mapping Exoplanet Coronagraphic Observer (PECO). Proceedings of SPIE - The International Society for Optical Engineering, 7440.
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  Abstract: The Pupil-mapping Exoplanet Coronagraphic Observer (PECO) mission concept uses a coronagraphic 1.4-m space-based telescope to both image and characterize extra-solar planetary systems at optical wavelengths. PECO delivers 10 -10 contrast at 2 λ/D separation (0.15") using a high-performance Phase-Induced Amplitude Apodization (PIAA) coronagraph which remaps the telescope pupil and uses nearly all of the light coming into the aperture. For exoplanet characterization, PECO acquires narrow field images simultaneously in 16 spectral bands over wavelengths from 0.4 to 0.9 μm, utilizing all available photons for maximum wavefront sensing and sensitivity for imaging and spectroscopy. The optical design is optimized for simultaneous low-resolution spectral characterization of both planets and dust disks using a moderate-sized telescope. PECO will image the habitable zones of about 20 known F, G, K stars at a spectral resolution of R≈15 with sensitivity sufficient to detect and characterize Earth-like planets and to map dust disks to within a fraction of our own zodiacal dust cloud brightness. The PIAA coronagraph adopted for PECO reduces the required telescope diameter by a factor of two compared with other coronagraph approaches that were considered for Terrestrial Planet Finder Coronagraph Flight Baseline 1, and would therefore also be highly valuable for larger telescope diameters. We report on ongoing laboratory activities to develop and mature key PECO technologies, as well as detailed analysis aimed at verifying PECO's wavefront and pointing stability requirement can be met without requiring development of new technologies. © 2009 Copyright SPIE - The International Society for Optical Engineering.
• Angel, R., Worden, S. P., Borra, E. F., Eisenstein, D. J., Foing, B., Hickson, P., Josset, J., Ma, K. B., Seddiki, O., Sivanandam, S., Thibault, S., & Susante, P. V. (2008). A cryogenic liquid-mirror telescope on the moon to study the early universe. Astrophysical Journal, 680(2), 1582-1594.
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  Abstract: We have studied the feasibility and scientific potential of zenith observing liquid-mirror telescopes having 20-100 m diameters located on the Moon. They would carry out deep infrared surveys to study the distant universe and follow up discoveries made with the 6 m James Webb Space Telescope (JWST), with more detailed images and spectroscopic studies. They could detect objects 100 times fainter than JWST, observing the first high-redshift stars in the early universe and their assembly into galaxies. We explored the scientific opportunities, key technologies, and optimum location of such telescopes. We have demonstrated critical technologies. For example, the primary mirror would necessitate a high-reflectivity liquid that does not evaporate in the lunar vacuum and remains liquid at less than 100 K. We have made a crucial demonstration by successfully coating an ionic liquid that has negligible vapor pressure. We also successfully experimented with a liquid mirror spinning on a superconducting bearing, as will be needed for the cryogenic, vacuum environment of the telescope. We have investigated issues related to lunar locations, concluding that locations within a few kilometers of a pole are ideal for deep sky cover and long integration times. We have located ridges and crater rims within 0.5°of the north pole that are illuminated for at least some sun angles during lunar winter, providing power and temperature control. We also have identified potential. problems, like lunar dust. Issues raised by our preliminary study demand additional in-depth analyses. These issues must be fully examined as part of a scientific debate that we hope to start with the present article. © 2008. The American Astronomical Society. All rights reserved.
• Guyon, O., Angel, J. R., Backman, D., Belikov, R., Gavel, D., Giveon, A., Greene, T., Kasdin, J., Kasting, J., Levine, M., Marley, M., Meyer, M., Schneider, G., Serabyn, G., Shaklan, S., Shao, M., Tamura, M., Tenerelli, D., Traub, W., , Trauger, J., et al. (2008). Pupil mapping exoplanet coronagraphic observer (PECO). Proceedings of SPIE - The International Society for Optical Engineering, 7010.
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  Abstract: The Pupil mapping Exoplanet Coronagraphic Observer (PECO) mission concept is a 1.4-m telescope aimed at imaging and characterizing extra-solar planetary systems at optical wavelengths. The coronagraphic method employed, Phase-Induced Amplitude Apodization or PIAA (a.k.a. pupil mapping) can deliver le-10 contrast at 2 lambda/D and uses almost all the starlight that passes through the aperture to maintain higher throughput and higher angular resolution than any other coronagraph or miller, making PECO the theoretically most efficient existing approach for imaging extra-solar planetary systems. PECO's instrument also incorporates deformable mirrors for high accuracy wavefront control. Our studies show that a probe-scale PECO mission with 1.4 m aperture is extremely powerful, with the capability of imaging at spectral resolution R ≈15 the habitable zones of already known F, G, K stars with sensitivity sufficient to detect planets down to Earth size, and to map dust clouds down to a fraction of our zodiacal cloud dust brightness. PECO will acquire narrow field images simultaneously in 10 to 20 spectral bands covering wavelengths from 0.4 to 1.0 μm and will utilize all available photons for maximum wavefront sensing and imaging/spectroscopy sensitivity. This approach is well suited for low-resolution spectral characterization of both planets and dust clouds with a moderately sized telescope. We also report on recent results obtained with the laboratory prototype of a coronagraphic low order wavefront sensor (CLOWFS) for PIAA coronagraph. The CLOWFS is a key part of PECO's design and will enable high contrast at the very small PECO inner working angle.
• Rabien, S., Ageorges, N., Angel, R., Brusa, G., Brynnel, J., Busoni, L., Davies, R., Deysenroth, M., Esposito, S., Gässlere, W., Genzel, R., Green, R., Haug, M., Hart, M. L., Hölzl, G., Masciadri, E., Pogge, R., Quirrenbach, A., Rademacher, M., , Rix, H. W., et al. (2008). The laser guide star program for the LBT. Proceedings of SPIE - The International Society for Optical Engineering, 7015.
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  Abstract: Laser guide star adaptive optics and interferometry are currently revolutionizing ground-based near-IR astronomy, as demonstrated at various large telescopes. The Large Binocular Telescope from the beginning included adaptive optics in the telescope design. With the deformable secondary mirrors and a suite of instruments taking advantage of the AO capabilities, the LBT will play an important role in addressing major scientific questions. Extending from a natural guide star based system, towards a laser guide stars will multiply the number of targets that can be observed. In this paper we present the laser guide star and wavefront sensor program as currently being planned for the LBT. This program will provide a multi Rayleigh guide star constellation for wide field ground layer correction taking advantage of the multi object spectrograph and imager LUCIFER in a first step. The already foreseen upgrade path will deliver an on axis diffraction limited mode with LGS AO based on tomography or additional sodium guide stars to even further enhance the scientific use of the LBT including the interferometric capabilities.
• Angel, R., Tae, K., Cuerden, B., Guyon, O., & Stahl, P. (2007). Active thermal figure control concept for the TOPS II primary mirror. Proceedings of SPIE - The International Society for Optical Engineering, 6693.
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  Abstract: TOPS (Telescope to Observe Planetary Systems) is the first coronagraphic telescope concept designed specifically to take advantage of Guyon's method of Phase Induced Amplitude Apodization PIAA).1 The TOPS primary mirror may incorporates active figure control to help achieve the desired wavefront control to approximately 1 ÅRMS accurate across the spectral bandwidth. Direct correction of the primary figure avoids the need for a separate small deformable mirror. Because of Fresnel propagation, correction at a separate surface can introduce serious chromatic errors unless it is precisely conjugated to the primary. Active primary control also reduces complexity and mass and increases system throughput, and will likely enable a full system test to the 10-10 level in the 1 g environment before launch. We plan to use thermal actuators with no mechanical disturbance, using radiative heating or cooling fingers distributed inside the cells of a honeycomb mirror. The glass would have very small but finite coefficient of expansion of ∼ 5×10 -8/C. Low order modes would be controlled by front-to-back gradients and high order modes by local rib expansion and contraction. Finite element models indicate that for a mirror with n cells up to n Zeraike modes can be corrected to better than 90% fidelity, with still higher accuracy for the lower modes. An initial demonstration has been made with a borosilicate honeycomb mirror. Interferometric measurements show a single cell influence function with 300 nm stroke and ∼5 minute time constant.
• Baranec, C., Lloyd-Hart, M., Milton, N. M., Stalcup, T., Snyder, M., Vaitheeswaran, V., McCarthy, D., & Angel, R. (2007). Astronomical imaging using ground-layer adaptive optics. Proceedings of SPIE - The International Society for Optical Engineering, 6691.
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  Abstract: Over the past several years, experiments in adaptive optics involving multiple natural and Rayleigh laser guide stars have been carried out by our group at the 1.5 m Kuiper telescope and the 6.5 m MMT telescope. From open-loop data we have calculated the performance gains anticipated from ground-layer adaptive optics (GLAO) and laser tomography adaptive optics corrections. In July 2007, the GLAO control loop was closed around the focus signal from all five laser guide stars at the MMT, leading to a reduction in the measured focus mode on the laser wavefront sensor by 60%. For the first time, we expect to close the full high order GLAO control loop around the five laser beacons and a tilt star at the MMT in October 2007, where we predict image quality of < 0.2 arc seconds FWHM in K band (λ = 2.2 μm) over a 2 arc minute field. We intend to explore the image quality, stability and sensitivity of GLAO correction as a function of waveband with the science instrument PISCES. PISCES is a 1-2.5 μm imager with a field of view of 110 arc seconds, at a scale of 0.11 arc seconds per pixel. This is well matched to the expected FWHM performance of the GLAO corrected field and will be able to examine PSF non-uniformity and temporal stability across a wide field.
• Borra, E. F., Seddiki, O., Angel, R., Eisenstein, D., Hickson, P., Seddon, K. R., & Worden, S. P. (2007). Deposition of metal films on an ionic liquid as a basis for a lunar telescope. Nature, 447(7147), 979-981.
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  PMID: 17581579;Abstract: An optical/infrared telescope of 20-100 m aperture located on the Moon would be able to observe objects 100 to 1,000 times fainter than the proposed next generation of space telescopes. The infrared region of the spectrum is particularly important for observations of objects at redshifts z > 7. The apparent simplicity and low mass of a liquid mirror telescope, compared with a traditional pointable glass mirror, suggest that the concept should be considered further. A previously proposed liquid mirror telescope, based upon a spinning liquid metallic alloy, is not appropriate for infrared applications, which will require a liquid below 130 K. Here we report the successful coating of an ionic liquid with silver. The surface is smooth and the silver coating is stable on a timescale of months. The underlying ionic liquid does not evaporate in a vacuum and remains liquid down to a temperature of 175 K. Given that there are ∼106 simple and ∼1018 ternary ionic liquids, it should be possible to synthesize liquids with even lower melting temperatures. ©2007 Nature Publishing Group.
• Guyon, O., R., J., Bowers, C., Burge, J., Burrows, A., Codona, J., Greene, T., Iye, M., Kasting, J., Martin, H., McCarthy Jr., D. W., Meadows, V., Meyer, M., Pluzhnik, E. A., Sleeps, N., Tamura, M., Tenerelli, D., Vanderbei, R., Woodgate, B., , Woodruffs, R. A., et al. (2007). TOPS: A small space telescope using phase induced-amplitude apodization (PIAA) to image rocky and giant exo-planets. Proceedings of SPIE - The International Society for Optical Engineering, 6693.
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  Abstract: The Telescope to Observe Planetary Systems (TOPS) is a proposed space mission to image planetary systems of nearby stars simultaneously in a few wide spectral bands covering the visible light (0.4-0.9 μm). It achieves its power by combining a high accuracy wavefront control system with a highly efficient Phase-Induced Amplitude Apodization (PIAA) coronagraph which provides strong suppression very close to the star (within 2 λ/D). The PIAA coronagraphic technique opens the possibility of imaging Earthlike planets in visible light with a smaller telescope than previously supposed. If sized at 1.2-m, TOPS would image and characterize many Jupiter-sized planets, and discover 2 RE rocky planets within habitable zones of the ≈ 10 most favorable stars. With a larger 2-m aperture, TOPS would have the sensitivity to reveal Earth-like planets in the habitable zone around ≈20 stars, and to characterize any found with low resolution spectroscopy. Unless the occurrence of Earth-like planets is very low (η⊕
• Kenworthy, M. A., Codona, J. L., Hinz, P. M., Roger, J., Angel, P., Heinze, A., & Sivanandam, S. (2007). First on-sky high-contrast imaging with an apodizing phase plate. Astrophysical Journal, 660(1 I), 762-769.
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  Abstract: We present the first astronomical observations obtained with an apodizing phase plate (APP). The plate is designed to suppress the stellar diffraction pattern by 5 mag from 2 - 9λ/D over a 180° region. Stellar images were obtained in the M′ band (λc = 4.85 μm) at the MMTO 6.5 m telescope, with adaptive wave-front correction made with a deformable secondary mirror designed for low thermal background observations. The measured point-spread function (PSF) shows a halo intensity of 0.1% of the stellar peak at 2λ/D (0.36″), tapering off as r-5/3 out to radius 9λ/D. Such a profile is consistent with residual errors predicted for servo lag in the AO system. We project a 5 σ contrast limit, set by residual atmospheric fluctuations, of 10.2 mag at 0.36″ separation for a 1 hr exposure. This can be realized if static and quasi-static aberrations are removed by differential imaging, and is close to the sensitivity level set by thermal background photon noise for target stars with M′ > 3. The advantage of using the phase plate is the removal of speckle noise caused by the residuals in the diffraction pattern that remain after PSF subtraction. The APP gives higher sensitivity over the range (2-5) λ/D than direct imaging techniques. © 2007. The American Astronomical Society. All rights reserved.
• Lloyd-Hart, M., Angel, R., Green, R., Stalcup, T., Milton, N. M., & Powell, K. (2007). Multi-laser-guided adaptive optics for the large binocular telescope. Proceedings of SPIE - The International Society for Optical Engineering, 6691.
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  Abstract: We describe the conceptual design of an advanced laser guide star facility (LGSF) for the Large Binocular Telescope (LBT), to be built in collaboration with the LBT's international partners. The highest priority goal for the facility is the correction of ground-layer turbulence, providing partial seeing compensation in the near IR bands over a 4′ field. In the H band, GLAO is projected to improve the median seeing from 0.55″ to 0.2″. The new facility will build on the LBT's natural guide star AO system, integrated into the telescope with correction by adaptive secondary mirrors, and will draw on Arizona's experience in the construction of the first multi-laser adaptive optics (AO) system at the 6.5 m MMT.1 The LGSF will use four Rayleigh beacons at 532 nm, projected to an altitude of 25 km, on each of the two 8.4 m component telescopes. Initial use of the system for ground layer correction will deliver image quality well matched to the LBT's two LUCIFER near IR instruments. They will be used for direct imaging over a 4′×4′ field and will offer a unique capability in high resolution multi-object spectroscopy. The LGSF is designed to include long-term upgrade paths. Coherent imaging at the combined focus of the two apertures will be exploited by the LBT Interferometer in the thermal IR. Using the same launch optics, an axial sodium or Rayleigh beacon can be added to each constellation, for tomographic wavefront reconstruction and diffraction limited imaging over the usual isoplanatic patch. In the longer term, a second DM conjugated to high altitude is foreseen for the LBT's LINC-NIRVANA instrument, which would extend the coherent diffraction-limited field to an arcminute in diameter with multi-conjugate AO.
• Stalcup Jr., T. E., Angel, R., Lloyd-Hart, M., & Rademacher, M. J. (2007). Status of the MMT observatory multiple laser beacon projector. Proceedings of SPIE - The International Society for Optical Engineering, 6691.
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  Abstract: The laser guidestar system at the MMT Observatory has produced its first closed loop results and should be producing ground-layer corrected closed loop images within a few months. The LGS system at the MMT is one of few in the world that uses atmospheric Rayleigh scattering from reliable, low-cost lasers, and is unique in its use of a dynamic refocus technique to increase the telescope depth of field for increased return flux. The resulting 10 km depth of field introduces additional constraints on the minimum spot size for the beam projector design. The short exposure spot size as measured at the telescope cassegrain focus is 0.65 arcseconds in 0.59 arcsecond seeing in the visible. Additionally, a method to correct for image motion due to telescope vibrations using accelerometer data has been successfully implemented.
• Angel, J., & Angel, J. R. (2006). Feasibility of cooling the Earth with a cloud of small spacecraft near the inner Lagrange point (L1). Proceedings of the National Academy of Sciences of the United States of America, 103(46).
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  If it were to become apparent that dangerous changes in global climate were inevitable, despite greenhouse gas controls, active methods to cool the Earth on an emergency basis might be desirable. The concept considered here is to block 1.8% of the solar flux with a space sunshade orbited near the inner Lagrange point (L1), in-line between the Earth and sun. Following the work of J. Early [Early, JT (1989) J Br Interplanet Soc 42:567-569], transparent material would be used to deflect the sunlight, rather than to absorb it, to minimize the shift in balance out from L1 caused by radiation pressure. Three advances aimed at practical implementation are presented. First is an optical design for a very thin refractive screen with low reflectivity, leading to a total sunshade mass of approximately 20 million tons. Second is a concept aimed at reducing transportation cost to 50 dollars/kg by using electromagnetic acceleration to escape Earth's gravity, followed by ion propulsion. Third is an implementation of the sunshade as a cloud of many spacecraft, autonomously stabilized by modulating solar radiation pressure. These meter-sized "flyers" would be assembled completely before launch, avoiding any need for construction or unfolding in space. They would weigh a gram each, be launched in stacks of 800,000, and remain for a projected lifetime of 50 years within a 100,000-km-long cloud. The concept builds on existing technologies. It seems feasible that it could be developed and deployed in approximately 25 years at a cost of a few trillion dollars,
• Angel, R. (2006). Feasibility of cooling the Earth with a cloud of small spacecraft near the inner Langrange point (L1). Proceedings of the National Academy of Sciences of the United States of America, 103(46), 17184-17189.
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  PMID: 17085589;PMCID: PMC1859907;Abstract: If it were to become apparent that dangerous changes in global climate were inevitable, despite greenhouse gas controls, active methods to cool the Earth on an emergency basis might be desirable. The concept considered here is to block 1.8% of the solar flux with a space sunshade orbited near the inner Lagrange point (L1), in-line between the Earth and sun. Following the work of J. Early [Early, JT (1989) J Br Interplanet Soc 42:567-569], transparent material would be used to deflect the sunlight, rather than to absorb it, to minimize the shift in balance out from L1 caused by radiation pressure. Three advances aimed at practical implementation are presented. First is an optical design for a very thin refractive screen with low reflectivity, leading to a total sunshade mass of ≈20 million tons. Second is a concept aimed at reducing transportation cost to $50/kg by using electromagnetic acceleration to escape Earth's gravity, followed by ion propulsion. Third is an implementation of the sunshade as a cloud of many spacecraft, autonomously stabilized by modulating solar radiation pressure. These meter-sized "flyers" would be assembled completely before launch, avoiding any need for construction or unfolding in space. They would weigh a gram each, be launched in stacks of 800,000, and remain for a projected lifetime of 50 years within a 100,000-km-long cloud. The concept builds on existing technologies. It seems feasible that it could be developed and deployed in ≈25 years at a cost of a few trillion dollars,
• Angel, R., Codona, J. L., Hinz, P., & Close, L. (2006). Exoplanet imaging with the giant magellan telescope. Proceedings of SPIE - The International Society for Optical Engineering, 6267 II.
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  Abstract: Over the next decade, we can expect that some imaging of extrasolar planets will be possible with the high-resolution LET and present 8 m class telescopes. But it will be limited by sensitivity and contrast ratio to self-luminous planets of the nearest young stars. When the Giant Magellan Telescope (GMT) comes on-line, it will have because of much larger light grasp and sharper PSF, the potential for imaging many new planets as well as, for the first time, imaging planets of known msini. It will also be capable of starting atmospheric studies through spectrophotometry. The full angular resolution of the GMT (that of D=24 m filled aperture) will be exploited with coronagraphy and nulling interferometry. The new coronagraphic technique of phase apodization being pioneered at the MMT will enable very high contrast at angular separations ≥3λ/D. To reach the highest contrast levels, the AO system is being designed not to minimize wavefront error, but to shape the corrected wavefront so as to cancel speckles in the search region. Interferometric measurements of complex amplitude in the focal plane make this possible, regardless of whether the speckles originate from errors in diffraction or phase. New control algorithms are being developed to minimize the decorrelation time as well as the intensity of residual speckles, so that they average out to the smoothest possible background halo. In this way, detections at 1.65 μm at the 5σ level of planets at 10 -8 contrast at 50 mas separation should be possible. The low background AO system of the GMT, made with its deformable secondary, will allow also high contrast imaging with high sensitivity at 5 μm, down to 100 mas separation.
• Angel, R., Eisenstei, D., Sivanandam, S., Worden, S. P., Burge, J., Borra, E., Gosselin, C., Seddiki, O., Hickson, P., Bui, M. K., Foing, B., Josset, J., Thibault, S., & Susante, P. V. (2006). A Lunar Liquid Mirror Telescope (LLMT) for deep-field infrared observations near the lunar pole. Proceedings of SPIE - The International Society for Optical Engineering, 6265 I.
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  Abstract: We have studied the feasibility and scientific potential of a 20 - 100 m aperture astronomical telescope at the lunar pole, with its primary mirror made of spinning liquid at less than 100K. Such a telescope, equipped with imaging and multiplexed spectroscopic instruments for a deep infrared survey, would be revolutionary in its power to study the distant universe, including the formation of the first stars and their assembly into galaxies. The LLMT could be used to follow up discoveries made with the 6 m James Webb Space Telescope, with more detailed images and spectroscopic studies, as well as to detect objects 100 times fainter, such as the first, high-red shift stars in the early universe. Our preliminary analysis based on SMART-1 AMIE images shows ridges and crater rims within 0.5° of the North Pole are illuminated for at least some sun angles during lunar winter. Locations near these points may prove to be ideal for the LLMT. Lunar dust deposited on the optics or in a thin atmosphere could be problematic. An in-situ site survey appears necessary to resolve the dust questions.
• Baranec, C., Lloyd-Hart, M., Milton, N. M., Stalcup, T., Snyder, M., & Angel, R. (2006). Tomographic reconstruction of stellar wavefronts from multiple laser guide stars. Proceedings of SPIE - The International Society for Optical Engineering, 6272 I.
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  Abstract: Experiments have been carried out at the MMT telescope in June 2005 and again in April 2006 to validate open loop tomographic wavefront reconstruction using five dynamically refocused Rayleigh laser beacons (RLGS) and multiple tilt natural guide stars (NGS). Wavefront sensing in this manner is recognized as a critical precursor to the development of adaptive optics for Extremely Large Telescopes. At the MMT, wavefronts from the laser beacons are recorded by five 60-element Shack-Hartmann sensors implemented on a single CCD. A wide-field camera measures image motion from multiple field stars to calculate global tilt and distinguish effects of contributions to second order aberrations from low and high altitude turbulence. Together, the signals from these sensors are used to estimate the first 45 Zernike modes in the wavefront of a star within the LGS constellation. The reconstruction is compared off line to simultaneous wavefront measurements made of the star with a separate Shack-Hartmann sensor. We will present the results in this paper and quantify the wavefront improvement expected from tomographic adaptive optics correction.
• Codona, J. L., Kenworthy, M. A., Hinz, P. M., Angel, J. R., & Woolf, N. J. (2006). A high-contrast coronagraph for the MMT using phase apodization: Design and observations at 5 microns and 2 λ/D radius. Proceedings of SPIE - The International Society for Optical Engineering, 6269 I.
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  Abstract: We report on the first implementation of phase apodization for high-contrast imaging at close inner working angle. It is designed for use in the 5 μm M band with the adaptive optics system at the MMT, which uses a deformable secondary for low thermal background and achieves a Strehl ratio of 90% at 5 μm. The method uses a diamond-turned ZnSe phase plate located at a cold pupil stop to diffract starlight into an "anti-halo" which suppresses the Airy diffraction pattern over a semi-circular region around the star. The design was optimized for strong suppression from the first bright Airy ring out to the control radius achievable with the MMT deformable secondary, about 9λ/D. The time-averaged PSF of a bright star agrees well with the design profile, the core with FWHM of 0.18 arcsec showing the diffraction-limited resolution of the full aperture. At 0.34 arcsec radius (2λ/D) the floor level is 3.5 × 10 -3 of the central peak, limited by residual atmospheric errors with 3 m wavelength across the aperture. The measured fluctuations at this radius averaged over 20 seconds are 2.5 × 10 -4 rms (9 magnitudes down from the peak). With the addition of active feedback to control residual speckles caused by static wavefront errors and with longer exposures, we project that exoplanet searches should reach 5σ sensitivity level > 10 magnitudes in an hour of integration.
• Fabricant, D., Hertz, E., Brown, W., McLeod, B., Angel, R., & Lloyd-Hart, M. (2006). A wide-field IR spectrograph for the Giant Magellan Telescope. Proceedings of SPIE - The International Society for Optical Engineering, 6269 I.
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  Abstract: The Giant Magellan Telescope, with seven 8.4 meter primary mirrors, is taking shape as one of the most powerful telescopes of the next generation. We describe a conceptual design for a powerful 0.85 to 2.50 μm imaging spectrograph that addresses a 7′ by 7′ field of view for imaging and a 5′ by 7′ field of view for spectroscopy at the GMTs f/8 Gregorian focus. The all-refractive optical design presses the limits of available lens blank diameters, but delivers excellent images (∼0.15Prime; 80% encircled energy) with just four collimator elements and five camera elements. The collimated beam diameter is 300 mm, and the detector is a 6K by 10K array. The spectrograph will use interchangeable slit masks, and an assortment of VPH and conventional surface relief gratings. Each of the entire J, H, or K bands can be observed with a resolution of 3000. The scientific potential of ground layer adaptive optics (GLAO) using a constellation of sodium laser guide stars appears to be very high in the near infrared. Simulations suggest that 0.2″ FWHM images may be achieved across the entire 7′ by 7′ field of view of the spectrograph. We describe the design of the GLAO system with a versatile opto-mechanical design that allows rapid changeover between GLAO and seeing-limited observations.
• Guyon, O., Angel, J. R., Bowers, C., Burge, J., Burrows, A., Codona, J., Greene, T., Iye, M., Kasting, J., Martin, H., McCarthy Jr., D. W., Meadows, V., Meyer, M., Pluzhnik, E. A., Sleep, N., Spears, T., Tamura, M., Tenerelli, D., Vanderbei, R., , Woodgate, B., et al. (2006). Telescope to Observe Planetary Systems (TOPS): A high throughput 1.2-m visible telescope with a small inner working angle. Proceedings of SPIE - The International Society for Optical Engineering, 6265 I.
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  Abstract: The Telescope to Observe Planetary Systems (TOPS) is a proposed space mission to image in the visible (0.4-0.9μzm) planetary systems of nearby stars simultaneously in 16 spectral bands (resolution R≈20). For the ≈10 most favorable stars, it will have the sensitivity to discover 2RE rocky planets within habitable zones and characterize their surfaces or atmospheres through spectrophotometry. Many more massive planets and debris discs will be imaged and characterized for the first time. With a 1.2m visible telescope, the proposed mission achieves its power by exploiting the most efficient and robust coronagraphic and wavefront control techniques. The Phase-Induced Amplitude Apodization (PIAA) coronagraph used by TOPS allows planet detection at 2λ/d with nearly 100% throughput and preserves the telescope angular resolution. An efficient focal plane wavefront sensing scheme accurately measures wavefront aberrations which are fed back to the telescope active primary mirror. Fine wavefront control is also performed independently in each of 4 spectral channels, resulting in a system that is robust to wavefront chromaticity.
• Hinz, P. M., Heinze, A. N., Sivanandam, S., Miller, D. L., Kenworthy, M. A., Brusa, G., Freed, M., & Angel, J. R. (2006). Thermal infrared constraint to a planetary companion of Vega with the MMT adaptive optics system. Astrophysical Journal, 653(2 I), 1486-1492.
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  Abstract: Vega may have a massive companion in a wide orbit, as evidenced by structure in its cold dust debris. We have tested this hypothesis by direct imaging with adaptive optics in the M band. The observations were made with a newly commissioned thermal infrared camera, Clio, on the 6.5 MMT AO system with low-background deformable secondary mirror. The observations constrain a planet to be less than 7 MJ at the approximate position angle expected from the dust structure and at a radius >20 AU (2″.5). This result is more stringent than similar previous near-infrared observations of Vega, which achieved limits of 20 and 10 MJ at separations of 7″. The higher sensitivity is due to both the more favorable contrast of gas giant planets at the M band and the higher Strehl ratio and more stable point spread function at longer wavelengths. Future L′ or M band observations could provide a powerful approach for wide-separation planet detection, especially for cooler and thus older or less massive planets. The natural best targets are nearby stars, where planets in the range of 5-15 MJ and as old as several Gyr are expected to be detectable with this technique. © 2006. The American Astronomical Society. All rights reserved.
• Kenworthy, M. A., Hinz, P. M., Roger, J., Heinze, A. N., & Sivanandam, S. (2006). Whack-a-speckle: Focal plane wavefront sensing in theory and practice with a deformable secondary mirror and 5-micron camera. Proceedings of SPIE - The International Society for Optical Engineering, 6272 II.
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  Abstract: Long exposures from adaptive optic systems show a diffraction limited core superimposed on a halo of uncorrected light from a science target, and the addition of various long-lived speckles that arise from uncorrected aberrations in the telescope system. The presence of these speckles limit the detection of extra-solar planets at a few diffraction widths from the primary source. Focal plane wavefront sensing uses the deformable secondary mirror of the MMT adaptive optics system to systematically remove the presence of long-lived speckles in a high-contrast image, and also test for the incoherent source that represents a separate astronomical target nearby. We use the Clio 5 micron camera (with its coronagraphic capabilities) to modulate long lived speckles and present initial on-sky results of this technique.
• Lloyd-Hart, M., Angel, R., Milton, N. M., Rademacher, M., & Codona, J. (2006). Design of the adaptive optics systems for GMT. Proceedings of SPIE - The International Society for Optical Engineering, 6272 I.
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  Abstract: The Giant Magellan Telescope (GMT) includes adaptive optics (AO) as an integral component of its design. Planned scientific applications of AO span an enormous parameter space: wavelengths from 1 to 25 μm, fields of view from 1 arcsec to 8 arcmin, and contrast ratio as high as 10 9. The integrated systems are designed about common core elements. The telescope's Gregorian adaptive secondary mirror, with seven segments matched to the primary mirror segments, will be used for wavefront correction in all AO modes, providing for high throughput and very low background in the thermal infrared. First light with AO will use wavefront reconstruction from a constellation of six continuous-wave sodium laser guide stars to provide ground-layer correction over 8 arcmin and diffraction-limited correction of small fields. Natural guide stars will be used for classical AO and high contrast imaging. The AO system is configured to feed both the initial instrument suite and ports for future expansion.
• Lloyd-Hart, M., Baranec, C., Milton, N. M., Snyder, M., Stalcup, T., & Roger, J. (2006). Experimental results of ground-layer and tomographic wavefront reconstruction from multiple laser guide stars. Optics Express, 14(17), 7541-7551.
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  PMID: 19529120;Abstract: We describe results from the first multi-laser wavefront sensing system designed to support tomographic modes of adaptive optics (AO). The system, now operating at the 6.5 m MMT telescope in Arizona, creates five beacons by Rayleigh scattering of laser beams at 532 nm integrated over a range from 20 to 29 km by dynamic refocus of the telescope optics. The return light is analyzed by a Shack-Hartmann sensor that places all five beacons on a single detector, with electronic shuttering to implement the beacon range gate. A separate high-order Shack-Hartmann sensor records simultaneous measurements of wavefronts from a natural star. From open-loop measurements, we find the average beacon wavefront gives a good estimate of ground layer aberration. We present results of full tomographic wavefront analysis, enabled by supplementing the laser data with simultaneous fast image motion measurements from three stars in the field. We describe plans for an early demonstration at the MMT of closed-loop ground layer AO, and later tomographic AO. © 2006 Optical Society of America.
• Putnam, N., Codona, J. L., & Roger, J. (2006). Laboratory demonstration of a focal plane coronagraphic interferometer designed for anti-halo apodization of starlight. Proceedings of SPIE - The International Society for Optical Engineering, 6272 II.
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  Abstract: The coronagraphic focal plane interferometer reflects away the core starlight with a mirror in the focal plane and uses it to form a coherent interferometric reference beam. This is used in a Mach-Zehnder configuration with phase shifting to measure the complex amplitude of the star halo speckles in the focal plane where the interference takes place. We present results from a laboratory prototype in which the speckles are suppressed over half the field by modifying the waverront in a pupil plane with a MEMS deformable mirror, based on a Fourier transform of the complex halo derived from the focal plane interferometric data. Even deeper suppression of the residual stellar halo over the full 360 degree field will be possible by explicitly constructing an "anti-halo" from the reference beam; a new technique for exoplanet imaging (Codona and Angel, 2004). We present the design and current status of a laboratory prototype to study antihalo apodization. The spatially-filtered core starlight will be modulated by deformable mirrors in a Michelson configuration to form a temporally-coherent copy of the measured residual complex halo, with the same amplitude but opposite phase (i.e. an "anti-halo"). Using components with only modest control accuracy, the method has the potential to reduce an already low residual halo by an additional two decades.
• Storey, J., Angel, R., Lawrence, J., Hinz, P., Ashley, M., & Burton, M. (2006). LAPCAT: The Large Antarctic Plateau Clear-Aperture Telescope. Proceedings of SPIE - The International Society for Optical Engineering, 6267 I.
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  Abstract: We present a proposal for an 8.4 metre off-axis optical/IR telescope to be located at Dome C, Antarctica. LAPCAT will use a mirror identical to the offset segment recently cast for the Giant Magellan Telescope (GMT) as a completely unobscured f/2.1 primary. With a cooled deformable Gregorian secondary in a dewar following prime focus, LAPCAT will allow for diffraction-limited imaging with only a single reflecting surface at ∼ 220K, and thus the lowest possible thermal background obtainable on earth. The exceptionally low atmospheric turbulence above Dome C enables very high contrast imaging in the thermal infrared, and diffraction limited imaging extending to optical wavelengths (20 mas at 800 nm, where Strehl ratios > 60% are projected). As an example, a deep 5 μm exoplanet imaging survey to complement current radial velocity methods could take advantage of both the low background and pupil remapping methods for apodization enabled by the clear aperture. Many new, young, giant planets (≥ 3Mj at 1 Gyr) would be detected in orbits ≥ 5 AU out to 20 pc. By providing a test bed for many of the GMT technologies in an Antarctic environment, LAPCAT also paves the way for the eventual construction of a second GMT at Dome C. Such a telescope would have unparalleled capabilities compared both to other ELTs in temperate sites and to JWST.
• Traub, W. A., Levine, M., Shaklan, S., Kasting, J., Angel, J. R., Brown, M. E., Brown, R. A., Burrows, C., Clamping, M., Dressler, A., Fergusor, H. C., Hammel, H. B., Heap, S. R., Horner, S. D., Illingworth, G. D., Kasdin, N. J., Kuchner, M. J., Lin, D., Marley, M. S., , Meadows, V., et al. (2006). TPF-C: Status and recent progress. Proceedings of SPIE - The International Society for Optical Engineering, 6268 I.
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  Abstract: The Terrestrial Planet Finder Coronagraph (TPF-C) is a deep space mission designed to detect and characterize Earth-like planets around nearby stars. TPF-C will be able to search for signs of life on these planets. TPF-C will use spectroscopy to measure basic properties including the presence of water or oxygen in the atmosphere, powerful signatures in the search for habitable worlds. This capability to characterize planets is what allows TPF-C to transcend other astronomy projects and become an historical endeavor on a par with the discovery voyages of the great navigators.
• Angel, R., Burge, J., & Worden, S. P. (2005). Testable lightweight telescopes for space. Proceedings of SPIE - The International Society for Optical Engineering, 5899, 1-6.
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  Abstract: The next generation of larger space optics will need lightweight and deployed mirror systems in order to control costs and fit within current and planned launch vehicle fairings. These will require active control based on wavefront sensing to establish and maintain their optical quality. Such control has been the enabling factor for the current generation of 8 m class ground-based telescopes, whose mirrors are either single monoliths with detailed shape control or have multiple rigid segments with control of relative position. They use actuator densities of typically a few per square meter. For active space systems it will be highly desirable to test the full deployed spacecraft in a vacuum test with a scene simulator, to validate before launch the optical performance of the complete system with its closed loop control systems. To enable such testing, the space mirror system must be designed from the start to work in a 1 g as well as zero g environment. The orientation we envisage has the spacecraft system pointed at the zenith, illuminated by a downward beam collimated with reference to a full aperture liquid flat. We consider here two space mirror systems. The first has rigid segments supported by position actuators to control only rigid body motions. Since the segments under test must hold their shape with an axial 1g load and no passive flotation supports, they must be smaller than for ground systems. If made of lightweighted silicon carbide or beryllium for diffraction limited imaging in the optical, they would have to be ∼ 30 cm in diameter. A mirror systems made from such segments will require about 40 actuators and wavefront sensor sub-apertures per square meter. The second system is a lightweight 3.5×8 m monolith for very high contrast imaging, as is envisaged for NASA's Terrestrial Planet Finder. High accuracy control of Fourier components down to ∼ 0.2 m period is required, requiring a deformable mirror with about 4000 actuators. If the primary itself is the deformable element, and has a 1 cm thick glass meniscus facesheet weighing 600 kg, the gravity-induced quilting during testing would be about 1 nm rms, low enough for ground testing of the complete system at the desired 10 -10 contrast level.
• Lloyd-Hart, M., Baranec, C., Milton, N. M., Stalcup, T., Snyder, M., Putnam, N., & Angel, J. R. (2005). First tests of wavefront sensing with a constellation of laser guide beacons. Astrophysical Journal Letters, 634(1 I), 679-686.
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  Abstract: Adaptive optics to correct current telescopes over wide fields, or even to correct future very large telescopes over narrow fields, will require real-time wavefront measurements made with a constellation of laser beacons. Here we report the first such measurements, made at the 6.5 m MMT with five Rayleigh beacons in a 2× pentagon. Each beacon is made with a pulsed beam at 532 nm of 4 W at the exit pupil of the projector. The return is range-gated from 20 to 29 km and recorded at 53 Hz by a 3 6-element Shack-Hartmann sensor. Wavefronts derived from the beacons are compared with simultaneous wavefronts obtained for individual natural stars within or near the constellation. Observations were made in seeing averaging 1.″0 with two-thirds of the aberration measured to be from a ground layer of mean height 380 m. Under these conditions, subtraction of the simple instantaneous average of the five beacon wavefronts from the stellar wavefronts yielded a 40% rms reduction in the measured modes of the distortion over a 2′ field. We discuss the use of multiple Rayleigh beacons as an alternative to single sodium beacons on 8 m telescopes and the impact of the new work on the design of a multi-sodium beacon system for the 25 m Giant Magellan Telescope. © 2005. The American Astronomical Society. All rights reserved.
• Angel, R. (2004). Buyer's guide to telescopes at the best sites: Dome A, L2 and shackleton rim. Proceedings of SPIE - The International Society for Optical Engineering, 5487(PART 1), 1-12.
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  Abstract: Buyer's guide to telescopes at the best sites including Dome A, L2 and Shackleton Rim, both on the ground and in space, and telescope concepts to take advantage of their complementary scientific potential are discussed. A telescope at L2 requires only a little fuel to stay on orbit and can be accurately pointed despite solar torques by well established active methods based on star trackers, gyros and reaction wheels. Solar shielding on the Moon requires a polar location such as the high rim of the Shackleton crater, adjacent to the south pole, where there is also nearly continuous solar power. The disposable character of space telescopes to data is in contrast to ground telescopes, which are built as long-lived observatories.
• Angel, R., Lawrence, J., & Storey, J. (2004). Concept for a second giant magellan telescope (GMT) in antarctica. Proceedings of SPIE - The International Society for Optical Engineering, 5382(PART 1), 76-84.
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  Abstract: While definitive winter measurements for Dome C must await until 2004, on the basis of existing data the Antarctic Dome sites promise the best conditions on the Earth for many astronomical observations. Because atmospheric turbulence is largely confined to a ∼ 100 m ground layer, adaptive correction with a single deformable mirror conjugated to this layer should yield an 8-arcminute field of view with 0.1 arcsec images at optical wavelengths. The ground layer wavefront aberration can likely be sensed with natural guide stars found over the wide field. In the infrared there is the added advantage of low thermal background from the cold atmosphere and telescope optics, as much as 50x reduction in the 3.5 μm L band. An ELT that fully exploited these advantages would provide a uniquely powerful ground-based complement to the James Webb Space Telescope, especially for spectroscopy. We consider here the concept of building a copy of the 21m Giant Magellan telescope (GMT) telescope (Johns, 2003) at Dome C. The optical design is ideal, with a very fast (f/0.7) primary mirror and direct Gregorian focus formed by a deformable secondary conjugated to the ground layer. In the thermal infrared, diffraction-limited images are produced with the low background of only two warm mirrors, the primary and secondary. There are also practical advantages. The enclosure is of modest size, by ELT standards, because the primary is very fast. Assembly, debugging and maintenance problems on-site are minimized by a primary mirror built from a small number of large, pre-tested segments. By building a copy of an already implemented ELT, engineering difficulties will be minimized, and experienced support staff will be available at the first GMT, where also instruments can be pre-tested.
• Baranec, C. J., Lloyd-Hart, M., Milton, N. M., Stalcup, T., A., J., Snyder, M., Putnam, N., & Angel, R. (2004). Progress towards tomographic wavefront reconstruction using dynamically refocused Rayleigh laser beacons. Proceedings of SPIE - The International Society for Optical Engineering, 5490(PART 3), 1129-1137.
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  Abstract: Simultaneous wavefront measurements are planned at the 6.5 m MMT telescope of five dynamically refocused Rayleigh laser beacons (RLGS) and a bright natural star to demonstrate tomographic wavefront reconstruction. In this paper, we summarize preliminary data recorded from the five laser beacons during the first telescope run at the MMT in June 2004. Beam projection is from behind the secondary of the MMT to form a regular pentagon of beacons on the sky with a radius of 60 arcseconds around the natural star. Beacon images are recorded over a range gate from 20 to 30 km, with dynamic refocus optics in the focal plane to remove perspective elongation (Stalcup, et. al., these proceedings). Separate externally synchronized Shack-Hartmann sensors record wavefront measurements of the beacons and the star, which will yield the first 33 Zernike modes from each wavefront measurement. A linear tomographic reconstructor, implemented as a matrix multiplication of the combined Zernike modal amplitudes from all five RLGS, has been computed to estimate contributions to the atmospheric aberration in two layers at 0 and 6 km. To validate the tomographic approach, the wavefront of the natural star will be predicted by computing the sum of the aberration in the direction of the star, and the prediction compared to simultaneous measurements recorded from the star directly.
• Clampin, M., Melnick, G., Lyon, R., Ford, H., Angel, R., Gezari, D., Golimowski, D., Hartig, G., Harwit, M., Holman, M., Illingworth, G., Kenyon, S., Lin, D., Marley, M., Olivier, S., Petro, L., Sasselov, D., Schneider, J., Seager, S., , Shao, M., et al. (2004). Extrasolar Planetary Imaging Coronagraph (EPIC). Proceedings of SPIE - The International Society for Optical Engineering, 5487(PART 3), 1538-1544.
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  Abstract: The Extrasolar Planetary Imaging Coronagraph (EPIC) will provide the first direct measurements of a broad range of fundamental physical characteristics of giant planets in other solar systems. These characteristics include orbital inclination, mass, brightness, color, the presence (or absence) of CH4 and H2O, and orbital or rotational-driven variability. EPIC utilizes a 1.5 meter telescope coupled to a Visible Nulling Coronagraph to achieve these science goals. EPIC has been proposed as a Discovery Mission.
• Codona, J. L., & Angel, R. (2004). Imaging extrasolar planets by Stellar Halo suppression in separately corrected color bands. Astrophysical Journal Letters, 604(2 II), L117-L120.
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  Abstract: Extrasolar planets have not been imaged directly with existing ground or space telescopes because they are too faint to be seen against the halo of the nearby bright star. Most techniques being explored to suppress the halo are achromatic, with separate correction of diffraction and wave-front errors. Residual speckle structure may be subtracted by differencing images taken through narrowband filters, but photon noise remains and ultimately limits sensitivity. Here we describe two ways to take advantage of narrow bands to reduce speckle photon flux and to obtain better control of systematic errors. Multiple images are formed in separate color bands of 5%-10% bandwidth and recorded by coronagraphic interferometers equipped with active control of wave-front phase and/or amplitude. In one method, a single deformable pupil mirror is used to actively correct both diffraction and wave-front components of the halo. This yields good diffraction suppression for complex pupil obscuration, with high throughput over half the focal plane. In a second method, the coronagraphic interferometer is used as a second stage after conventional apodization. The halo from uncontrollable residual errors in the pupil mask or wave front is removed by destructive interference made directly at the detector focal plane with an "antihalo," synthesized by spatial light modulators in the reference arm of the interferometer. In this way very deep suppression may be achieved by control elements with greatly relaxed, and thus achievable, tolerances. In both examples, systematic errors are minimized because the planet imaging cameras themselves also provide the error-sensing data.
• Hinz, P. M., Connors, T., McMahon, T., Cheng, A., Peng, C. Y., Hoffmann, W. F., McCarthy Jr., D. W., & Roger, J. (2004). Large Binocular Telescope Interferometer: The universal beam combiner. Proceedings of SPIE - The International Society for Optical Engineering, 5491(PART 2), 787-797.
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  Abstract: The Large Binocular Telescope with its single mount design and adaptive optics integrated into the secondary mirrors, I provides a unique platform for mid-infrared interferometry. The Large Binocular Telescope Interferometer is designed to take advantage of this platform, specifically for extrasolar planet detection in preparation for the Terrestrial Planet Finder mission. The instrument consists of three components: a general purpose or Universal Beam Combiner (UBC) which preserves the sine condition of the array, a nulling interferometer for the LBT (NIL) to overlap the two beams and sense phase variations, and a milling-optimized mid-infrared camera (NOMIC) for detection of the final images. Here we focus on the design and tolerancing of the UBC. The components of the system are currently being fabricated and the instrument is planned to be integrated with the LBT in 2006.
• Johns, M., Angel, R., Shectman, S., Bernstein, R., Fabricant, D., McCarthy, P., & Phillips, M. (2004). Status of the Giant Magellan Telescope (GMT) project. Proceedings of SPIE - The International Society for Optical Engineering, 5489(PART 1), 441-453.
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  Abstract: The Giant Magellan Telescope (GMT) is a joint project of a consortium of universities and research institutions to build and operate a 21.5-m equivalent aperture astronomical telescope for use at visible and IR wavelengths. This paper briefly summarizes the science goals for the project and provides an overview of the preliminary telescope and enclosure concepts and site test program. The telescope is a Gregorian design with a fast, f/0.7, primary mirror that allows a compact and stiff mount structure. The 25.3-meter diameter primary mirror consists of six off-axis 8.4-meter circular mirrors arranged in a hexagon around a center 8.4-meter mirror. The Gregorian secondary mirror is adaptive allowing two-mirror, wide-field adaptive optics. Several corrector designs have been studied for wide-field applications and one such design is shown. Instruments being considered for GMT provide a wide range of scientific capabilities. Instruments mount below the primary mirror on an instrument platform. Instrument mounting and servicing provisions are summarized.
• Liu, W. M., Hinz, P. M., Hoffmann, W. F., Brusa, G., Wildi, F., Miller, D., Lloyd-Hart, M., Kenworthy, M. A., McGuire, P. C., & Angel, J. R. (2004). Adaptive optics nulling interferometric constraints on the mid-infrared exozodiacal dust emission around vega. Astrophysical Journal Letters, 610(2 II), L125-L128.
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  Abstract: We present the results of mid-infrared nulling interferometric observations of the main-sequence star α Lyr (Vega) using the 6.5 m MMT with its adaptive secondary mirror. From the observations at 10.6 μm, we find that there is no resolved emission from the circumstellar environment (at separations greater than 0.8 AU) above 2.1% (3 σ limit) of the level of the stellar photospheric emission. Thus, we are able to place an upper limit on the density of dust in the inner system of 650 times that of our own solar system's zodiacal cloud. This limit is roughly 2.8 times better than those determined with photometric excess observations such as those by IRAS. Comparison with far-infrared observations by IRAS shows that the density of warm dust in the inner system (80%) of the material in the outer system is ice.
• Putnam, N., Snyder, M., Stalcup, T., & Angel, R. (2004). Manufacture and use of a Shack-Hartmann sensor with a multifaceted prism for simultaneous sensing of multiple wavefronts. Proceedings of SPIE - The International Society for Optical Engineering, 5490(PART 3), 1138-1145.
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  Abstract: A new requirement for astronomical adaptive optics is the simultaneous measurement of wavefronts of multiple natural or laser guide stars. We have devised a new implementation of the Shack-Hartmann method to image multiple spot patterns on a single imaging array. An image of the telescope pupil is formed on a multifaceted prism with rings of subapertures. All beacons in the field are then imaged by a camera lens to form the same spot pattern repeated over the detector format. The facets are fly-cut in polycarbonate, tangent to a convex surface. In order to minimize scattering and aid manufacturing, the prism angles are exaggerated, and an index-matching fluid is used to reduce the refracted angles by a factor of 15. Results from lab and telescope tests are presented.
• Stalcup Jr., T. E., A., J., Snyder, M., Baranec, C., Putnam, N., Milton, M., Angel, R., & Lloyd-Hart, M. (2004). Field tests of wavefront sensing with multiple Rayleigh laser guide stars and dynamic refocus. Proceedings of SPIE - The International Society for Optical Engineering, 5490(PART 2), 1021-1032.
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  Abstract: A demonstration of tomographic wavefront sensing has been designed, fabricated, and tested. The last of the initial testing of the dynamic refocus system at the 61" telescope on Mt. Bigelow, Arizona is presented, along with the first results from the system after its transfer to the 6.5 m MMT on Mt. Hopkins, Arizona. This system consists of a laser beam projector, and a wavefront sensor at the telescope's Cassegrain focus. The projector transmits 5 pulsed 532 nm beams in a regular pentagon of 2 arcminutes diameter from behind the telescope's secondary mirror that in good seeing can yield sub-arcsecond beacons over a 20-30 km altitude range. The wavefront sensor incorporates a dynamic refocus unit to track each returning laser pulse, and a multiple laser beacon Shack-Hartmann wavefront sensor using a novel substitute for the traditional lenslet array. A natural guide star wavefront sensor was also fielded to collect ground-truth data to compare with wavefronts reconstructed from the laser wavefront sensor measurements. All of the subsystems were shown to work, but bad weather ended the testing before the final data could be collected.
• A., J., Stalcup, T., Angel, R., & Mallik, P. (2003). Field tests of dynamic refocus of Rayleigh laser beacons. Proceedings of SPIE - The International Society for Optical Engineering, 5169, 137-148.
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  Abstract: Dynamically refocusing the Rayleigh backscatter of a modestly powered laser beacon is a concept for increasing LGS brightness by 10 times1,2,3. Dynamic refocus will allow for high photon return from multiple Rayleigh beacons enabling MCAO for wide field correction of the MMT and Magellan telescopes 2,3,4,5,6. In a system without dynamic refocus, light from a beacon integrated from 20 to 30km is blurred to a length of 14arcsecs. In a system with dynamic refocus, the bow tie is restored to a spot limited only by atmospheric seeing. The dynamic refocus system has been designed to deliver images with
• Angel, R. (2003). Direct detection of terrestrial exoplanets: Comparing the potential for space and ground telescopes. European Space Agency, (Special Publication) ESA SP, 221-230.
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  Abstract: Telescopes of various different designs are potentially capable of detecting extrasolar terrestrial planets. We analyze here in a consistent way the limiting sensitivities set by photon noise from the background underlying the planet signal, which may be of thermal, zodiacal or stellar origin. The strength of the unsuppressed stellar halo is itself set by photon noise in wavefront measurement. While optical telescopes have potentially higher limiting sensitivity, thermal detection is more secure, At 11 μm wavelength, the planet/star contrast is 1000 times more favorable than in the optical. Together with the longer wavelength, this leads to a 500 times more relaxed tolerance for star suppression, one that can be met by a fast servo based on the bright star flux sensed at shorter wavelengths. Either Darwin or a 100 m ground telescope should be capable of thermal detection of the earth in a solar system twin at 10 pc at 5 to 10σ in 24 hr. At optical wavelengths, the limiting sensitivity for space telescopes is set at the 10-30σ level by photon noise in the zodiacal background. Reaching this limit, as do the deep fields of the Hubble Space Telescope, will require extreme coronagraphic suppression of the bright star at 0.1 arcsec separation. The ∼1 m-scale Fourier components of the wavefront would need to have stable amplitude ≤2 picometers, a severe challenge. On the ground, fast atmospheric correction at the photon noise limit will leave residual Fourier amplitudes of 20-60 pm, for a halo background 100-1000 times zodiacal. But given larger apertures and stronger fluxes, optical sensitivity can still be high, provided the photon noise limit of short halo exposures can be maintained in a long-term average. If this challenge can be met, detection in 24 hr would be at the 5σ level for a 20 m Antarctic telescope, ∼50σ for the 100 m OWL. If a terrestrial planet were detected at 10 pc, a spectrum that could reveal water and oxygen would be of great interest. Thermal features can be accessed only from space, where broad spectral cover is possible. Optical spectroscopy could be undertaken with a 100 m telescope on the ground. An Antarctic location would give high sensitivity to water, and the added benefit of thermal imaging with high sensitivity and resolution.
• Angel, R. (2003). Space: Telescopes reveal the way forward [1]. Nature, 422(6930), 373-.
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  PMID: 12660751;
• Burge, J. H., & Angel, J. R. (2003). Wide field telescope using spherical mirrors. Proceedings of SPIE - The International Society for Optical Engineering, 5174, 83-92.
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  Abstract: A new class of optical telescope is required to obtain high resolution spectra of many faint, distant galaxies. These dim objects require apertures approaching 30 meters in addition to many hours of integration per object, and simultaneous observation of as many galaxies as possible. Several astronomical telescopes of 20, 30, 50, even 100 meters are being proposed for general purpose astronomy. We present a different concept here with a 30-m telescope optimized for wide field, multi-object spectroscopy. The optical design uses a fully steerable, quasi-Cassegrain telescope in which the primary and secondary mirrors are parts of concentric spheres, imaging a 3° field of view onto a spherical surface. The spherical aberration from the mirrors is large (about 2 arc minutes) but it is constant across the field. Our system design uses numerous correctors, placed at the Cassegrain focus, each of which corrects over a small field of a few arc seconds. These can be used for integral field spectroscopy or for direct imaging using adaptive optics. Hundreds of these units could be placed on the focal surface during the day to allow all-night exposures of the desired regions. We believe that this design offers an economical system that can be dedicated for several important types of astronomical observation.
• Close, L. M., Biller, B., Hoffmann, W. F., Hinz, P. M., Bieging, J. H., Wildi, F., Lloyd-Hart, M., Brusa, G., Fisher, D., Miller, D., & Angel, R. (2003). Mid-infrared imaging of the post-asymptotic giant branch star AC Herculis with the Multiple Mirror Telescope adaptive optics system. Astrophysical Journal Letters, 598(1 II), L35-L38.
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  Abstract: We utilized the unique 6.5 m Multiplie Mirror Telescope deformable secondary adaptive optics (AO) system to produce high-resolution (FWHM = 0″.3), very high Strehl mid-infrared (9.8, 11.7, and 18 μm) images of the post-asymptotic giant branch star AC Her. The very high (98% ± 2%) Strehls achieved with mid-IR AO led naturally to an ultrastable point-spread function (PSF) independent of air mass, seeing, or location on the sky. We find no significant difference between AC Her's morphology and our unresolved PSF calibration stars (μ UMa and α Her) at 9.8, 11.7, and 18 μm. Our current observations do not confirm any extended mid-IR structure around AC Her. These observations are in conflict with previously reported Keck (seeing-limited) 11.7 and 18 μm images that suggested the presence of a resolved ∼0″.6 edge-on circumbinary disk. We conclude that AC Her has no extended mid-IR structure on scales greater than 0″.2 (R < 75 AU). These first results of mid-IR AO science are very encouraging for future high-accuracy mid-IR imaging with this technique.
• Close, L. M., Wildi, F., Lloyd-Hart, M., Brusa, G., Fisher, D., Miller, D., Riccardi, A., Salinari, P., McCarthy, D. W., Angel, R., Allen, R., Martin, H. M., Sosa, R. G., Montoya, M., Rademacher, M., Rascon, M., Curley, D., Siegler, N., & Duschl, W. J. (2003). High-resolution images of orbital motion in the trapezium cluster: First scientific results from the multiple mirror telescope deformable secondary mirror adaptive optics system. Astrophysical Journal Letters, 599(1 I), 537-547.
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  Abstract: We present the first scientific images obtained with a deformable secondary mirror adaptive optics (AO) system. We utilized the 6.5 m Multiple Mirror Telescope adaptive optics system to produce high-resolution (FWHM = 0″.07) near-infrared (1.6 μm) images of the young (∼1 Myr) Orion Trapezium θ1 Ori cluster members. A combination of high spatial resolution and high signal-to-noise ratio allowed the positions of these stars to be measured to within ∼0″.003 accuracies. We also present slightly lower resolution (FWHM ∼ 0″.085) images from Gemini with the Hokupa'a AO system as well. Including previous speckle data from Weigelt et al., we analyze a 6 yr baseline of high-resolution observations of this cluster. Over this baseline we are sensitive to relative proper motions of only ∼0.002 yr-1 (4.2 km s-1 at 450 pc). At such sensitivities we detect orbital motion in the very tight θ1 Ori B2-B3 (52 AU separation) and θ1 Ori A1-A2 (94 AU separation) systems. The relative velocity in the θ1 Ori B2-B3 system is 4.2 ± 2.1 km s-1. We observe 16.5 ± 5.7 km s-1 of relative motion in the θ1 Ori A1-A2 system. These velocities are consistent with those independently observed by Schertl et al. with speckle interferometry, giving us confidence that these very small (∼0.002 yr-1) orbital motions are real. All five members of the θ1 Ori B system appear likely gravitationally bound (B2-B3 is moving at ∼1.4 km s-1 in the plane of the sky with respect to B1, where Vesc ∼ 6 km s-1 for the B group). The very lowest mass member of the θ1 Ori B system (B4) has K′ ∼ 11.66 and an estimated mass of ∼0.2 M⊙. Very little motion (4 ± 15 km s-1) of B4 was detected with respect to B1 or B2; hence, B4 is possibly part of the θ1 Ori B group. We suspect that if this very low mass member is physically associated, it most likely is in an unstable (nonhierarchical) orbital position and will soon be ejected from the group. The θ1 Ori B system appears to be a good example of a star formation "minicluster," which may eject the lowest mass members of the cluster in the near future. This "ejection" process could play a major role in the formation of low-mass stars and brown dwarfs.
• Milton, N. M., Lloyd-Hart, M., Cheng, A., A., J., & Angel, J. R. (2003). Design and expected performance of an MCAO system for the giant Magellan telescope. Proceedings of SPIE - The International Society for Optical Engineering, 5169, 238-249.
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  Abstract: Adaptive optics will play a crucial role in achieving the full potential of the next generation of large diameter telescopes. In this paper, we present an optical design for a multi-conjugate adaptive optics system for the Giant Magellan Telescope, a 25.7 m telescope with a primary mirror consisting of seven 8.4 m segments. The tri-conjugate MCAO optics is based on adaptive secondary technology developed for the MMT telescope and incorporates dynamic refocus optics for the laser guide star wavefront sensors. We use the results of analytic (non-Monte-Carlo) numerical simulations to determine the optimal configuration of deformable mirrors as well as laser and natural guide stars. The simulation results are extended to include and quantify the effects of wavefront sensor and control loop delay noise as well as dynamic refocus and fitting error on the expected system performance and sky coverage.
• Velusamy, T., Angel, R. P., Eatchel, A., Tenerelli, D., & Woolf, N. J. (2003). Single and double Bracewell nulling interferometer in space. European Space Agency, (Special Publication) ESA SP, 631-636.
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  Abstract: As part of a NASA NRA study we have examined the design and use of a small Bracewell nulling interferometer in space for observations of Jovian and terrestrial planets in the 4-12 μm spectral region. At these wavelengths, the contrast is typically 10-7 for most planets while younger and/or more massive Jovian planets have a contrast 100 times better. For planets farther from their star than Earth, a short baseline can provide good null depth and still have adequate resolution. With the single Bracewell nulling interferometer described here, massive/young giant planets far from their star are not difficult to observe and young Jovian mass planetary systems as far as 100 pc can be probed. A program to study planetary systems like the solar system is much harder. In the case of older and smaller planets, distinguishing the planet signal from the star and local zodiacal background is much harder and requires both high stability of the gain for the IR detectors, and nulling over periods of hours. The difficulties of a 2-telescope, single Bracewell nuller are overcome using a dual Bracewell nuller consisting of four telescopes, and phase chopping between two null beams. Double Bracewell nulling described here will have the potential to detect Earth's around 20 nearby stars.
• A., J., Mallik, P., Stalcup, T., Roger, J., & Sarlot, R. (2002). Design and testing of a dynamic refocus system for Rayleigh laser beacons. Proceedings of SPIE - The International Society for Optical Engineering, 4839(1), 473-483.
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  Abstract: We present the design, laboratory tests and preliminary field tests of a dynamic refocus system for 351nm Rayleigh beacon laser guide stars. The purpose of dynamic refocus is to increase the beacon signal from a pulsed laser, by maintaining focus in a fixed plane while the laser pulse travels through the atmosphere over an extended height range. The focusing element in our system is a moving concave mirror. The optics have been designed and built to focus on a ring of 5 beacons at 1 arc minute radius at the 6.5 m MMT, covering the range 18-40 km. Laboratory tests of image quality resulted in 0.5 arcsec refocused images corresponding to the height range 22 - 28 km, free from spherical aberration. Preliminary field tests were performed on the Mt. Bigelow Observatory 1.5 m telescope, with a frequency-tripled, Q-switched YLF laser beam projected from a 25 cm telescope. To simulate an off-axis sub-aperture of the MMT, the laser and telescope axes were set 3 m apart and reimaging optics were placed ahead of the refocus unit to image at the same plate scale as the MMT (500 μm/arcsec). Returns from different heights were selected by gating the detector with a Pockels cell. Returns over a 10 km height range from 8km to 18km were brought into focus for a total mirror motion measured to be 900 μm. The system is now ready for testing dynamic refocus, which will be accomplished by attaching the mirror to a metal resonator tuned to the laser pulse frequency. The range from 23 to 35 km to be used will require a motion of 500 μm.
• Angel, R. (2002). Sensitivity of optical interferometers with coherent image combination. Proceedings of SPIE - The International Society for Optical Engineering, 4838(1), 126-133.
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  Abstract: When multiple, adaptively-corrected telescopes are coherently combined at a common focal plane, interference appears as spatial structure in the point spread function (PSF). True images of deep fields can be reconstructed at resolution λ/b from multiple images recorded to cover the u-v plane with baselines up to b. Assuming no losses or detector noise, sensitivity improves with the number of elements, n, and of combined images, N. The formalism of PSF fitting is used here to determine signal/noise ratio for faint point sources seen against sky background noise. For two-element and dilute, non-redundant arrays the PSF fit is dominated not by any sharp peak, but by fringes or weak speckles, and the signal/noise ratio is proportional to √N(2n-1). For close packed or highly redundant arrays the sensitivity increases as n√N, and is the same as for a single dish with the same total collecting area and integration time. It follows that the signal/noise ratio for a two-element array is √3/2 = 86% of this maximum limit. We show that the requirement for PSF sampling with negligible detector read noise can be met with available optical and infrared array detectors, for baselines up to 10 times the element diameter. Low-loss combination could be realized in practice by an interferometer with two large moving elements, with baselines set up sequentially to sample the full u-v plane. In the 20/20 concept, two 21 m telescopes move continuously around a circular track during an integration, to keep the baseline oriented perpendicular to the source. Coherent combination is made at a station held midway between, to obtain aperture synthesis of images covering the full, one-arcminute field corrected by multiconjugate adaptive optics. With 16 images taken over 8 hours to fully cover baselines up to 100 m, the resolution in the K band is 4 mas, and the 10a limiting magnitude for point sources will be 27.7.
• Angel, R., Burge, J., Codona, J. L., Davison, W., & Martin, B. (2002). 20 and 30 m telescope designs with potential for subsequent incorporation into a track-mounted pair (20/20 or 30/30). Proceedings of SPIE - The International Society for Optical Engineering, 4840, 183-193.
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  Abstract: Any future giant ground-based telescope must, at a minimum, provide foci for seeing-limited imaging over a wide field and for diffraction-limited imaging over ∼1 arcminute fields corrected by adaptive optics (AO). While this is possible with a number of design concepts, our choices are constrained if we anticipate wanting to later add a second telescope for imaging with still higher resolution, and very high contrast imaging for exoplanet studies. This paper explores designs that allow for such future development. Higher resolution imaging by interferometric combination of the AO-corrected fields of two telescopes is possible without loss of point-source sensitivity or field of view, as long as the baseline can be held perpendicular to the source and varied in length. This requirement is made practical even for very large telescopes, provided both can move continuously on a circular track. The 20/20 telescope1 illustrates this concept. Telescopes so mounted can additionally be operated as a Bracewell nulling interferometer with low thermal background, making possible the thermal detection of planets that would have been unresolvable by a single 20 m aperture. In practice, limits set by funding and engineering experience will likely require a single 20 or 30 m telescope be built first. This would be on a conventional alt-az mount, but it should be at a site with enough room for later addition of a companion and track. In anticipation of future motion it should be compact and stiff, with a fast primary focal ratio. We envisage the use of large, highly aspheric, off-axis segments, manufactured using the figuring methods for strong aspherics already proven for 8 m class primaries. A compact giant telescope built under these guidelines should be able to perform well on its own for a broad range of astronomical observations, with good resistance to wind buffeting and simple alignment and control of its few, large segments. We compare here configurations with adjacent hexagonal segments and close-packed circular segments. For given segment parent size and number, the largest effective aperture is achieved if the segments are left as circles, when also the sensitivity and resolution for diffraction-limited operation with AO is higher. Large round segments can also be individually apodized for high-contrast imaging of exoplanets with the entire telescope-for example 8.4 m segments will yield 10-6 suppression 0.05 arcsec from a star at 1 μm wavelength, and 0.25 arcsec at 5 μm.
• Burge, J. H., & Angel, J. R. (2002). A 30 meter cassegrain telescope with spherical optics and a 3° field. Proceedings of SPIE - The International Society for Optical Engineering, 4840, 574-582.
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  Abstract: We present a concept is for a fully steerable, quasi-Cassegrain telescope in which the 36 m primary and secondary mirrors are parts of concentric spheres. The 15-m secondary, supported 36 meters above the primary yields a 3°, Cassegrain focal surface, 7 meters in diameter. Though the spherical aberration is large (about 2 arcminutes), many small fields are fully corrected locally. Multiple small units are placed in the focal surface at regions of interest to correct a field of view of a few arcseconds. These can be used for integral field spectroscopy of for direct imaging using adaptive optics. Hundreds of these units could be placed on the focal surface during the day to allow all-night exposures of the desired regions. We believe that this design offers an economical system that can be dedicated for several important types of astronomical observation.
• Davison, W., & Angel, R. (2002). Large synoptic survey telescope mechanical structure and design. Proceedings of SPIE - The International Society for Optical Engineering, 4836, 104-110.
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  Abstract: The Large-aperture Synoptic Survey Telescope, LSST, will have an effective aperture of ∼6.5 m and a 3 degree field of view. Its 3-mirror optical system with 8.4 m primary, 3.5 m secondary, 4.2 meter tertiary mirrors and a trapped focus offer unique telescope design challenges. The operation of this telescope will require quick slewing, accurate tracking and alignment maintained actively for 0.25 arcsec images in the presence of wind and gravity perturbations. We describe our current design for which finite element models show a lowest frequency resonance above 7 Hertz. Further refinement promises an even suffer structure. The design has been optimized for low mass (230 tons), minimal inertia (2.4×106 kg-m2 in elevation, 3.2×106 kg-m2 in azimuth) for fast response. It takes advantage of several concepts proven in the Large Binocular Telescope mount, which has shown high performance at low cost. These include elevation motion on C rings placed under the primary mirror, a primary mirror cell built as an integral part of the structure, and the elevation axis placed behind and off to the side of the primary vertex, to achieve balance with minimum mass.
• Davison, W., & Angel, R. (2002). Mechanical concepts for 30 m class telescopes. Proceedings of SPIE - The International Society for Optical Engineering, 4840, 206-213.
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  Abstract: The 20 20 Telescope is a 30 meter class telescope comprised of two 21.2 m collector telescopes on a 100 m circular track. Each collector telescope has a focal ratio of F: 0.7 and is comprised of seven 8.4 m segments. There is an instrument bridge that carries the combining instrument. The proposal for 20 20 is to have discrete combiner stations for 30, 60, and 100 meter baselines. Additional focal stations are implemented for Nasmyth and bent Cassegrain. The Track has the same segmented construction and tracking motion on hydrostatic bearings as LBT. The collector telescope buildings will co-track and co-rotate on separate tracks. The 30 m design has the same basic shape as a single 21 meter Collector but many aspects are different. The 30 meter telescope is a single hexagonal aperture with a primary at F: 0.5. There are 13 that are 8.74m hexagons and 6 half hexagons. The 30 m telescope has primarily Nasmyth platforms behind the primary mirror. Both telescopes have a 30 meter equivalent circular aperture. Both telescopes have high structural performance, at 6.5 Hz and 5.3 Hz respectively. Both are balanced, and use similar designed components. Comparison of their characteristics and design differences can show the strengths and weaknesses of each.
• Davison, W., Woolf, N., & Angel, R. (2002). Design and analysis of 20 m track mounted and 30 m telescopes. Proceedings of SPIE - The International Society for Optical Engineering, 4840, 533-540.
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  Abstract: This paper presents designs of compact 21 and 30 m aperture telescopes with primary focal off/0.7 and f/0.56. The 20 20 telescope moves on three axes; the elevation axis (which is below the primary vertex), the azimuth axis, and a tracking axis at the center of 100 m diameter tracks. The 30 m telescope has an elevation and azimuth axis. All of the axes move on hydrostatic bearings. A primary requirement for such large telescopes is stiffness against deformation by wind gusts. The mass and stiffness needed for the structure is substantially independent of the primary mirror mass, which can therefore be set by thermal and diffraction issues. For the 21 m design, whose primary has seven 8.4 m glass segments weighing 128 tons, the total moving mass is 905 tons, and the lowest resonant frequency 6.5 Hz. For the 30 m design, whose primary has, 13 whole and 6 half, glass segments 8.7 m, across the points, weighing 256 tons, the total moving mass is 3,460 tons, and the lowest resonant frequency 5.3 Hz. These practical designs offer two versatile telescopes with high performance.
• Errico, S., Angel, R., Calvert, P., & Woof, N. (2002). Harnessing solar pressure to slew and point large infrared space telescopes. Proceedings of SPIE - The International Society for Optical Engineering, 4850(2), 1080-1090.
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  Abstract: Large astronomical Gossamer telescopes in space will need to employ large solar shields to safeguard the optics from solar radiation. These types of telescopes demand accurate controls to maintain telescope pointing over long integration periods. We propose an active solar shield system that harnesses radiation pressure to accurately slew and acquire new targets without the need for reaction wheels or thrusters. To provide the required torques, the solar shield is configured as an inverted, 4-sided pyramidal roof. The sloped roof interior surfaces are covered with hinged "tiles" made from piezoelectric film bimorphs with specular metallized surfaces. Nominally, the tiles lie flat against the roof and the sunlight is reflected outward equally from all sloped surfaces. However, when the tiles on one roof pitch are raised, the pressure balance is upset and the sunshade is pushed to one side. By judicious selection of the tiles and control of their lift angle, the solar pressure can be harvested to stabilize the spacecraft orientation or to change its angular momentum. A first order conceptual design performance analysis and the results from the experimental design, fabrication and testing of piezoelectric bimorph hinge elements will be presented. Next phase challenges in engineering design, materials technology, and systems testing will be discussed.
• Errico, S., Angel, R., Stamper, B., Burge, J., & Connors, T. (2002). Stretched membrane with electrostatic curvature (SMEC) mirrors: A new technology for large lightweight space telescopes. Proceedings of SPIE - The International Society for Optical Engineering, 4849, 356-364.
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  Abstract: Exploration of faint distant objects in space has been limited by the power of telescopes. Currently our only option for studying these remote objects is to build larger and better telescopes. These giant telescopes are often constrained by system mass, which is dominated by the primary mirror. It appears that the evolutionary path of using conventional technology to build giant mirrors will not be sufficient to meet the small areal density of approximately 1.5 kg/m2. Therefore the development of large primary mirrors for space is dependent on innovative approaches and new technology. One approach to building a large primary reflector is to use smaller individual segments and place them along a curve approximating a paraboloid. These smaller segments could be comprised of either flat or curved thin membrane mirrors. These thin membrane mirrors have the potential of meeting the small areal density requirement. We have started development on a thin membrane mirror. We have built and are testing a 6 inch stretched membrane mirror prototype that uses electrostatic pressure to pull the nominally flat mirror to a 32 m radius of curvature and adaptively correct for aberrations. Preliminary test results of the flat membrane are promising. The surface error for the flat membrane was measured to better than λ/10 rms for the center four inches and λ/20 rms over the central three inches. The interferograms for the curved membrane show a residual figure-eight pattern of high order astigmatism, most likely due to tension anisotropy in the mirror. Analysis on the fully curved mirror is still on-going. This paper discusses the SMEC design, development, test results, and current on-going activities.
• Hinz, P. M., Roger, J., McCarthy, D. W., Hoffmann, W. F., & Peng, C. Y. (2002). The large binocular telescope interferometer. Proceedings of SPIE - The International Society for Optical Engineering, 4838(1), 108-112.
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  Abstract: The Large Binocular Telescope (LET), with dual 8.4 m optics on a common mount, is unique among the large-aperture interferometers. Deformable secondaries on the telescope capable of adaptive atmospheric correction allow beam combination after only three warm reflections. The design allows the implementation of two powerful uses of interferometry: suppression of starlight (or nulling interferometry) and wide-field imaging (or Fizeau interferometry). Nulling will allow detection of extrasolar planetary systems (from either zodiacal emission or giant planets) down to solar system-equivalent levels for nearby stars. This will dramatically increase our knowledge of the prevalence and make-up of extrasolar planetary systems. Fizeau interferometry will allow imaging of even complex structure at the resolution of a 22.8 m telescope. To implement these two powerful techniques the University of Arizona and NASA are collaborating to build the Large Binocular Telescope Interferometer (LBTI) a cryogenic instrument capable of sensitive interferometric observations in the infrared.
• Jian, G. e., Angel, J. R., Jacobsen, B., Woolf, N., Fugate, R. Q., Black, J. H., & Lloyd-Hart, M. (2002). An optical ultrahigh-resolution cross-dispersed echelle spectrograph with adaptive optics. Publications of the Astronomical Society of the Pacific, 114(798), 879-891.
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  Abstract: A prototype cross-dispersed optical echelle spectrograph of very high resolution has been designed and built at Steward Observatory and tested at the Starfire Optical Range 1.5 m telescope. It is the first spectrograph to take advantage of diffraction-limited images provided by adaptive optics in order to achieve a potential resolving power of R ∼ 600,000. The wavelength coverage in a single exposure is about 300 Å, which is approximately 100 times that of conventional spectrographs operating at comparable resolution. This was achieved by recording 60 cross-dispersed orders across the 18 × 18 mm2 area of the CCD detector. The total efficiency of the system, including the sky and telescope transmission, spectrograph, and CCD detector, is measured to be 1.3% at peak, much higher than that of other ultrahigh-resolution spectrographs. Sample stellar spectra with R ∼ 250,000 are presented.
• Lloyd-Hart, M., Georges, J., Angel, R., Brusa, G., & Young, P. (2002). Dynamically refocused Rayleigh laser beacons for atmospheric tomography. Proceedings of SPIE - The International Society for Optical Engineering, 4494, 259-270.
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  Abstract: We report initial results from a prototype system to generate multiple Rayleigh laser guide stars for adaptive optics from a single pulse laser at 354 nm wavelength. A 3.2 mW laser pulsed at 630 Hz was used to project three beams on the sky simultaneously, each pulsed at 210 Hz. A spinning mirror was used to direct the pulses in three directions at the vertices of an equilateral triangle 90 arcsec across. Laser pulses were triggered by a synchronising electrical pulse from the motor. Dynamic focusing optics in the receiving telescope will in future be used to hold such beacons from more powerful lasers in focus over a height range of many kilometers. Multiple beacons can be used to derive tomographic information on the vertical distribution of the aberration. We show initial analytical and numerical work on how the unique features of refocused Rayleigh beacons can improve the tomographic wavefront measurement for multi-conjugate adaptive optics.
• Angel, R. (2001). Future optical and infrared telescopes. Nature, 409(6818), 427-430.
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  PMID: 11201760;Abstract: New telescopes, new detectors and new regions of the electromagnetic spectrum have often revealed totally unsuspected aspects of the Universe. How can we maximize the chances of such serendipity in the future?
• Burge, J., DeRigne, S., Angel, R., Cuerden, B., Clapp, S., Rivlis, G., Woida, P., Gohman, P., Bell, S., & Kingston, R. (2001). NGST mirror system demonstrator from the University of Arizona. Proceedings of SPIE - The International Society for Optical Engineering, 4451, 27-38.
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  Abstract: Future space telescopes require primary mirrors that are much lighter than those currently being manufactured. They also must maintain optical tolerances while operating at cryogenic temperatures. We present a Mirror System Demonstrator for the Next Generation Space Telescope (NGST) that uses a thin glass facesheet with active control to achieve low mass and high surface quality. A 2 mm thick glass facesheet is controlled by miniature actuators and held together by a rigid carbon fiber frame. The 2-m diameter mirror system weighs only 13 kg/m2, including the glass, supports, actuators, support structure, and cabling. We present the status of the development and testing of this revolutionary mirror.
• Lloyd-Hart, M., Jefferies, S. M., Roger, J., Angel, P., & Hege, E. K. (2001). Wave-front sensing with time-of-flight phase diversity. Optics Letters, 26(7), 402-404.
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  Abstract: We present a new way to sense atmospheric wave-front phase distortion. Short collimated pulses of laser light at ∼350 nm are projected from a small auxilliary telescope. Rayleigh scattering from each pulse is recorded over a wide range of height through the main telescope aperture in a continuous sequence of fast video frames by a detector conjugate to mid-height. Phase diversity is thus naturally introduced as the pulses approach and pass through focus. We show that an iterative algorithm can extract the phase structure from the recorded images and do so with a much higher signal-to-noise ratio than is possible with existing techniques. If the requirements for real-time data recording and reduction can be met, the new method will address the need for tomographic wave-front sensing at planned 30-m-class telescopes. © 2001 Optical Society of America.
• Rhoadarmer, T. A., & Roger, J. (2001). Low-cost, broadband static phase plate for generating atmosphericlike turbulence. Applied Optics, 40(18), 2946-2955.
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  PMID: 18357312;Abstract: Over the past decade adaptive optics (AO) has proved its worth as AO systems have been used successfully on several telescopes to improve image resolution. As scientists and engineers push the technological state of the art in an effort to make bigger, faster, and better systems, it has become more and more important to test and verify the operation of these systems in a controlled laboratory setting. To perform full-system tests in the presence of atmospheric turbulence, some sort of turbulence generator is needed. We describe a simple, low-cost approach to making static phase plates that generate atmosphericlike wave-front aberrations. These plates have several advantages over traditional heated-air turbulence generators and, as such, are better suited for well-controlled, detailed testing of an AO system. © 2001 Optical Society of America.
• Stamper, B., Angel, R., Burge, J., Connors, T., Duffy, B., & Woolf, N. (2001). Stretched membrane with electrostatic curvature (SMEC) mirrors for extremely large space telescopes. Proceedings of SPIE - The International Society for Optical Engineering, 4451, 105-113.
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  Abstract: Very lightweight mirrors can be constructed by stretching a membrane to form a flat surface. Adding tension to the membrane, making it flat, can be done by discrete attachment points, or by using a continuous boundary. Such lightweight mirrors are very attractive for space telescopes where a 100m aperture can be made up of smaller mirror segments. Adding a slight curvature to each segments simplifies the optical train. This article looks at the making of a curved membrane mirror, and demonstrates its use. Measurements of the flat membrane, and the curved figure will be shown.
• Angel, R. (2000). Future very large space telescopes. Space 2000 Conference and Exposition.
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  Abstract: Future very large space telescopes will require low density mirrors, with nevertheless very high surface accuracy to yield diffraction limited images at optical wavelengths. Carbon fiber dishes faced with glass a mm thick should be practical in sizes up to 25 m. Still larger apertures could be made of stretched reflective membrane, with weak concave curvature induced by electrostatic pressure. © 2000 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.
• Angel, R., & Fugate, B. (2000). Adaptive optics. Science, 288(5465), 455-456.
• Angel, R., & Lloyd-Hart, M. (2000). Atmospheric tomography with Rayleigh laser beacons for correction of wide fields and 30 m class telescopes. Proceedings of SPIE - The International Society for Optical Engineering, 4007, I/-.
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  Abstract: The laser beacons produced by Rayleigh scattering have the potential to correct 6-10 m class telescopes over a small field of view. The laser can give brighter beacon and information about atmospheric turbulence. Atmospheric tomography uses short pulses and the number of brighter beacons or stars increases with number of layers included in the tomographic solution. These can be used in very large telescopes as it shows the minimum reliance over natural stars for the space research.
• Angel, R., Burge, J., Hege, K., Kenworthy, M., & Woolf, N. (2000). Stretched membrane with electrostatic curvature (SMEC): A new technology for ultra-lightweight space telescopes. Proceedings of SPIE - The International Society for Optical Engineering, 4013, 699-705.
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  Abstract: Very large space telescopes with primary mirrors made of flat segments have been recently proposed. The segments would be extremely lightweight, made like pellicles from stretched, reflective membranes. Here we consider the use of such membrane primary mirrors in which slight concave curvature is induced by electrostatic force, by application of a potential difference between the membrane and a control electrode behind. In this way segmented spherical or paraboloidal primaries of long focal length can be made directly, eliminating the correction optics needed when flat segments are used. The electric potential would be spatially and temporally controlled to obtain uniform curvature despite non-uniformity in membrane tension, to create slight asphericity if needed and to provide active damping of vibrations. We report the operation of a small prototype telescope with a SMEC primary. A design for a 3.2-m space prototype is described, based on the two-mirror anastigmat of Schwartzschild and Couder. A 400 m radius primary is combined with a single concave secondary flying in formation at a distance of 190 m. Together they form a wide-field, well-corrected focus at f/30. A larger telescope with much higher diffraction limited resolution could be made simply by adding more segments of the 27 m diameter parent primary to the formation. The TPF nulling interferomer could be made with similarly sized elements in a 100 m formation, configured as segments of a kilometer focal length parent. A very large SMEC telescope might be made from a continuous 10×1000 m membrane, rolled up for launch. Cryogenic cooling for all these configurations would be accomplished by additional spacecraft in the formation to block direct solar illumination of the telescope elements.
• Burge, J. H., Cuerden, B., & Angel, J. R. (2000). Active mirror technology for large space telescopes. Proceedings of SPIE - The International Society for Optical Engineering, 4013, 640-648.
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  Abstract: Future space telescopes rely on advances in technology to enable fabrication of primary mirrors with orders of magnitude more area, yet similar mass as current mirrors. This requires a shift of paradigm from the concept of the mirror as a rigid, stable unit, to the idea of the mirror as a system that uses active control to maintain the figure of a flexible surface. We discuss issues for this new class of optics and present status on a 2-m prototype mirror for NGST.
• Burge, J. H., Sabatke, E., Angel, J. R., & Woolf, N. J. (2000). Optical design of giant telescopes for space. Proceedings of SPIE - The International Society for Optical Engineering, 4092, 82-93.
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  Abstract: Increased performance for optical telescopes has historically come from larger apertures, from technological advances for the telescope components, such as detectors, and from access to better sites, such as space. Little has changed in the basic telescope design for a century. These conventional designs have served us well and will continue to do so with the Next Generation Space Telescope. There is an upper limit to the size of this type of telescope, set by the capacity to launch the required mass. For future space telescopes of 50, 100, 500 meter apertures, we have developed a new type of optical design. We use a primary reflector made from segments of flat and near-flat membranes. The secondary reflector and subsequent optics are supported in separate spacecraft, flying in formation with the primary reflector. In addition, each spacecraft maintains sunshields to keep the optics shaded from the sun. This paper explores optical design issues for this type of giant space telescope.
• Ford, H. C., Angel, J. R., Burrows, C. J., Morse, J. A., Trauger, J. T., & Dufford, D. A. (2000). HST to HST10X: A second revolution in space science. Proceedings of SPIE - The International Society for Optical Engineering, 4013, 595-604.
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  Abstract: The Hubble Space Telescope is arguably one of the most important and successful scientific endeavors undertaken in the twentieth century. Hubble, a modest-sized 2.4-m telescope, outperforms much larger terrestrial telescopes because it is diffraction limited, and because the sky seen from orbit is darker than the terrestrial night sky. If we increase the diameter of Hubble to 8.4-m, a diameter comparable to Keck and the VLT, the increase in capability will be comparable to that which, was first achieved by Hubble's launch and subsequent repair. HST10X will allow a fast track solution of outstanding problems in astronomy. Perhaps foremost among these is the detection of earth-like planets orbiting nearby stars. HST10X can detect earth-like planets around stars at distances up to 10 parsecs. Furthermore, HST10X will enable spectroscopic examination of earth-like planets to search for atmospheric oxygen, a certain sign of life. A one-orbit F814W HST10X exposure will be deeper than the HDF and have 14 times more spatial information per galaxy. HST10X will allow us to measure the spectra and spectra energy distributions of high redshift galaxies that cannot be reached with the Keck and VLT telescopes.
• Hinz, P., Angel, R., Woolf, N., Hoffmann, B., & McCarthy, D. (2000). BLINC: A testbed for nulling interferometry in the thermal infrared. Proceedings of SPIE - The International Society for Optical Engineering, 4006, I/-.
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  Abstract: A key technology in NASA's plans for a Terrestrial Planet Finder (TPF) is nulling interferometry in the thermal infrared. This technique suppresses the overwhelming light from a star in order to study its immediate surroundings. To further develop nulling interferometry we have built the BracewelL Infrared Nulling Cryostat (BLINC). The instrument is designed to achieve high precision cancellation of an artificial source in the lab and of starlight on the telescope. Our goal is to achieve suppression of >10,000 both with a laser source and a broadband source over a 20% bandwidth. This is sufficient for ground-based observations with even short baseline interferometers since the finite diameter of the star does not allow suppression greater than that for most nearby sources. BLINC uses two parts of the MMT pupil to create an interferometer of 2.7 m diameter elements separated by 4 m. Active compensation for phase variations between the two apertures will be used to maintain the cancellation of the starlight in the presence of atmospheric turbulence. When combined with the adaptive secondary of the MMT to remove high order aberrations, BLINC will be able to achieve suppression of 10,000. This will allow detection of zodiacal dust around nearby stars as faint as 10 times the solar level and detection of companions large than 10 Jupiter masses for systems less than one billion years old. BLINC serves as a prototype for nulling with the Large Binocular Telescope which will be able to see zodiacal dust as faint as solar level and Jupiter mass or larger companions. Thus both in technological and scientific background BLINC will help begin the search for Earth-like planets.
• Langlois, M. P., Angel, J. P., & Lloyd-Hart, M. (2000). Prime focus coma corrector for the MMT with 'off the shelf' components. Proceedings of SPIE - The International Society for Optical Engineering, 4008, II/-.
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  Abstract: A small prime focus corrector for the new 6.5 m MMT telescopes has been built using small off-the-shelf optical components. Its purpose is to aid in initial tests of the primary mirror. At f/1.25 the paraboloidal primary is by far the fastest of any large telescope, and without correction the field and the plate scale are both inconveniently small. The corrector design is based on the Maksutov camera, with a concave spherical relay mirror and a refractive meniscus corrector to subtract the coma introduced by the primary mirror and balance out the spherical aberration from the relay mirror. The result is a 48 arcsec field at f/7 (220 μm/arcsec) with 80% encircled energy within 0.25 arcsec diameter (the uncorrected field is 6 arcsec). The instrument includes a CCD with 500×500 20 micron pixels (0.09 inches per pixel), a guide camera and provision for a fast wavefront sensor to measure mirror figure and atmospheric turbulence. The instrument was tested at the telescope before implementation of the active control of mirror support forces or the mirror air conditioning system. Nevertheless images with FWHM 0.53 arcsec were recorded.
• Lloyd-Hart, M., & Angel, R. (2000). Imaging giant extra-solar planets from the ground. Optics and Photonics News, 11(10), 42-46.
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  Abstract: The imaging of giant extra-solar planets from the ground was discussed. The NASA's Terrestrial Planet Finder (TPF) will extend the search to earth-like planets. For planet searchers, each extra optical element contributes to the noisy thermal background. Nulling interferometry and adaptive correction to very high order were also elaborated.
• Lloyd-Hart, M., Jefferies, S. M., Hege, E. K., & Angel, J. R. (2000). New approach to Rayleigh guide beacons. Proceedings of SPIE - The International Society for Optical Engineering, 4007, I/-.
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  Abstract: We present analysis and numerical simulations of a new method to sense atmospheric wavefront distortion in real time with Rayleigh beacons. Multiple range-gated images of a single pulse from the laser are used to determine each phase map, providing an advantage over other methods in that photon noise is substantially reduced for a given brightness of the beacon. A laser at about 350 nm projects collimated pulses of light adjacent to the telescope. Rayleigh-scattered light from each pulse is recorded through the full telescope aperture in a sequence of video frames, each a few μs long. Images are captured as the pulse approaches and passes through the height at which the camera is focused. Phase diversity is thus naturally introduced between the frames. An iterative algorithm is used to extract the pupil-plane phases from the recorded intensity distributions. We anticipate that such beacons are likely to be valuable in future advanced systems for adaptive optics on very large telescopes with multiple laser beacons and deformable mirrors that aim to provide a large corrected field of view by tomography of the atmospheric turbulence.
• Lloyd-Hart, M., Wildi, F., Martin, H., McGuire, P., Kenworthy, M., Johnson, R., Fitz-Patrick, B., Angeli, G., Miller, S., & Angel, R. (2000). Adaptive optics for the 6.5 m MMT. Proceedings of SPIE - The International Society for Optical Engineering, 4007, I/-.
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  Abstract: The adaptive optics system for the 6.5 m MMT conversion telescope will be the first to compensate the aberrated wavefront at the telescope's secondary mirror. This approach has unique advantages in terms of optical simplicity, high throughput and low emissivity. We report here the present state of construction, and the results of static and dynamic performance tests of the Cassegrain optical package.
• McGuire, P. C., Langlois, M. P., Lloyd-Hart, M., Rhoadarmer, T. A., & Angel, J. P. (2000). Measurement of atmospheric turbulence with a Shack-Hartmann wavefront sensor at the new MMT's prime focus. Proceedings of SPIE - The International Society for Optical Engineering, 4007, II/-.
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  Abstract: In order to prepare for the adaptive secondary program at the new 6.5 meter MMT, we have begun a campaign to measure atmospheric turbulence with a Shack-Hartmann wavefront sensor (WFS) camera. Our first efforts, prior to second aluminization of the primary, consisted of taking turbulence data with our WFS camera with special coma-correcting optics at the MMT prime focus, without a Cassegrain secondary mirror. Our first measurements consisted of 1000 frames of approximately 10 millisecond duration, taken approximately 3 seconds apart. We measure a 5/3 power law structure function, suggesting Kolmogorov turbulence, with an r0 = 15 cm, but with possible hints of an outer scale and tracking jitter in the structure function. At the end of our data acquisition, we deliberately put 2 μm of astigmatism into the primary mirror with its actuators, and in our analysis, we recover 1.7±0.3 μm of astigmatism. A brief analysis of the low-order modal amplitudes with the 3 second frame delay shows that there are significant self-correlations of the low-order modes even on this long time-scale.
• McGuire, P. C., Rhoadarmer, T. A., Coy, H., Roger, J., Angel, P., & Lloyd-Hart, M. (2000). Linear zonal atmospheric prediction for adaptive optics. Proceedings of SPIE - The International Society for Optical Engineering, 4007, II/-.
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  Abstract: We compare linear zonal predictors of atmospheric turbulence for adaptive optics. Zonal prediction has the possible advantage of being able to interpret and utilize wind-velocity information from the wavefront sensor better than modal prediction. For simulated open-loop atmospheric data for a 2-meter 16-subaperture AO telescope with 5 millisecond prediction and a lookback of 4 slope-vectors, we find that Widrow-Hoff Delta-Rule training of linear nets and Back-Propagation training of non-linear multilayer neural networks is quite slow, getting stuck on plateaus or in local minima. Recursive Least Squares training of linear predictors is two orders of magnitude faster and it also converges to the solution with global minimum error. We have successfully implemented Amari's Adaptive Natural Gradient Learning (ANGL) technique for a linear zonal predictor, which premultiplies the Delta-Rule gradients with a matrix that orthogonalizes the parameter space and speeds up the training by two orders of magnitude, like the Recursive Least Squares predictor. This shows that the simple Widrow-Hoff Delta-Rule's slow convergence is not a fluke. In the case of bright guidestars, the ANGL, RLS, and standard matrix-inversion least-squares (MILS) algorithms all converge to the same global minimum linear total phase error (approximately 0.18 rad2), which is only approximately 5% higher than the spatial phase error (approximately 0.17 rad2), and is approximately 33% lower than the total `naive' phase error without prediction (approximately 0.27 rad2). ANGL can, in principle, also be extended to make nonlinear neural network training feasible for these large networks, with the potential to lower the predictor error below the linear predictor error. We will soon scale our linear work to the approximately 108-subaperture MMT AO system, both with simulations and real wavefront sensor data from prime focus.
• Stamper, B., Angel, R., Burge, J., & Woolf, N. (2000). Flat membrane mirrors for space telescopes. Proceedings of SPIE - The International Society for Optical Engineering, 4091, 126-136.
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  Abstract: Current work concentrates on making optically flat mirrors using stretched membranes. Very lightweight mirrors can be made that only require a rigid support at the perimeter of the membrane. Contact with the membrane need not be continuous, only discrete attachment points are required to tension the material. Initial results of useable area as a function of the number of attach points will be given. Experimental fixtures demonstrating methods of forming a flat membrane are shown. The potential for nearly flat mirrors is mentioned including one method of implementation. Surface measurements are also contrasted for different materials.
• Woolf, N. J., & Angel, J. R. (2000). Passive cooling of gossamer telescopes. Proceedings of SPIE - The International Society for Optical Engineering, 4091, 39-42.
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  Abstract: We discuss the rationale for gossamer telescopes and why the initial ones will be used for infrared wavelengths and this necessitates a cold telescope. The sunshield presents major problems for gossamer telescopes and may limit the potential for large IR telescopes in space. Astronomical sources of heat other than sun, moon and earth would set the telescope temperature at 4.5 K. We discuss the sunshield problem and suggest that gossamer telescopes at 1AU are more likely to be limited to approximately 10 K. We discuss the spacing of the telescope and the sunshield. The optimum spacing is 100 times the telescope size. Such spacing will require constantly firing ion engines to keep the sunshield and telescope moving round the sun with the same angular velocity. Alternately it is possible to attach the sunshield to the telescope with a compression member. This will require the telescope and sunshield to be closer together. A single layer sunshield will bring the telescope temperature to approximately 25 K. Heat sources on the telescope will limit the cooling, and so far as possible heat sources must be off-loaded to the vicinity of the sunshield.
• Burrows, A., & Angel, R. (1999). Direct detection at last. Nature, 402(6763), 732-733.
• Jian, G. e., Ciarlo, D., Kuzmenko, P., Alcock, C., Macintosh, B., Angel, R., Woolf, N., Lloyd-Hart, M., Fugate, R. Q., & Najita, J. (1999). Adaptive optics high resolution spectroscopy: present status and future direction. Proceedings of SPIE - The International Society for Optical Engineering, 3762, 174-183.
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  Abstract: High resolution spectroscopy experiments with visible adaptive optics (AO) telescopes at Starfire Optical Range and Mt. Wilson have demonstrated that spectral resolution can be routinely improved by a factor of approx. 10 over the seeing-limited case with no extra light losses at visible wavelengths. An account is given on the progress on a newly developed AO optimized high resolution optical spectrograph at Steward Observatory. The progress on a compact infrared (IR) high resolution spectrograph with silicon immersion echelle gratings being developed at LLNL is also discussed.
• Johnson, R. L., Angel, J. R., Lloyd-Hart, M., & Angeli, G. Z. (1999). Miniature instrument for the measurement of gap thickness using poly-chromatic interferometry. Proceedings of SPIE - The International Society for Optical Engineering, 3762, 245-253.
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  Abstract: An optical instrument for measuring the thickness of a thin gap or transparent film is presented. It will be used to calibrate the adaptive secondary mirror under development for the Steward Observatory 6.5 m Multiple Mirror Telescope. Capacitive sensors in the mirror assembly measure dynamically the thickness of the nominally 50 μm air gap between the deformable mirror and a glass reference body. The miniature interferometer has been developed to accurately determine the gap thickness so that the capacitive sensors may be calibrated. Interference fringes are produced by illuminating an air gap, which is between two reflective surfaces, with monochromatic plane waves and observing the reflected light. Intensity variations are measured as the wavelength of illumination is varied over an octave. The film thickness is determined by correlating the observed fringes with those modeled for different gaps. Absolute measurement to an accuracy of a small fraction of a wavelength is possible.
• Langlois, M. P., Angel, J. P., Lloyd-Hart, M., Miller, S., & Angeli, G. (1999). High order adaptive optics system with a high density spherical membrane deformable mirror. Proceedings of SPIE - The International Society for Optical Engineering, 3762, 50-57.
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  Abstract: We have designed a new type of curved deformable mirror with 3000 actuators. It will be used at the 6.5 m MMT telescope in an adaptive optics system to correct the wavefront on a scale of 12 cm, for diffraction limited changes in visible light (0.02 arcsec) and for high contrast imaging with a coronagraph in the near infrared. The mirror's 51 cm diameter glass faceplate, 400 microns thick, will be actuated by tubular piezoceramic actuators on 8 mm centers. The mirror is made concave so it can be used in a simple, compact optical relay with no need for large collimator or camera elements. A prototype deformable mirror with 300 elements that tests the support and assembly techniques is being built for lab tests and will be tested in a closed loop adaptive optics system at the Steward Observatory 2.3m telescope.
• McGuire, P. C., Lloyd-Hart, M., Angel, J. R., Angeli, G. Z., Johnson, R. L., Fitz-Patrick, B., Davison, W. B., Sarlot, R. J., Bresloff, C. J., Hughes, J. M., Miller, S. M., Schaller, P., Wildi, F. P., Kenworthy, M. A., Cordova, R. M., & al., e. (1999). Full-system laboratory testing of the F/15 deformable secondary mirror for the new MMT adaptive optics system. Proceedings of SPIE - The International Society for Optical Engineering, 3762, 28-37.
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  Abstract: We will present a system to perform closed-loop optical tests of the 64 cm diameter, 336 actuator adaptive secondary made at the Steward Observatory Mirror Laboratory. Testing will include Shack-Hartmann wavefront sensing and modal correction of static and dynamic aberrated wavefronts. The test optical system (called the 'Shimmulator') is designed so that experiments can be made with both the focal plane instrument and secondary installed in their normal configuration at the MMT, or with the same 9 m spacing in a laboratory test tower. The convex secondary will be illuminated at normal incidence through two 70 nm diameter lenses mounted just below. The artificial, aberrated star is projected from near the wavefront sensor in the Cassegrain focus assembly. Computer generated holograms correct for spherical aberration in the relay optics at the test wavelengths of 0.594 and 1.5 μm. Atmospheric turbulence is reproduced by two spinning transmission plates imprinted with Kolmogorov turbulence. The Shimmulator will give us the opportunity to test fully the adaptive optics system before installation at the new MMT, hence saving much precious telescope time.
• McGuire, P. C., Rhoadarmer, T. A., Lloyd-Hart, M., Shelton, J. C., Lesser, M. P., Angel, J. R., Angeli, G. Z., Hughes, J. M., Fitz-Patrick, B., Rademacher, M. L., Schaller, P., Kenworthy, M. A., Wildi, F. P., Capara, J. G., & Ouellete, D. B. (1999). Construction and testing of the wavefront sensor camera for the new MMT adaptive optics system. Proceedings of SPIE - The International Society for Optical Engineering, 3762, 269-282.
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  Abstract: This paper describes the construction and testing of the Shack-Hartmann wavefront sensor (WFS) camera for the new MMT adaptive optics system. Construction and use of the sensor is greatly simplified by having the 12 × 12 lenslet array permanently glued to the detector array, obviating the need for any further realignment. The detector is a frame transfer CCD made by EEV with 80 × 80 pixels, each 24 microns square, and 4 output amplifiers operated simultaneously. 3 × 3 pixel binning is used to create in effect an array of quad-cells, each centered on a spot formed by a lenslet. Centration of the lenslet images is measured to have an accuracy of 1 μm (0.02 arcsec) rms. The maximum frame rate in the binned mode is 625 Hz, when the rms noise is 4.5 - 5 electrons. In use at the telescope, the guide star entering the wavefront sensor passes through a 2.4 arcsec square field stop matched to the quad-cell size, and each lenslet samples a 54 cm square segment of the atmospherically aberrated wavefront to form a guide star image at a plate scale of 60 μm/arcsec. Charge diffusion between adjacent detector pixels is small: the signal modulation in 0.7 arcsec seeing is reduced by only 10% compared to an ideal quad-cell with perfectly sharp boundaries.
• Rhoadarmer, T. A., McGuire, P. C., Hughes, J. M., Lloyd-Hart, M., Angel, J. R., Schaller, S., & Kenworthy, M. A. (1999). Laboratory adaptive optics system for testing the wave front sensor for the new MMT. Proceedings of SPIE - The International Society for Optical Engineering, 3762, 161-173.
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  Abstract: A laboratory adaptive optics system has been built for testing the wave front sensor hardware and software for the new Multiple Mirror Telescope adaptive optics system. The system will also allow different wave front reconstruction and prediction schemes to be examined. The laboratory system contains a silicon micromachined adaptive mirror with 37 electro-static actuators as well as a novel approach for generating atmospheric turbulence. The design and initial testing of the system are presented.
• Angel, R., & Burge, J. (1998). Verifying mirror technology for NGST with a space-qualified, cryogenic 3.5 m mirror. European Space Agency, (Special Publication) ESA SP, SP-429, 181-188.
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  Abstract: A cryogenic 3.5 m mirror is being developed which weighs 140 kg, that could be incorporated into a telescope and launched to a high orbit by the Shuttle or number of other vehicles. Such a space mission would complement those already planned for scaled down tests of the unfolding of the next generation space telescope (NGST) mirror segments and the sunshade. Much would be learned by running the mirror at cryogenic temperature, with a new infrared array to make preliminary observations in the 2-4 μm dark sky window and a charge coupled devices to check optical image quality.
• Angel, R., Burge, J., & Wolf, N. (1998). Toward NGST. Proceedings of SPIE - The International Society for Optical Engineering, 3356(1), 185-186.
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  Abstract: A telescope to follow the HST does not necessarily have to be a lot bigger to make a dramatic advance in observing capability. Provided it is cooled to allow exploration the 2-4 μm window of very dark sky background, and has several times the 0.85 m aperture of SIRTF, it would have already unique capability to resolve and study the crowded fields of the most strongly red-shifted, distant galaxies. Such a telescope could test many of the advanced technologies needed for the mirror elements in future deployed telescopes or interferometers of very large size.
• Barrett, T. K., Bruns, D. G., Brinkley, T. J., Sandler, D. G., FitzPatrick, B. C., Angel, J. R., & Rhoadarmer, T. (1998). Adaptive secondary mirror for the 6.5-m MMT. Proceedings of SPIE - The International Society for Optical Engineering, 3353(2), 754-763.
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  Abstract: We report the latest progress on the design, fabrication and testing of the adaptive secondary mirror to be used in the adaptive optics system to for the 6.5m upgrade to the Steward Observatory's MMT. The adaptive secondary will use electromagnetic force actuators in conjunction with a rigid reference structure to deform a thin and flexible glass facesheet. The facesheet is fabricated with figure accuracy comparable to the surface of a traditional static secondary mirror. The flexible facesheet can however, be deformed by the actuators to conjugate the changing atmospheric aberration. Capacitive position sensors are placed at each actuator and are used to rapidly measure the position of the glass facesheet relative to the rigid reference structure. These measurements are used as feedback in a servo control-loop which maintains the desired figure of the adaptive secondary facesheet. In the proposed design the mechanical interface between the facesheet and the reference structure is limited to a small hub in the center of the facesheet. Due to heat dissipation in the electromagnetic voice-coils a temperature control system is required to maintain the facesheet of the adaptive secondary near the ambient temperature of the atmosphere. We report on laboratory tests of a nearly full size 60 actuator adaptive secondary prototype. We include tests of actuator stroke and position accuracy, control-loop stability, and closed-loop bandwidth.
• Burge, J. H., Angel, J. R., Cuerden, B., Martin, H. M., Miller, S. M., & Sandler, D. G. (1998). Lightweight mirror technology using a thin facesheet with active rigid support. Proceedings of SPIE - The International Society for Optical Engineering, 3356(2), 690-701.
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  Abstract: The next generation of space telescopes will require primary mirrors that push beyond the current state of technology for mirror fabrication. These mirrors are large, up to 8 meters in diameter, have low mass per unit area, less than 15 kg/m2 and must maintain diffraction limited performance at cryogenic temperatures. To meet these requirements, we have developed an active mirror that has a thin membrane as the optical surface, which is attached to a stiff lightweight support structure through a set of screw-type actuators. This system allows periodic adjustments with the actuators to maintain the surface figure as measured from star light. The optical surface accuracy and stability are maintained by the active system, so the support structure does not have to be optically stable and can be made using light weight carbon fiber laminates to economically provide stiffness. The key technologies for implementing this technology are now in place. We have performed two critical demonstrations using 2-mm glass membranes - diffraction limited optical performance of a 0.5-m diameter mirror and launch survival of a 1-m diameter mirror. We have also built and tested a prototype actuator that achieves 25 nm resolution at cryogenic temperatures. We are now building a 2-m mirror as a prototype for the Next Generation Space Telescope. This mirror will have mass of only 40 kg, including support structure, actuators, and control electronics. It will be actively controlled and interferometrically measured at 35 K.
• Carleton, N. P., Traub, W. A., & Roger, J. (1998). Interferometric capability for the Magellan Project. Proceedings of SPIE - The International Society for Optical Engineering, 3350, 733-735.
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  Abstract: The Magellan Project is building two 6.5-m telescopes, 60 m apart, at the Las Campanas Observatory in Chile. There are on-going plans to combine the beams of the two main telescopes, and of smaller auxiliary telescopes, for interferometric measurements. In this paper we consider the array of auxiliary telescopes as a stand-alone instrument, recognizing that it will operate as such for some large fraction of the time. Our interest is sharpened by the availability of six 1.8-m optical systems, retired from the Smithsonian-Arizona Multiple-Mirror Telescope in preparation for the installation of a single-mirror 6.5-m system. We have completed a design for a 1.8-m telescope, in which the MMT components are supported on a proven tripod mount. The optics-support uses steel for stiffness, and low-thermal- expansion rods for passive stability. This array will be a powerful tool for the investigation of stellar limb darkening, surface features, and changes of diameter in pulsations, as well as dust disks, shells, and binary companions. The 1.8-m telescopes on good sites such as Magellan's should be able to operate at full aperture for interferometry at 2.2μm. They should therefore be able to reach to magnitude K = 10 or so, and thus to cover substantial samples of both main-sequence and pre-main- sequence stars, and of fully evolved stars as well. ©2004 Copyright SPIE - The International Society for Optical Engineering.
• Hinz, P. M., Roger, J., Hoffmann, W. F., McCarthy Jr., D. W., McGuire, P. C., Cheselka, M., Hora, J. L., & Woolf, N. J. (1998). First results of nulling interferometry with the Multiple Mirror Telescope. Proceedings of SPIE - The International Society for Optical Engineering, 3350, 439-447.
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  Abstract: We have successfully used nulling interferometry at 10 μm wavelength to interferometrically suppress a star's radiation. This technique was first proposed by Bracewell 20 years ago to image extra-solar planets and is now the basis for proposed space-borne instruments to search for Earth-like extra-solar planets and their spectroscopic signatures of habitability and life. In our experiment, the beams from two 1.8 m telescopes of the Multiple Mirror Telescope were brought into registration at a semi-transparent beamsplitter, and the images made coincident on an infrared array detector capable of taking rapid short exposure images. The atmospheric fluctuations caused the phase difference between the beams to fluctuate, changing the total flux of the star seen in the image plane. When the atmosphere caused the wavefronts to be exactly out of phase the entire stellar Airy pattern disappeared. For the unresolved star α Tauri the cancellation was such that a companion only 0.2 arcsec from the star and 25 times fainter would appear equal in intensity to the nulled star. The residual flux was spread into a wide halo suggesting the cause of this flux was imperfect cancellation of the aberrated wavefronts. To increase the precision of nulling beyond this first step several sources of error need to be addressed. We discuss the control of errors due to amplitude, polarization, chromatic differences, stellar leak, and sampling time. Improvements such as active phase tracking, adaptive optics, and cooled optics will increase the achievable gain of nulling interferometry and allow it to be used on fainter objects. ©2004 Copyright SPIE - The International Society for Optical Engineering.
• Hinz, P. M., Roger, J., Hoffmann, W. F., McCarthy Jr., D. W., McGuire, P. C., Cheselka, M., Hora, J. L., & Woolf, N. J. (1998). Imaging circumstellar environments with a nulling interferometer. Nature, 395(6699), 251-253.
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  Abstract: Extrasolar planets must be imaged directly if their nature is to be better understood. But this will be difficult, as the bright light from the parent star (or rather its diffracted halo in the imaging apparatus) can easily overwhelm nearby faint sources. Bracewell has proposed a way of selectively removing starlight before detection, by superposing the light from two telescopes so that the stellar wavefronts interfere destructively. Such a 'nulling' interferometer could be used in space to search for extrasolar Earth-like planets through their thermal emission and to determine through spectroscopic analysis if they possess the atmospheric signatures of life. Here we report mid-infrared observations using two co-mounted telescopes of the Multiple Mirror Telescope that demonstrate the viability of this technique. Images of unresolved stars are seen to disappear almost completely, while light from a nearby source as close as 0.2 arcsec remains, as shown by images of Betelgeuse. With this star cancelled, there remains the thermal image of its surrounding, small dust nebula. In the future, larger ground-based interferometers that correct for atmospheric distortions (using adaptive optics) should achieve better cancellation, allowing direct detection of warm, Jupiter-size planets and faint zodiacal dust around other nearby stars.
• Jian, G. e., Angel, R., & Shelton, C. (1998). Optical spectroscopy with a near single-mode fiber feed and adaptive optics. Proceedings of SPIE - The International Society for Optical Engineering, 3355, 253-263.
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  Abstract: We report on first astronomical results with a cross-dispersed optical échelle spectrograph fed by a near single-mode fiber. We also present on a novel design of a new adaptive optics (AO) optimized fiber-fed cross-dispersed échelle spectrograph. The spectrograph is designed to match with AO corrected images in the optical bands provided by such as the Mt. Wilson 100 inch, Starfire Optical Range 3.5 m AO telescopes. Ultimately, it will be installed at the 6.5 m MMT, when this has high resolution AO correcting the optical spectrum. The spectrograph, fed by a 10 micron fused silica fiber, is unique in that the entire spectrum from 0.4 micron to 1.0 micron will be almost completely covered at resolution 200,000 in one exposure. The detector is a 2k×4k AR coated back illuminated CCD with 15 micron pixel size. The close order spacing allowed by the sharp AO image makes the full cover possible. A 250×125 mm2 Milton Roy R2 echelle grating with 23.2 grooves mm-1 and a blaze angle of 63.5 deg provides main dispersion. A double pass BK7 prism with 21 deg wedge angle provides cross dispersion, covering the spectrum from order 193 to 77. The spectrograph is used in the quasi-Littrow configuration with an off-axis Maksutov collimator/camera. The fiber feeds the AO corrected beams from the telescope Cassegrain focus to the spectrograph, which is set up on an optical bench. The spectrograph will be used mainly to study line profiles of solar type stars, to explore problems of indirect detection of planets and also study interstellar medium, circumstellar medium and metal abundance and isotopic ratios of extremely metal-poor stars.
• Jian, G. e., Jacobsen, B. P., Roger, J., McGuire, P. C., Roberts, T., McLeod, B. A., & Lloyd-Hart, M. (1998). Simultaneous measurements of sodium column density and laser guide star brightness. Proceedings of SPIE - The International Society for Optical Engineering, 3353, 242-253.
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  Abstract: We report on new results of simultaneous measurements of sodium layer column density and the absolute return flux from laser guide stars created by a monochromatic approximately 1 W CW laser, tuned to the peak of the sodium D2 hyperfine structure. The return was measured at the MMT while the sodium abundance was measured at the CFA 60 inch telescope, about 1 km away, with the Advanced Fiber Optic Echelle spectrograph. The laser frequency stability, which can greatly affect the return flux, was monitored at the same time in order to improve the measurement accuracy. After the correction for laser frequency jitter and atmospheric transmission, the absolute flux return above the atmosphere for circularly polarized light is 1.2 X 106 photons s -1 m-2 per watt launched above the atmosphere, per unit column density, which we taken as our measured mean over the year of N(Na) equals 3.7 X 109 cm-2 at Tucson. The solidification of a final well-determined relationship between the sodium laser guide star brightness and sodium layer column density is pivotal in the design of the next generation laser guide star adaptive optics systems. We also report the measurements and analysis of the relationship between the projected beam waist of the sodium laser and the resultant spot size on the sodium layer under typical atmospheric conditions. Since wavefront measured error is proportional to spot size, and also to 1/(root) power, minimum spot size is crucial for lowest laser power requirement. By projecting the laser through diffraction limited optics of 0.5 m diameter, roughly 3 r0, we have achieved the smallest artificial beacon yet recorded, about 0.8 arcsec. ©2003 Copyright SPIE - The International Society for Optical Engineering.
• Lloyd-Hart, M., Roger, J., Sandler, D. G., Barrett, T. K., McGuire, P. C., Rhoadarmer, T. A., Bruns, D. G., M., S., McCarthy Jr., D. W., & Cheselka, M. (1998). Infrared adaptive optics system for the 6.5 m MMT: System status and prototype results. Proceedings of SPIE - The International Society for Optical Engineering, 3353, 82-93.
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  Abstract: The Multiple Mirror Telescope (MMT) is now being replaced by a single 6.5m telescope which will have an integral adaptive optics system optimized for near IR operation. We illustrate key results obtained with low-order adaptive optics at the MMT and report on progress on the major components of the system for the new telescope. We also give a brief update on the status of the telescope itself. First light for the adaptive system is expected in mid 1999. ©2003 Copyright SPIE - The International Society for Optical Engineering.
• McCarthy Jr., D. W., Burge, J. H., Roger, J., Jian, G. e., Sarlot, R. J., Fitz-Patrick, B. C., & Hinz, J. L. (1998). ARIES: Arizona infrared imager and echelle spectrograph. Proceedings of SPIE - The International Society for Optical Engineering, 3354, 750-754.
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  Abstract: ARIES, a new 1-5 μm camera/spectrograph, is designed to capitalize on the exceptionally low thermal background and high optical throughput offered by the f/15 adaptive secondary system being built for the upgraded 6.5m MMT. With two state-of-the-art IR arrays, ARIES will provide diffraction-limited imaging in the JHKLM atmospheric windows and also echelle, long-slit spectroscopy at resolutions of 2,000 and 30,000. ARIES will also supply global wavefront tip/tilt information to the adaptive system using cryogenic pick-off mirrors to access field stars over a 50 arcsec diameter field at wavelengths from 1-2 μm. ©2003 Copyright SPIE - The International Society for Optical Engineering.
• Nelson, A. F., & Angel, J. R. (1998). The range of masses and periods explored by radial velocity searches for planetary companions. Astrophysical Journal Letters, 500(2 PART I), 940-957.
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  Abstract: Radial velocity measurements have proven a powerful tool for finding planets in short-period orbits around other stars. In this paper we develop an analytical expression relating the sensitivity to a periodic signal to the duration and accuracy of a given set of data. The effects of windowing and the sensitivity to periods longer than the total length of observations are explored. We show that current observations are not yet long or accurate enough to make unambiguous detection of planets with the same mass and period as Jupiter. However, if measurements are continued at the current levels of accuracy (5 m s-1) for a decade, then planets of Jovian mass and brown dwarfs will either be detected or ruled out for orbits with periods less than ∼15 yr. As specific examples, we outline the performance of our technique on large-amplitude and large-eccentricity radial velocity signals recently discussed in the literature, and we delineate the region explored by the measurements of 14 single stars made over a 12 yr period by Walker et al. Had any of these stars shown motion like that caused by the exoplanets recently detected, it would have been easily detected. The data set interesting limits on the presence of brown dwarfs at orbital radii of 5-10 AU. The most significant features in the Walker et al. data are apparent long-term velocity trends in 36 UMa and β Vir, consistent with super planets of mass of 2 MJ in a 10 yr period, or 20-30 MJ in a 50 yr period. If the data are free of long-term systematic errors, the probability of just one of the 14 stars showing this signal by chance is about 15%. Finally, we suggest an observing strategy for future large radial velocity surveys that, if implemented, will allow coverage of the largest range of parameter space with the smallest amount of observing time per star. We suggest that about 10-15 measurements be made of each star in the first 2 yr of the survey, then 2-3 measurements yr-1 thereafter, provided no (or slow) variation is observed. More frequent observations would of course be indicated if such variations were present. © 1998. The American Astronomical Society. All rights reserved.
• Rhoadarmer, T. A., McGuire, P. C., Lloyd-Hart, M., Roger, J., Cuerden, B., Fitz-Patrick, B. C., & Sandler, D. G. (1998). Design of the first generation wave front sensor and actuator geometry for the 6.5 m MMT adaptive-optical system. Proceedings of SPIE - The International Society for Optical Engineering, 3353, 579-590.
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  Abstract: A description is given for the geometry of the first generation adaptive-optical (AO) system being developed for the 6.5m single-mirror telescope that will replace the Multiple Mirror Telescope on Mt. Hopkins. The AO system consists of an adaptive secondary mirror with 330-actuators and a wave front senor composed of a 13 by 13 square subaperture array. The deformable mirror actuator vibrational modes were determined from a finite element model of the adaptive-secondary mirror. Numerical simulations in based on the finite element results were carried out to determine the expected performance of the system. Two reconstruction algorithms were compared - a least squares reconstructor and a modal equalization technique developed for this AO geometry. Strehl ratios are reported for the two algorithms for various guide star magnitudes and number of corrected wave front modes. ©2003 Copyright SPIE - The International Society for Optical Engineering.
• Roberts Jr., W. T., Murray, J. T., Austin, W. L., Powell, R. C., & Roger, J. (1998). Solid-state Raman laser for MMT sodium guide star. Proceedings of SPIE - The International Society for Optical Engineering, 3353, 347-355.
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  Abstract: Generation of sodium guide stars for adaptive optics requires very precise control of the frequency and bandwidth of the laser to maximize the brightness of the generated guide star. The ruggedness, efficiency and ease of use of a solid state system has great potential for improving the reliability and power of the laser guide star over the dye laser system currently used. The dearth of solid state transitions at the precise wavelength required for exciting resonance scattering in sodium drives us toward Raman shifting to downshift a nearby solid-state transition line tuned to work with the Raman-shifting material. The system being developed for the 6.5 meter multiple mirror telescope (MMT) takes two approaches to creating the sodium guide star: one uses YGAG to maximize the Raman-shifted output at the sodium D2 resonance. The second approach is to thermally tune the output of YAG to reach the appropriate wavelength for Raman shifting to 589 nm. Initial results from YGAG indicate that it will not be a suitable material for creating the sodium guide star laser. Initial results from the YGAG laser is presented, along with a discussion of the potential of the technology. ©2003 Copyright SPIE - The International Society for Optical Engineering.
• Woolf, N. J., Roger, J., Beichman, C. A., Burge, J. H., Shao, M., & Tenerelli, D. J. (1998). Planet Discoverer Interferometer I: PDI, a potential precursor to Terrestrial Planet Finder. Proceedings of SPIE - The International Society for Optical Engineering, 3350, 683-689.
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  Abstract: We consider a possible precursor interferometer to Terrestrial Planet Finder. The precursor called Planet Discoverer Interferometer (PDI) would search for broadband 10 μm radiation from possible terrestrial planets orbiting stars out to a distance of 8-10pc and at an angular separation of at least 0.1 arcseconds. There are about 20 stars of types A,F,G and K around which an Earth-analog might be detected. PDI would be able to confirm such planets by seeing their orbital motion. PDI would also be able to observe 5 μm radiation from the more massive and younger gas-giant planets around stars up to distances ∼ 150 pc, separated from their star by more than 0.05 arc seconds. It would also see the re-radiated thermal radiation of Jupiter-like planets at temperatures above ∼130K. The device would be a 15m long truss with four SIRTF-like telescopes. It would need to be in a SIRTF-like Earth-trailing orbit, and would be radiatively cooled. A very preliminary design suggest that PDI could fit into the shroud of a Delta II rocket. Similar preliminary calculations suggest that the total lifetime cost of such a mission would be under $300M. Detailed studies of this concept are in process. ©2004 Copyright SPIE - The International Society for Optical Engineering.
• Woolf, N., & Angel, J. R. (1998). Astronomical searches for Earth-like planets and signs of life. Annual Review of Astronomy and Astrophysics, 36(1), 507-537.
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  Abstract: If Earth-like planets orbit nearby stars, they could be detectable with specially designed telescopes. Direct observations would be very revealing, particularly low resolution infrared spectra, which could establish habitability on the basis of temperature and atmospheric water. Abundant, primitive life based on organized molecular structure might reveal itself, as on Earth, by an atmospheric composition modified in ways unlikely to be from inorganic processes. The technical challenge is to detect and obtain spectra of an object with Mbol∼28 that is very close to a star and some 5 × 109 times less luminous. Indirect methods, used to detect Jupiter-mass planets, do not seem to offer an easy intermediate step to finding Earth-like planets. However, the direct detection techniques needed for spectroscopy also offer a viable method for discovery by imaging. Thermal infrared wavelengths, in which a planet emits most energy, are the most favorable. A robust search for planets of ∼100 nearby solar-type stars, with spectroscopic follow-up of Earth-like candidates, could be made with an interferometer ∼75 m in length. In visible light, the Next Generation Space Telescope (NGST) could, with the addition of a high resolution correction instrument, see Earth-like planets around a dozen or so of the nearest stars. Both infrared and optical instruments are possible within the range of current space agency plans.
• Angel, J. R., & Woolf, N. J. (1997). An imaging nulling interferometer to study extrasolar planets. Astrophysical Journal Letters, 475(1 PART I), 373-379.
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  Abstract: Interferometric techniques offer two advantages for the detection and analysis of thermal radiation from planets: destructive interference to strongly suppress the stellar emission, and the possibility of high-resolution imaging to resolve planets and distinguish them from dust emission. This paper presents a new interferometric configuration in which the conflicting requirements for these goals are reconciled. It realizes a very strong, broad interference null, so high-resolution fringes can be used while maintaining good suppression of the stellar disk. Complex phase measurement is precluded by the need for destructive interference, but we find that a cross-correlation technique analogous to aperture synthesis can recover true images. When operated 5 AU from the Sun to escape background emission from local zodiacal dust, the interferometer's sensitivity will be limited fundamentally by noise in the photon flux from warm zodiacal dust in the planetary system under observation. In order to scale the interferometer for adequate sensitivity, the 10 μm emission from such dust could be determined early on by a ground-based interferometer. If stars at 10 pc distance have zodiacal clouds like our own, a 50 m long space interferometer with four 1 m elements should see individual planets like the Earth in images taken over 10 hours. Simultaneous infrared spectra of planets like Earth, Venus, Jupiter, and Saturn could be obtained during a 3 month integration, with the sensitivity to detect carbon dioxide, water, and ozone at the levels seen in Earth's spectrum. © 1997. The American Astronomical Society. All rights reserved.
• Angel, J. R., Burge, J. H., & Woolf, N. J. (1997). Detection and spectroscopy of exo-planets like Earth. Proceedings of SPIE - The International Society for Optical Engineering, 2871, 516-519.
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  Abstract: Planets with mass similar to Jupiter's are now known to orbit nearby stars. Are there also planets like Earth? If so, their thermal emission should be directly detectable, and thermal spectra could identify the strong features of carbon dioxide, water and ozone at the levels seen in Earth. But the very close angular separation (approximately 0.1 arcsec) and huge brightness difference (approximately 107) between a star and such a planet present a technical challenge. Space interferometry could in principle solve both problems, by using destructive interference to cancel out the stellar emission, and aperture synthesis to recover high angular resolution images. We show how these two functions conflict, and point to a new interferometer design which allows them to be reconciled. One key technical challenge is to combine beams with strictly controlled amplitude and achromatic phase inversions, so as to cancel the stellar disc flux by a factor of a million. We show how refractive elements analogous to an achromatic lens can be used for this purpose.
• Bruns, D. G., Barrett, T. K., Brinkley, T. J., Goyena, D., Sandler, D. G., Martin, H. M., Brusa, G., & Angel, J. R. (1997). Final prototype design for the adaptive secondary mirror of the 6.5-m MMT. Proceedings of SPIE - The International Society for Optical Engineering, 3126, 164-172.
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  Abstract: The upgraded 6.5 m MMT in AZ will use an adaptive secondary to optimize performance in the near infrared spectral region. The secondary mirror is a 2 mm thick, 640 mm diameter Zerodur shell suspended only by a flexible center hub. Three hundred voice coil actuators installed in an aluminum reference surface deform the shell according to commands from a wavefront sensor. Capacitor position sensors surrounding each actuator provide feedback in an inner servo loop, much faster than the exterior wavefront sensor control bandwidth. A 60 actuator prototype, nearly identical to the final adaptive secondary size, has been built and is currently being tested.
• Bruns, D. G., Barrett, T. K., Sandler, D. G., Brusa, G., Martin, H. M., & Angel, J. R. (1997). MMT adaptive secondary mirror prototype performance. Proceedings of SPIE - The International Society for Optical Engineering, 2871, 890-896.
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  Abstract: The new 6.5 m single mirror multiple mirror telescope (MMT) will be equipped with adaptive optics capabilities to enhance high resolution infrared astronomy. Before we build the 64 cm diameter adaptive secondary, we fabricated a smaller prototype mirror. The adaptive secondary uses voice coil force actuators with an average spacing of 30 mm. Surrounding each actuator is an analog capacitor position sensor operating in a digital closed loop at 10 kHz. This allows the force actuators to be controlled as if they were position actuators. The adaptive secondary configuration and performance test results are presented, followed by the changes to be incorporated into the next curved shell prototype.
• Jian, G. e., Angel, R., Sandler, D., Shelton, C., McCarthy, D., & Burge, J. (1997). Adaptive optics spectroscopy: Preliminary theoretical results. Proceedings of SPIE - The International Society for Optical Engineering, 3126, 343-354.
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  Abstract: Preliminary theoretical results of adaptive optics (AO) spectroscopy were presented. Simulations for infrared spectroscopy at the MMT 6.5 m with laser guide star AO were performed which provides almost full sky coverage. The results showed that about 40-60% of the photons from a unresolved source can be expected within 0.2 arcsec diameter circle for J, H, K, L and M bands under typical atmospheric seeing condition. Natural guide star AO spectroscopy was also studied.
• Lloyd-Hart, M., Angel, J. R., Sandler, D. G., Groesbeck, T. D., Martinez, T., & Jacobsen, B. P. (1997). Design of the 6.5-m MMT Adaptive Optics System and results from its prototype system FASTTRAC II. Proceedings of SPIE - The International Society for Optical Engineering, 2871, 880-889.
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  Abstract: The first images of astronomical objects have been obtained with a telescope exploiting wavefront compensation with adaptive optics where the reference beacon was generated by laser excitation of mesospheric sodium. This was done using the FASTTRAC II low-order adaptive optics system at the multiple mirror telescope (MMT). FASTTRAC II is a prototype for a full-scale adaptive optics system under construction for the 6.5 m telescope that will replace the MMT in late 1997. The 6.5 m system is designed to provide correction to the diffraction limit of resolution in the near infrared (1 - 5 micrometer) with high Strehl ratio and excellent sky coverage. This paper describes the new system and its expected performance in view of the achieved performance of FASTTRAC II.
• Lloyd-Hart, M., Angel, R., Groesbeck, T., McGuire, P., Sandler, D., McCarthy, D., Martinez, T., Jacobsen, B., Roberts, T., Hinz, P., Ge, J., McLeod, B., Brusa, G., Hege, K., & Hooper, E. (1997). Final review of adaptive optics results from the pre-conversion MMT. Proceedings of SPIE - The International Society for Optical Engineering, 3126, 44-54.
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  Abstract: The FASTTRAC II adaptive optics instrument has been used at the Multiple Mirror Telescope (MMT) for the past 2 years to provide improved image resolution in the near infrared. Results have been obtained using both natural guide stars and an artificial sodium laser beacon. With the imminent closure of the MMT prior to its conversion to a single-mirror 6.5 m telescope, FASTTRAC II has come to the end of its life. The instrument has been to the telescope for a total of 8 runs, and during that time it has been of enormous value both as a learning aid, demonstrating the requirements of its successor on the 6.5 m, and as a scientific tool. At this meeting, we present a selection of astrophysical data derived from FASTTRAC II, including the first closed-loop demonstration of an adaptive optics system using a sodium laser beacon. The sodium laser has been used to obtain near diffraction-limited near-infrared images of the core of M13, allowing the construction of a color-magnitude diagram to below the main sequence turnoff. Results have also been obtained from several gravitationally lensed quasars, and the cores of nearby galaxies in the local group. We also summarize work characterizing atmospheric conditions at the site. These studies have proceeded in two areas - understanding the behavior of the phase perturbation with field angle and time, and characterizing the return from the sodium resonance beacon.
• Miller, S., Angel, R., Martin, B., Kapp, J., Ketelsen, D., & Dettmann, L. (1997). Fabrication of ultra thin mirrors for adaptive and space optics. Proceedings of SPIE - The International Society for Optical Engineering, 3126, 391-396.
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  Abstract: We describe the optical fabrication of thin glass shells which will be combined with rigid active supports for adaptive secondary mirrors and for space optics. These applications require glass shells about 2 mm thick, with diameters up to about 1 m for adaptive optics and possibly 6 m for space optics. The extreme flexibility presents unique fabrication challenges which are overcome by a simple adaptation of traditional glassworking techniques. Here we describe the fabrication of concave spherical shells 20 cm and 55 cm diameter. A method of handling and supporting the thin substrates for loose abrasive grinding and polishing is demonstrated and some variations on this approach are compared. Extension of the technique to aspheric adaptive secondary mirrors and to ultra-light mirrors up to 6 meters in diameter is discussed. The subsequent integration and optical testing of a 55 cm shell with a 36 point active support is reported.
• Sandler, D. G., & Angel, J. R. (1997). Direct imaging of extra-solar planets from the ground using adaptive optics. Proceedings of SPIE - The International Society for Optical Engineering, 2871, 842-849.
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  Abstract: The recent discoveries of Jupiter-mass planets around nearby stars by measurement of stellar reaction motion (Mayor et al., 1995; Marcy and Butler, 1996) may be viewed as the beginning of a new era for ground-based astronomy. The next step is to obtain direct images of giant planets around nearby stars. In this paper, we show that this goal can be met by using adaptive optics (AO) on the new large telescopes with very smooth primary mirrors. Detailed simulations of an advanced AO system show that a Jupiter twin at 10 pc can be detected at 5 standard deviations above the residual halo noise in a single night of observation. With Gatewood's recent discovery (Gatewood, 1996) of a Jupiter mass planet at 2.5 AU orbiting Lalande 21185, there is now a perfect target for the first application of the new technique. This and other nearby stars will be imaged in a survey planned for the new single-mirror 6.5 m MMT and its twin Magellan telescope in Chile.
• Shelton, C., Lloyd-Hart, M., Angel, R., & Sandler, D. (1997). The 6.5 m MMT laser-guided adaptive optics system: Overview and progress report II. Proceedings of SPIE - The International Society for Optical Engineering, 3126, 2-7.
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  Abstract: We report on the development and testing of the adaptive optics system for the 6.5 m single-mirror telescope that replaces the Multiple Mirror Telescope (MMT). This system features an adaptive secondary mirror and other innovations intended to give diffraction-limited infrared imaging with full sky coverage and the ultimate in low optical losses and thermal emissivity. In this paper, we first describe the system from a global perspective and then dwell particularly on the optical features of the system, the science package with integral tip/tilt guiding, and the solid-state sodium guide star laser. Other papers in this conference describe in more detail the innovative 300-actuator adaptive secondary, the control processor architecture and performance, and results from Fasttrac II, the immediate precursor to this system.
• Angel, R., Martin, B., Sandler, D. G., Wolf, N., Bely, P. Y., Benvenuti, P., Fosbury, R., Laurance, R. J., Crocker, J. H., & Giacconi, R. (1996). Next generation space telescope: a monolithic mirror candidate. Proceedings of SPIE - The International Society for Optical Engineering, 2807, 354-356.
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  Abstract: We present a concept for a Next Generation Space Telescope with a monolithic 8 × 4 meter primary, optimized for the near infrared region (2 - 5 microns). The observatory is radiatively cooled to about 35 K and would be launched on an Ariane 5 to the Lagrange Point L2.
• Angel, R., & Burrows, A. (1995). Seeking planets around nearby stars. Nature, 374(6524), 678-679.
• Gray, P. M., Lloyd-Hart, M., Angel, J. P., McCarthy Jr., D. W., Sandler, D. G., Martinez, T., Close, L. M., Brusa, G., Bruns, D. G., McLeod, B. A., Ryan, P. T., Groesbeck, T. D., Wittman, D. M., & Jacobsen, B. P. (1995). FASTTRAC II near-IR adaptive optics system for the Multiple Mirror Telescope: description and preliminary results. Proceedings of SPIE - The International Society for Optical Engineering, 2534, 2-16.
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  Abstract: A new adaptive optics system has been constructed for moderately high resolution in the near infrared at the Multiple Mirror Telescope (MMT). The system, called FASTTRAC II, has been designed to combine the highest throughput with the lowest possible background emission by making the adaptive optical element be an existing and necessary part of the telescope, and by eliminating all warm surfaces between the telescope and the science camera's dewar. At present, only natural guide stars are supported, but by the end of 1995, we will add the capability to use a single sodium resonance beacon derived from a laser beam projected nearly coaxially with the telescope. In this paper, we present a description of FASTTRAC II, and show results from its first test run at the telescope in April 1995.
• Hege, E., Angel, J. P., Cheselka, M., & Lloyd-Hart, M. (1995). Simulation of aperture synthesis with the Large Binocular Telescope. Proceedings of SPIE - The International Society for Optical Engineering, 2566, 144-155.
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  Abstract: The large binocular telescope (LBT) will have two 8.4 m apertures spaced 14.4 m from center to center. Adaptive optics will be used to recover deep, long exposure diffraction-limited images in the infrared. The LBT configuration has a diffraction-limited resolution equivalent to a 22.8 m telescope along the center-to-center baseline. Using simulated LBT images and an iterative blind deconvolution algorithm (IBD - Jefferies and Christou, 1993) a sequence of three exposures, at sufficiently different parallactic angles, allows recovery of imagery nearly equivalent to that of the circumscribing 22.8 m circular aperture. To establish a credibility basis for these simulations we have studied the performance of IBD for image constructions of several examples of atmospherically perturbed and partially corrected stellar and galactic data. IBD is robust against influences of real, non-ideal data obtained from large astronomical telescopes, including partial anisoplanicity and Poisson noise from object, sky, and thermal background. For faint objects, which are sky-background and photon-statistics limited, the use of adaptive optics is presumed in these simulations. IBD removes the dilute aperture point spread function effects in the set of parallactic angle-diverse images linearly combined to produce the circumscribed aperture result. Optimal image combination strategy is considered for multi-aperture imaging array configurations.
• Lloyd-Hart, M., Angel, J. R., Jacobsen, B., Wittman, D., Dekany, R., Mccarthy, D., Kibblewhite, E., Wild, W., Carter, B., & Beletic, J. (1995). Adaptive optics experiments using sodium laser guide stars. Astrophysical Journal Letters, 439(1), 455-473.
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  Abstract: The Multiple Mirror Telescope (MMT) has been used in experiments with sodium laser guide stars to sense and correct atmospheric image distortion. The major goal was to develop and test concepts for a full adaptative optics system, to be used in 2 yr when the present array of six 1.8 m telescopes will be replaced with a single 6.5 m mirror The guide star, produced by a continuous-wave dye laser beam projected out along the optical axis of the telescope, was as bright in the V band as a natural star of mv = 10.4. Our tests culminated in the first demonstration of a sodium laser guide star used to improve the image of an astronomical telescope in this case formed by two of the six 1.8 m apertures. Two adaptive servo loops were closed simultaneously The laser beacon provided a measure of the differential wave-front tilt between the two apertures, and a natural guide star was used to measure the overall wave-front tilt. A factor of 2 improvement in the K-band Strehl ratio was measured, and the resolution improved from 0″.58 to 0″.41. The experiment demonstrated all the features needed for correction of the 6.5 m telescope to the diffraction limit using a sodium The accuracy with which the laser beacon measures the atmospheric aberration of starlight across the full 6.9 m aperture of the MMT was examined. This was done with the artificial beacon and a coaxial natural star and using the six elements of the MMT as a large Shack-Hartmann wave-front sensor to measure the shape of both wave fronts simultaneously. The small difference between the wave fronts, caused by focus anisoplanatism was analyzed in terms of Zernike coefficients and was found to correspond to a Strehl ratio of 77% in the K band over the full aperture sampled, despite poor seeing during this measurement. From more extensive measurements of binary star wave fronts, we deduce that focus anisoplanatism for the 6.5 m telescope will correspond to a Strehl ratio of typically 88% at K under normal seeing conditions. In a laser-based adaptive system, a natural guide star is still required to sense overall wave-front slope. Our measurements of binary stars also yielded the image degradation to be expected from differences in the overall slopes between the wave fronts from the object of scientific interest and the natural guide star. A Strehl ratio of 80% at K was deduced for an offset of 40″, implying that good sky coverage will be possible. In general, our results are consistent with calculations based on measurements of atmospheric turbulence at the best sites. Our direct measurements over such a large aperture show clearly the effects of a finite and variable outer scale of turbulence.
• Martinez, T., & Angel, J. P. (1995). Astigmatic-waist TEMoo-output dye laser design. Proceedings of SPIE - The International Society for Optical Engineering, 2537, 316-320.
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  Abstract: An interesting optical design problem is to design a laser cavity which will couple an astigmatic waist to a TEMoo output. The astigmatic waist is used to reduce the thermal effects introduced in a dye laser when pumped at high power levels. The use of components which are inexpensive and readily available was one of the requirements in the design. We looked at the problem from an aberration stand point. We present two designs for a dye laser cavity which couples an astigmatic waist to a TEMoo output. Each design uses a tilted spherical mirror to introduce the astigmatism needed to produce the astigmatic waist. A second mirror tilted in an orthogonal direction is used to remove the astigmatism. The tilted mirrors also introduce coma into the system. One design uses symmetry to remove the coma. The second design uses an asymmetric design which removes the coma and requires fewer elements.
• Ryan, P. T., & Angel, J. P. (1995). Single-exposure wide-wavelength-coverage echelle spectrograph. Proceedings of SPIE - The International Society for Optical Engineering, 2537, 129-138.
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  Abstract: We present the design for an echelle spectrograph for 6.5 m telescopes: i.e. the converted MMT and Magellan. We incorporate fused silica prism cross dispersion for higher transmission, and use a single camera that records simultaneously full 300 - 1100 nm wavelength range of CCD detectors. The broad wavelength coverage in a single exposure will make this a powerful instrument for many scientific problems: i.e. the study of the metallicity, evolution and clustering of the primordial Lyman alpha forest, tracing the nucleosynthetic history and element depletion pattern in the ISM of high redshift galaxies, determining if dark-matter halos are ubiquitous, and studying metal abundance analyses of stars. Six detectors are placed tangent to the focal sphere of a Schmidt camera. Each has its own thin field flattener. In this way excellent image quality is maintained while the obscuration by the detector array is minimal, even though it is big enough to cover the entire cross dispersed spectrum. Each back-illuminated CCD is dedicated to a limited spectral range, and can be given appropriately optimized coatings for highest efficiency. Designs for 20 and 30 cm beams have been produced, both using 60 cm Schmidt plates and achieve resolving power slit width products of 50,000 arc seconds. The 2048 square CCDs yield spectral resolution sampling of 200,000/pixel. The `point and shoot' character of the spectrograph should result in simplified controls and software.
• Sandler, D. G., Lloyd-Hart, M., Martinez, T., Gray, P. M., Angel, J. P., Barrett, T. K., Bruns, D. G., & Stahl, S. M. (1995). 6.5-m MMT infrared adaptive optics system: detailed design and progress report. Proceedings of SPIE - The International Society for Optical Engineering, 2534, 372-377.
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  Abstract: We present an overview of the new adaptive system under development for the conversion of the Multiple Mirror Telescope (MMT) to a 6.5 m continuous primary mirror. The system is optimized for diffraction-limited imaging from 1.6 to 2.2 micrometer wavelength, using an adaptive secondary mirror which directly feeds an infrared science detector at f/15 Cassegrain focus. Nearly full sky coverage will be obtained using a low-power, continuous wave (cw) sodium laser beacon to sense high-order wavefront errors, with image motion sensing using a quadrant detector sensitive to infrared field star photons in the 1.2 - 1.6 micrometer band. Components are currently under development, so that the adaptive instrument can be integrated with the new 6.5 m telescope soon after first light.
• Sandler, D. G., Stahl, S. M., & Angel, J. P. (1995). Adaptive optics system for direct imaging of extra-solar planets from the ground. Proceedings of SPIE - The International Society for Optical Engineering, 2534, 378-385.
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  Abstract: Direct detection of planets of neighboring stars by imaging from the ground is extremely challenging, and requires bringing together and extending all that has been learned in the past two decades in adaptive optics experiments and development. From a distance of 30 light years, the planet Jupiter would be 10 9 times dimmer than the sun, and at 0.5 arcsec separation would be lost in the strong glare of scattered light from the central star. In this paper, we lay out the requirements for adaptive optics to allow direct detection with a large telescope. The stellar halo must be suppressed by several orders of magnitude, and speckle noise caused by correlated wavefront errors must be severely reduced to allow efficient smoothing of the halo through averaging of random fluctuations caused by photon noise. For a 6.5 m telescope imaging near 1 micrometer wavelength, suppression of the stellar halo to 10 -6 of the peak intensity allows direct detection of Jupiter-like planets in several hours of integration. A deformable mirror with approximately 10,000 correction elements is needed, updated at 0.5 millisec intervals using a wavefront sensor optimized for use with bright stellar sources. Local filtering of wavefront sensor data is required to overcome correlated errors arising from time delay between sensing and imaging. Correction of the strongest amplitude errors caused by scintillation allows the required integration times to be decreased by a factor of 2. We present results of detailed simulations for an adaptive system which achieves the above goals, for imaging at 1 micrometer wavelength with a 6.5 meter telescope. A simulated image of a solar system twin at 8 parsecs shows Jupiter at the 5 σ level for a 5 hour integration. We plan to develop and use a similar system to conduct a two- hemisphere survey of bright nearby stars on the twin 6.5 m MMT and Magellan telescopes.
• Smith, P. S., Schmidt, G. D., Allen, R. G., & Angel, J. R. (1995). The polarization and ultraviolet spectrum of Markarian 231. Astrophysical Journal Letters, 444(1), 146-156.
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  Abstract: Ultraviolet spectropolarimetry acquired with the Hubble Space Telescope of the peculiar Seyfert galaxy Mrk 231 is combined with new high-quality ground-based measurements to provide the first, nearly complete, record of its linear polarization from 1575 to 7900 Å. The accompanying ultraviolet spectrum portrays the heavily extinguished emission-line spectrum of the active nucleus plus the emergence of a blue continuum shortward of ∼2400 Å. In addition, absorption features due to He I λ3188, Mg I λ12853, Mg II λ12798, and especially several resonance multiplets of Fe n are identified with a well-known optical absorption system blueshifted ∼4600 km s-1 with respect to the emission lines. The continuum is attributed to ∼ 105 hot, young stars surrounding the nucleus. This component dilutes the polarized nuclear light, implying that the intrinsic polarization of the active galactic nucleus (AGN) spectrum approaches 20% at 2800 A. The rapid decline in degree of polarization toward longer wavelengths is best explained by the strongly frequency-dependent scattering cross section of dust grains coupled with modest starlight dilution. Peculiar S-shaped inflections in both the degree and position angle of polarization through Hα and other major emission lines are interpreted as effects of scattering from two regions offset in velocity by several hundred km s-1. A third source of (weakly) polarized flux is required to explain a nearly 40° rotation in position angle between 3200 and 1800 Å. The displaced absorption features, polarimetry, and optical/infrared properties of Mrk 231 all point to its classification as a low-ionization, or "Mg H" broad absorption line quasar, in which most, if not all, lines of sight to the active nucleus are heavily obscured by dust and low-ionization gas clouds.
• Angel, J. P. (1994). Wavefront reconstruction by machine learning using the delta rule. Proceedings of SPIE - The International Society for Optical Engineering, 2201, 629-635.
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  Abstract: In this paper we use phase screen models to illustrate the power of the delta rule, by obtaining the optimum reconstructor for a Shack-Hartmann sensor with just 6 subapertures in the form of pie segments. The dependence of the matrix elements and residual error on measurement noise is determined, and the accuracy compared with theoretical limits. Reconstructors for more complex problems involving time dependence and multiple laser spots are ideal applications for the method.
• Angel, J. R. (1994). Ground-based imaging of extrasolar planets using adaptive optics. Nature, 368(6468), 203-207.
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  Abstract: The detection of extrasolar planets by direct imaging presents an extraordinary technical challenge. They must be identified against background light scattered from a star close by and about a billion times brighter. It has been supposed that a near-perfect space telescope would be required to avoid atmospheric blurring. But by using adaptive optics operating at fundamental performance limits, the new generation of large ground-based telescopes has the potential to detect planets orbiting nearby stars.
• Dekany, R. G., Cheselka, M., Hege, E. K., Angel, J. P., & Beletic, J. W. (1994). Atmospheric limitations on speckle astrometry with large telescopes. Proceedings of SPIE - The International Society for Optical Engineering, 2200, 422-432.
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  Abstract: Traditional differential astrometric techniques are limited in precision by the atmosphere in a way that does not show much improvement with increased telescope aperture. However, greatly improved astrometric precision may be obtainable by exploiting the strong aperture dependence of the spatial correlation between simultaneously recorded specklegrams within the speckle isoplanatic angle. The cross-correlation of two speckle images of a binary star pair may yield higher astrometric precision in the measurement of the binary separation than centroid differences. The degree of this improvement, however, depends strongly upon the effective thickness of the turbulence in the atmosphere. A 5- minute observation using a large-format, rapid-readout CCD at a 2.3-m telescope has demonstrated 1-milliarcsec precision in the determination of the separation of a 7.3 arcsec binary star pair when processed with speckle techniques.
• Dekany, R., Angel, R., Hege, K., & Wittman, D. (1994). Searching for planets by differential astrometry with large telescopes. Astrophysics and Space Science, 212(1-2), 299-319.
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  Abstract: Traditional astrometric methods are limited in accuracy by the atmosphere in a way that does not show much improvement with increased telescope aperture. However, there is the potential for very high accuracy with large telescopes if advantage can be taken of these factors: First, the differential atmospheric distortion of images of closely adjacent stars is less with larger aperture; second, the diffraction limit is sharper, and third, photon statistics are improved. In this paper we analyze and give experimental tests of techniques that could be applied to the detection of planets with the mass of Jupiter or Uranus, if they are present in nearby binary star systems. The atmospheric perturbation of the relative position of the energy centroids measured in short exposure images of binary stars depends on the effective height of the turbulent distortion. For a 4-meter telescope, the error in centroid determination of a 4-arcsec binary can be as small as 20 milliarcsec (mas) in a single 20-millisecond (msec) exposure. The relative position measured by cross-correlation of short exposure speckle images, as suggested by McAlister (1977b), may give even higher accuracy. In this case, Roddier (Roddier et al., 1980) has shown that the atmospheric error depends on the thickness rather than the height of the layers that make the dominant contribution to the turbulence. Through Monte Carlo analysis we show that on occasions when the turbulence arises largely in a thin layer, a single 20-msec exposure of a 4-arcsec binary taken with a 4-m aperture can yield an astrometric accuracy of order 0.5 mas. We report on experiments made at the Steward Observatory 2.3-m telescope which achieved accuracies corresponding to 1.7 mas in a 2.24-arcsec binary and 16.1 mas in a 6.0-arcsec binary with only 15 and 18 specklegram pairs respectively. We plan to use the 6.5-m converted MMT to obtain much higher performance, between 4.0 mas and 0.40 mas per independent specklegram pair, depending upon atmospheric conditions, for binaries of 4-arcsec separation. By cycling rapidly through perhaps 100 binaries, thus calibrating systematic errors through the average change in binary separation, Jupiter-mass planets may be detectable with small but regular access to the telescope. © 1994 Kluwer Academic Publishers.
• Gleckler, A. D., Angel, J. P., McCarthy Jr., D. W., Ulich, B. L., Pflibsen, K. P., & Wizinowich, P. L. (1994). First light on an edge-matched segmented adaptive mirror at the McMath Telescope. Proceedings of SPIE - The International Society for Optical Engineering, 2201, 394-406.
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  Abstract: The first astronomical results using a seven segment edge-matched adaptive mirror are presented. The mirror is a prototype for adaptive secondary mirrors, consisting of seven flat hexagonal mirrors, with inductive edge sensors to measure piston errors, and piezoelectric actuators. A Hartmann- Shack wavefront sensor uses the visible light for centroiding information while the IR image is formed on a 58 × 62 InSb array at 2.2 microns. In addition, a number of mathematical models are presented that illustrate how well the system should perform when correcting atmospheric turbulence.
• Jacobsen, B. P., Martinez, T., Angel, J. P., Lloyd-Hart, M., Benda, S., Middleton, D., Friedman, H. W., & Erbert, G. (1994). Field evaluation of two new continuous-wave dye laser systems optimized for sodium beacon excitation. Proceedings of SPIE - The International Society for Optical Engineering, 2201, 342-351.
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  Abstract: Adaptive correction of infrared images formed by large telescopes will be possible with the aid of a single laser guide star projected along the telescope axis. The fundamental limit set by focus anisoplanatism of a sodium beacon allows correction to the diffraction limit in the 1.65 and 2.2 micron bands under typical conditions at a good site. A 1 arcsec `star' of V magnitude ≈ 9 is then required to reduce photon noise to negligible level. In this paper we report on recent tests of continuous wave dye lasers at wavelength 589 nm, used to create sodium beacons above the Multiple Mirror Telescope. A coherent ring dye laser operating at 2.5 W average power and projected as circularly polarized light yielded a beacon of mv = 9.85, and a Livermore-designed standing wave laser operating at 1.7 W and projected as linearly polarized light yielded mv = 10.4. Beacon sizes close to 1 arcsec were achieved. The 9th magnitude goal should be achieved by using more advanced cw dye lasers with 20% conversion efficiency of a 25 W argon ion pump laser, and from improvements in beam projection efficiency. We describe a refractive beam expander under construction for the 6.5 m conversion of the MMT.
• Lioyd-Hart, M., DeKany, R., Sandler, D., Wittman, D., Angel, R., & McCarthy, D. (1994). Progress in diffraction-limited imaging at the Multiple mirror telescope with adaptive optics. Journal of the Optical Society of America A: Optics and Image Science, and Vision, 11(2), 846-857.
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  Abstract: One of the main goals of adaptive-optics program at the Multiple Mirror Telescope (MMT) is the development of ways to reconstruct phase errors across large scales, taking advantage of 6.9m baseline of the telescope. Application to adative-optics program in the visible and the near infrared to large filled-aperture telescopes such as the 6.5m MMT upgrade is indicated.
• Lloyd-Hart, M., Angel, J. P., Jacobsen, B. P., Wittman, D. M., McCarthy Jr., D. W., Kibblewhite, E. J., Carter, B., & Wild, W. J. (1994). Preliminary closed-loop results from an adaptive optics system using a sodium resonance guide star. Proceedings of SPIE - The International Society for Optical Engineering, 2201, 364-372.
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  Abstract: A sodium guide star has been used to sense and correct atmospheric aberration during two runs at the Multiple Mirror Telescope (MMT). For the first run in 1993 May, the artificial star was created by a 0.5 W beam from a continuous- wave dye laser tuned to the D2 resonance line, projected from a telescope centered and coaxial with the main array of six 1.8 m mirrors. Scattering by the mesospheric sodium layer produced an artificial beacon equivalent in brightness to a natural star of visual magnitude 12.5, and of angular extent 1'.2 full width at half maximum (FWHM). During the second run in 1994 February, a 1.7 W dye laser was used to generate an artificial guide star of visual magnitude 10.4, and 1'.1 FWHM. In each case, the beacon was used by the MMT adaptive optics system to compensate in real time for atmospherically- induced differential image motion between the six mirror elements, at correction rates of up to 76 Hz. In the latter experiment, global wavefront tilt correction using a natural reference star was added, giving complete adaptive control. Simultaneously recorded images of a natural star coincident with the laser beacon show significantly reduced width and an increase in Strehl ratio of almost a factor of two.
• Lloyd-Hart, M., Jacobsen, B. P., Angel, J. P., Dekany, R. G., Carter, B., Wild, W. J., Kibblewhite, E. J., & Beletic, J. W. (1994). Measurement of focus and off-axis anisoplanatism using a sodium resonance beacon and binary stars. Proceedings of SPIE - The International Society for Optical Engineering, 2201, 284-294.
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  Abstract: We have obtained the first measurements with a sodium laser beacon of focus anisoplanatism over large aperture, at the Multiple Mirror Telescope. In complementary studies, the atmospheric turbulence at the high altitude and on the large scale responsible for the measured focus anisoplanatism was explored by observations of binary stars of different separations. We confirm the predictions of Kolmogorov theory, and derive an effective height for the turbulence of 5050 m above the telescope. These results confirm that the sodium laser guide star planned for use with the 6.5 m telescope conversion of the MMT in 1996 will allow diffraction limited infrared observations in the H and K bands.
• Sandler, D. G., Stahl, S. M., Angel, J. P., & Lloyd-Hart, M. (1994). Adaptive optics for the 6.5-m single mirror conversion of the Multiple Mirror Telescope. Proceedings of SPIE - The International Society for Optical Engineering, 2201, 407-413.
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  Abstract: For the past three years, Steward Observatory's adaptive optics program has been directed toward developing and testing techniques required to correct large telescopes to the diffraction limit. Previously, we have reported on a specific design for an 8 m sodium-laser adaptive system capable of achieving a strong diffraction-limited core at imaging wavelengths of 1.6 - 2.2 microns, with very good sky coverage for image motion sensing using infrared field stars. Recent experiments at the 6.9 m Multiple Mirror Telescope (MMT) have provided the first large aperture measurements of off-axis anisoplanatism for natural stars and focus anisoplanatism for an artificial sodium beacon. This paper summarizes predicted performance for adaptive optics planned for the upgraded MMT, after conversion in 1996 to a single-mirror telescope using the 6.5 m primary mirror developed at Steward's Mirror Laboratory. Performance estimates are obtained using the real MMT atmospheric data and lead to component specifications which are realizable in the near term, permitting the system to be developed and integrated for operation soon after the telescope upgrade is completed.
• Sandler, D. G., Stahl, S., Angel, J. R., Lloyd-Hart, M., & McCarthy, D. (1994). Adaptive optics for diffraction-limited infrared imaging with 8-m telescopes. Journal of the Optical Society of America A: Optics and Image Science, and Vision, 11(2), 925-945.
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  Abstract: It is shown that adaptive optics is likely to have a profound effect on the use of an 8-m class telescopes in the infrared. The combination of huge light grasp and sharp images provided by a large telescope working at its diffraction limit is a big advantage. The isoplanatic angle, being telescope size independent is highly resolved by big telescopes.
• Allen, R. G., Smith, P. S., Angel, J. R., Miller, B. W., Anderson, S. F., & Maroon, B. (1993). Ultraviolet polarimetry and spectroscopy of the BL lacertae object PKS 2155-304. Astrophysical Journal Letters, 403(2), 610-620.
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  Abstract: We present ultraviolet spectropolarimetry of the BL Lac object PKS 2155-304. These observations were made using the Hubble Space Telescope and the Faint Object Spectrograph and represent the first opportunity to study the wavelength dependence of the UV (1360-3300 Å) linear polarization in a BL Lac object. Nearly simultaneous ground-based optical polarization and flux measurements were also taken of PKS 2155-304. During these observations the object was faint (V = 13.7-13.8) and lowly polarized (P = 2%-4%). The UV/optical polarization increased smoothly by about a factor 2 from 8000 Å to ∼ 1400 Å. This wavelength dependence in the polarization is stronger than typically observed for this object and other BL Lac objects; however, the plane of polarization was constant with wavelength. The UV polarimetry yields strong evidence that the UV polarized flux is produced by the same synchrotron mechanism that is responsible for the optical polarization, and that both the UV and optical emission are produced in the same region of the source. There is also no evidence from these measurements to support models of PKS 2155-304 that have an accretion disk or other thermal source contributing significantly to its UV continuum. We also present an ultraviolet spectrum from 1150 Å to 1600 Å of PKS 2155-304. Absorption lines from the Galactic halo are present, but we also find several prominent absorption lines which are not obviously associated with the halo or with the object, thought to be at z ∼ 0.1. The absorptions are consistent with strong (up to 0.8 Å rest equivalent width) Lyα forest lines from intervening clouds, similar to those recently found in HST spectra of 3C 273. Thus our observations both confirm and exacerbate the surprise that Lyα absorption systems are present at low redshifts.
• Lloyd-Hart, M., Dekany, R., Mcleod, B., Wittman, D., Colucci, D., Mccarthy, D., & Angel, R. (1993). Direct 75 milliarcsecond images from the Multiple Mirror Telescope with adaptive optics. Astrophysical Journal Letters, 402(2 PART 2), L81-L84.
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  Abstract: We report results from an adaptive optics system designed to provide imaging at the diffraction limit of resolution in the near-infrared at the Multiple Mirror Telescope (MMT). For the present experiment, the aperture consisted of five of the six primary mirrors of the MMT, operating as a coherently phased array. The largest components of the atmospherically induced wave-front aberration are the fluctuations in mean phase between the segments. These errors were derived in real time from the Fourier transform of short-exposure stellar images at 2.2 μm and corrected at an image of the telescope pupil with piston motion from a segmented adaptive mirror. At a correction rate of 43 Hz, this level of adaptive control resulted in an integrated image with a clear diffraction-limited component of 0″.075 FWHM. This stabilized component is present directly in the light arriving at the detector and is not the result of postprocessing. We discuss future improvements to our adaptive wave-front control and its application to astronomical observations.
• Lloyd-Hart, M., Wlzinowich, P., Mcleod, B., Wittman, D., Colucci, D., Dekany, R., Mccarthy, D., Angel, J. R., & Sandler, D. (1993). First results of an on-line adaptive optics system with atmospheric wavefront sensing by an artificial neural network. Astrophysical Journal Letters, 390(1 PART 2), L41-L44.
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  Abstract: We report the first results from an adaptive optics system operating on-line at the telescope with the wavefront aberration sensed by a trained artificial neural network. Star images were formed at 2.2 μn wavelength by two coherently phased apertures of the Multiple Mirror Telescope (MMT), and analyzed by the neural net. The net derives wavefront parameters in a few milliseconds, and the system performance is fast enough that the aberration is nearly frozen during the time needed to make a correction. With the servo loop in operation, the corrected image shows significant power at the diffraction limit of 0″.1. We discuss some areas of scientific interest which will benefit from the application of adaptive wavefront correction.
• Smith, P. S., Allen, R. G., & Angel, J. R. (1993). First ultraviolet spectropolarimetry of radio-selected BL Lacertae objects. Astrophysical Journal Letters, 415(2 PART 2), L83-L86.
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  Abstract: We present ultraviolet spectropolarimetry of the BL Lac objects OJ 287 and 0754+100 (OI 090.4) acquired with the Faint Object Spectrograph of the Hubble Space Telescope. These are the first such observations of radio-selected BL Lac objects and the polarimetry spans the wavelength range 1580-3300 Å in the observer's frame. Neither object exhibited emission-line features in their UV spectra. OJ 287 was very faint during the UV observations (V ∼ 16). The UV linear polarization of this object was ∼20%. There is no indication that the degree of polarization or the polarization position angle varies with wavelength. Nearly simultaneous optical ground-based measurements show that the polarization does not significantly change with wavelength out to ∼8000 Å. The UV and optical polarization of 0754+100 was ∼8% and also wavelength-independent during the epoch of observation. These observations confirm that the synchrotron emission that dominates the optical continuum also dominates in the ultraviolet in these two BL Lac objects. There is no evidence for significant contributions to the UV/optical flux by other emission components, and we set limits on the brightness of nonsynchrotron continuum components.
• Colucci, D., Lloyd-Hart, M., Wizinowich, P. L., & Angel, J. R. (1992). Atmospheric modeling with the intent of training a neural net wavefront sensor. Proceedings of SPIE - The International Society for Optical Engineering, 1688, 527-535.
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  Abstract: At Steward Observatory we are developing an adaptive optics program for the Multiple Mirror Telescope (MMT) initially based in the near infrared. Using a neural network to recognize the wavefront aberrations in real time from a pair of in and out of focus images has proven itself a promising new method of wavefront sensing especially with the revolution of low noise, fast read out IR detectors. It takes a neural net on the order of 10,000 training image pairs to learn to recognize wavefront aberrations of a new, previously unseen image. Training begins with aberrated images created by the adaptive instrument itself, but since correction is over a region of approximately 2ro (Fried's parameter), the high spatial frequency components of real atmospheric turbulence are absent in these training images. We use computer simulated image pairs generated by atmospheric models based on Kolmogorov turbulence theory to further train the neural nets for the real conditions encountered when observing. Recently we have expanded our atmospheric modeling to include the stratification of turbulent layers. Using knife-edge and phase structure function measurements, we have begun to model temporal characteristics caused by atmospheric winds. The motivation for this modeling is to eventually train nets to separate the various turbulent layers allowing for multi-conjugate wavefront correction, a method which greatly extends the isoplanatic patch. Presented here are descriptions of our modeling techniques as well as results of our modeling including comparisons between stratified and single layer models.
• Lloyd-Hart, M., Wizinowich, P. L., Wittman, D., Colucci, D., McLeod, B. A., Dekany, R. G., Angel, J. R., McCarthy Jr., D. W., Rieke, M. J., & McCaughrean, M. J. (1992). High-resolution imaging at the Multiple Mirror Telescope using adaptive optics. Proceedings of SPIE - The International Society for Optical Engineering, 1688, 442-452.
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  Abstract: The next generation of 6 to 10 m class telescopes is being planned to include the capability for adaptive wavefront correction. The Multiple Mirror Telescope (MMT) with its 7 m baseline, provides an ideal testbed for novel techniques of adaptive optics. Using a new instrument based on a six segment adaptive mirror, a number of wavefront sensing algorithms including an artificial neural network have been implemented to demonstrate the high resolution imaging capability of the telescope. These algorithms rely on a freely available property of starlight, namely its coherence over large scales, to sense directly atmospheric and instrumental phase errors across large distances. In this paper, we report results obtained so far with resolutions between 0.08 and 0.3 arcseconds at 2.2 μm wavelength. We also show data indicating that at the level of 0.1 arcsec imaging in good seeing, the isoplanatic patch at this wavelength is at least 20 arcsec across.
• Martin, B., Hill, J. M., & Angel, R. (1991). The new ground-based optical telescopes. Physics Today, 44(3), 22-30.
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  Abstract: The next round of advances in ground-based optical and infrared astronomy can only be made with larger telescopes or arrays of telescopes. New mirror designs make possible telescopes much larger than any in existence.
• Angel, J. R. (1990). Filled aperture telescopes in the next millennium. Proceedings of SPIE - The International Society for Optical Engineering, 1236 pt 1, 204-205.
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  Abstract: An outline is given for the design of a 32 m diameter ground based telescope with an adaptive primary.
• Angel, J. R., Davison, W. B., Hill, J. M., Mannery, E. J., & Martin, H. M. (1990). Progress toward making lightweight 8 m mirrors of short focal length. Proceedings of SPIE - The International Society for Optical Engineering, 1236 pt 2, 636-640.
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  Abstract: The ideal 8 m primary mirror for astronomical telescopes should be rigid, lightweight, thermally responsive and have short focal length. To satisfy this need, a technique for casting glass in the form of honeycomb structure has been developed at the Steward Observatory Mirror Lab. This form is preferred for stiffness and light weight, and also lends itself to thermal control, by forced ventilation of the honeycomb cells. The fabrication method has been proven by the successful casting of three blanks of intermediate size, 3.5 m diameter. The furnace is now being increased in size to accomodate 8 m castings. It is expected that the first large blank, of diameter 6.5 meters, will be cast in February, 1991, and the first 8 m blank a year later. No mirror as large as 8 m has ever been polished. The challenge is not only the very large size, but also the higher accuracy levels and shorter focal ratios now demanded for telescope optics. A new technique, stressed lap polishing, has been developed to meet this challenge. The 8 m mirrors will be polished in a newly built extension to the Mirror Laboratory that will house an existing 8 m vertical lathe for numerically controlled diamond grinding, and a new 8 m polishing machine to be tailored for the stressed lap method.
• Angel, J. R., Wizinowich, P., Lloyd-Hart, M., & Sandler, D. (1990). Adaptive optics for array telescopes using neural-network techniques. Nature, 348(6298), 221-224.
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  Abstract: IMAGES formed by ground-based telescopes are marred by atmospheric 'seeing'. The plane wavefront from an unresolved star is distorted by continually changing turbulent fluctuations in the air's refractive index. Diffraction-limited performance can in principle be recovered through the methods of adaptive optics, in which the instantaneous wavefront shape is sensed and corrected in real-time by deformable optics that cancel the distortion1,2. The highest resolution will be achieved when this technique is applied to multiple-telescope arrays. For such arrays, the biggest errors caused by seeing at infrared wavelengths are the variations in pathlength and wavefront tilt between array elements. We show here that these errors can be derived by an artificial neural network, given only a pair of simultaneous in-focus and out-of-focus images of a reference star formed at the combined focus of all the array elements. We have optimized a neural network appropriate for 2.2-μm wavelength imaging at the Multiple Mirror Telescope in Arizona. Corrections made by moving the beam-combining mirrors will largely recover the diffraction-limited profile, with a resolution of 0.06 arcsec.
• Martin, H. M., Anderson, D. S., Angel, J. R., Nagel, R. H., West, S. C., & Young, R. S. (1990). Progress in the stressed-lap polishing of a 1.8-m f/1 mirror. Proceedings of SPIE - The International Society for Optical Engineering, 1236 pt 2, 682-690.
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  Abstract: We are in the process of polishing a 1.8-m f/1 ellipsoid with an actively stressed lap. As a preliminary exercise, we have polished the mirror as a sphere using a rigid subdiameter lap. The overall surface error was 25 nm rms, and the surface met a specification corresponding to 1/8-arcsec image quality. A stressed lap 600 mm in diameter was designed and built to polish the mirror as an f/1 ellipsoid. It consists of an aluminum disk which changes shape continuously under the influence of 12 moment-generating actuators. These actuators are programmed to produce the shape changes necessary to make the lap fit the mirror surface as it moves across that surface and rotates. In this paper we describe the principles and design of the lap, test results, and progress to date in polishing the 1.8-m mirror.
• Angel, J. R. (1986). OPTICIANS AS ASTRONAUTS.. Proceedings of SPIE - The International Society for Optical Engineering, 571, 40-41.
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  Abstract: One of the most useful tasks to be carried out at the space station will be the making of large precision telescopes. It will become possible to assemble optics bigger than can be launched in one piece. A further step would be to take advantage of extraordinarily favorable conditions in space for testing and even manufacturing optics. In this short paper we will consider these two aspects.
• Angel, J. R., Cheng, A. Y., & Woolf, N. J. (1986). A space telescope for infrared spectroscopy of Earth-like planets. Nature, 322(6077), 341-343.
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  Abstract: Owen1 has reviewed the potential for detecting life on Earth-like planets of nearby stars, from atmospheric spectra. The presence of oxygen, as revealed, for example, by the 7,600-Å absorption band, would be of particular significance. But even the direct detection of a Jupiter-like planet around the nearest star is a formidable task, perhaps just possible with the Hubble Space Telescope2,3 or with a Michelson interferometer operating at 40-μ wavelength4. Earth-like planets, being fainter and closer in, are still more difficult. Here we show that a space telescope of 16 m diameter, apodized in a new way, could image and measure oxygen in the thermal infrared spectra of earthlike planets up to 4 pc away. Several interesting candidate stars lie within this distance. © 1986 Nature Publishing Group.
• Angel, J. R., Cheng, A. Y., & Woolf, N. J. (1986). STEPS TOWARD 8m HONEYCOMB MIRRORS. VI. THERMAL CONTROL.. Proceedings of SPIE - The International Society for Optical Engineering, 571, 123-130.
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  Abstract: Under the best atmospheric conditions mountain based telescopes have the potential to realize images with 0. 25 arc second resolution. However, close thermal control of the observatory is needed to prevent local seeing from spoiling this quality. In particular it is very desirable to control the primary mirror surface temperature by removing heat from behind. In this paper we obtain expressions for the temperature of simple solid and honeycomb mirrors under a realistic observatory thermal environment and with different rates of heat removal. Heat transfer within a honeycomb structure by ventilation with air at ambient temperature is characterized by an efficiency and the mass transfer rate. Experimental values for efficiency are presented for a simple flow geometry. It is shown that readily achievable flow rates can produce the required thermal equilibrium of an 8m honeycomb mirror.
• Angel, R., Cheng, A., & Woolf, N. (1986). Detecting Earth-like planets. Nature, 324(6097), 518-.
• Cheng, A. Y., & Angel, J. R. (1986). STEPS TOWARD 8M HONEYCOMB MIRRORS VIII: DESIGN AND DEMONSTRATION OF A SYSTEM OF THERMAL CONTROL.. Proceedings of SPIE - The International Society for Optical Engineering, 628, 536-544.
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  Abstract: If they are to achieve the best possible images, honeycomb borosilicate telescope mirrors must follow ambient temperature changes while maintaining low internal temperature gradients. This is best done by cooling or heating the internal, edge and back mirror surfaces at the same rate established by convection on the front surface. We propose to use directed jets of air at ambient temperature, arranged with more air flow on thicker sections so as to match cooling rates. A full scale glass thermal model of one honeycomb cell of an 8m mirror was built, along with a system to flow in air at controlled temperature. We find that the air jets realize high thermal coupling efficiency, and allow good control of internal gradients. With the air cooling steadily at 0. 25 degree C/hour, typical of nighttime cooling at good sites, internal gradients were less than 0. 1 degree C and the overall lag between air and glass temperature was 0. 25 degree C. This performance, achieved with a flow rate of 6-10 litres/sec per cell, will ensure negligible image degradation from convection at the mirror surface (mirror seeing) or from thermal distortion of the mirror substrate.
• Goble, L., Angel, J. R., & Hill, J. M. (1986). STEPS TOWARD 8m HONEYCOMB MIRRORS. VII. SPIN CASTING AN EXPERIMENTAL f/1 1. 8m HONEYCOMB BLANK OF BOROSILICATE GLASS.. Proceedings of SPIE - The International Society for Optical Engineering, 571, 92-100.
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  Abstract: In March 1985 a 1. 8m diameter, f/1 focal ratio, lightweighted honeycomb sandwich mirror blank was cast at the Steward Observatory Mirror Laboratory. The front surface curvature was formed by spinning molten borosilicate glass at 15. 6 rpm. The purpose of the casting was to test the method to be used to make 8m honeycomb mirrors. The blank will be used to test a polishing method for f/1 paraboloids.
• Lesser, M. P., Leach, R. W., & Angel, J. R. (1986). THINNING AND MOUNTING A TEXAS INSTRUMENTS 3-PHASE CCD.. Proceedings of SPIE - The International Society for Optical Engineering, 627(pt 2), 517-523.
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  Abstract: By having the ability to thin CCDs with precise control of thickness and surface quality astronomers will be able to optimize chips for specific applications. Traditional chemical etching is used to reduce subsurface damage of the silicon crystal lattice. We demonstrate a means of mechanically thinning a TI 800 multiplied by 800 CCD with an abrasive slurry of aluminum oxide. Using the same techniques we can replace the abrasives with a chemical solution to eliminate subsurface damage. We also demonstrate a technique of mounting the CCD which retains the high quality surface generated during thinning. This requires the backside of the chip to be bonded to a glass window which closely matches silicon's thermal expansion properties. Thinned CCDs require backside treatment to enhance blue and UV quantum efficiency. Two methods are discussed which may be effective with this mounting system. Due to silicon's high index of refraction in the visible spectral region up to 50 percent of the radiation incident onto a CCD can be lost to reflection. We briefly discuss the improvements to be gained from AR coating the backside.
• Melugin, R. K., Miller, J. H., Angel, J. R., Wangsness, P. A., Parks, R. E., & Ketelsen, D. A. (1986). DEVELOPMENT OF LIGHTWEIGHT, GLASS MIRROR SEGMENTS FOR THE LARGE DEPLOYABLE REFLECTOR.. Proceedings of SPIE - The International Society for Optical Engineering, 571, 101-114.
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  Abstract: Accomplishments in the development of lightweight, honeycomb-core, sandwich mirror blanks made of borosilicate and high-silica glasses at the University of Arizona for the Large Deployable Reflector program are described. In this paper, work spanning the last 2 years is reported, highlighting a new mirror blank fabrication technique that permits the fabrication of the honeycomb core integrally with the front and back plates of the blank in a single furnace cycle. Two types of mirror blanks made by this method, an off-axis, aspheric segment and a smaller Vycor circular piece, are described. The fabrication of two off-axis, aspheric mirror segments is also described. Cryogenic test results are included on the test of a 38-cm diameter, lightweight, honey-comb core, sandwich mirror made of Pyrex.
• Wangsness, P. A., Angel, J. R., & Cannon, J. (1986). ULTRALIGHTWEIGHT MIRROR SUBSTRATES OF VYCOR GLASS.. Proceedings of SPIE - The International Society for Optical Engineering, 628, 535-.
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  Abstract: This work focused on the fabrication of an ultralightweight honeycomb sandwich mirror substrate 30 cm in diameter, of 96% fused silica glass. The technique used was the gas pressure inflation method developed at Steward Observatory's Mirror Lab at the University of Arizona. The method was first developed with borosilicate glass that fuses at 850 degrees C. The new mirror is of Corning's Vycor glass which has much lower thermal expansion and fuses at 1600 degrees C. In order to use this glass we have had to find new refractory materials compatible with the process. In particular, we have had to control outgassing and chemical interaction between the glass and the mold. The techniques developed should now be applicable to refractory glasses with zero expansion coefficient. The density of the present blank is kg/m**2. Densities as low as 20kg/m**2 should be readily accomplished. This work is supported by NASA under grants NAGW-121 and Ames NAG-161.
• Angel, J. R. (1985). GLASS MIRRORS FOR SPACE TELESCOPES.. Proceedings of SPIE - The International Society for Optical Engineering, 542, 32-34.
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  Abstract: The Hubble Space Telescope uses a primary mirror of area 5 m**2. We consider the mirrors needed for two possible future telescopes, of 50 m**2 and 500 m**2 in area. The former would be an 8 m diameter monolithic mirror of 'zero' expansion glass, and would be diffraction limited at optical wavelengths (0. 015 arcsecond images). The 500 m**2 mirror, the area proposed for the Large Deployable Reflector, would be diffraction limited down to 30 mu wavelength (0. 3 arcsecond images). Both types of mirror could be of honeycomb sandwich construction made by a new air inflation method. The larger reflector would consist of ultra lightweight glass panels.
• Angel, J. R. (1984). STEPS TOWARDS 8M HONEYCOMB MIRRORS: V. A METHOD FOR POLISHING ASPHERES AS FAST AS F/1.. Array, 11-21.
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  Abstract: A method is proposed for polishing fast aspherics with a lap whose shape is continuously changed under computer control as it moves over a rigid mirror blank. The required changes of radius, astigmatism and coma in a circular lap are made with edge bending levers and tensioning members with screw actuators. This method of bending has been demonstrated in the laboratory.
• Angel, J. R., & Hill, J. M. (1984). STEPS TOWARD 8m HONEYCOMB MIRROR BLANKS: III 1. 8m HONEYCOMB SANDWICH BLANKS CAST FROM BOROSILICATE GLASS.. Proceedings of SPIE - The International Society for Optical Engineering, 444, 184-193.
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  Abstract: As part of a program leading to the production of 8m honeycomb mirrors, the authors have recently made two 1. 8m blanks. These have honeycomb sandwich form, with hexagonal honeycomb ribs sandwiched between front and back plates. Each is cast in one piece from borosilicate glass, using techniques that can be extended to larger sizes. Both blanks are of high quality, free from cracks and voids, and with an adequately low bubble content. The second and better blank, made of Ohara's E6 glass, is now to be figured to high precision, 0. 25 arcsecond images, and is to be tested for an extended period in the Multiple Mirror Telescope.
• Angel, J. R., Woolf, N. J., Hill, J. M., & Goble, L. (1984). STEPS TOWARD 8m HONEYCOMB MIRRORS IV: SOME ASPECTS OF DESIGN AND FABRICATION.. Proceedings of SPIE - The International Society for Optical Engineering, 444, 194-199.
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  Abstract: A facility is to be built to make 8m diameter glass honeycomb mirror blanks by casting, in the same way that has been demonstrated with 1. 8m blanks. The only major difference is that the larger furnace will be rotated on a turntable so as to preform the deep parabolic surface needed for f/2 mirrors. This paper explores the tolerances in glass homogeneity, thermal control and support of the blank necesssary to meet the stringent imaging requirements of telescopes in the best ground based sites. In round numbers, homogenity in expansion coefficient of 10** minus **8/ degree C and thermal equilibriation to 0. 1 degree C are required. Laboratory measurements show that both can be met by a ventilated honeycomb of borosilicate or similar glass. Adequate resistance to wind pressure and buffeting can be achieved by an axial support that responds to pressure on the three defining points.
• Barr, L. D., Lynds, C. R., Angel, J. R., Woolf, N. J., Mast, T. S., & Nelson, J. E. (1984). 15-METER NATIONAL NEW TECHNOLOGY TELESCOPE (NNTT): THE TWO DESIGN CONCEPTS.. Proceedings of SPIE - The International Society for Optical Engineering, 444, 37-47.
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  Abstract: A brief background and description is given of the plan to build a 15-meter telescope. The two design concepts currently under consideration are presented and contrasted. A choice will be made in the near future.
• Epps, H. W., Angel, J. R., & Anderson, E. (1984). ADVANCED WIDE-FIELD BROAD-PASSBAND REFRACTING FIELD CORRECTORS FOR LARGE TELESCOPES.. Array, 519-548.
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  Abstract: This paper describes a preliminary 30-arcmin prime focus (f/2. 0) refracting field corrector system for the University of California Ten-Meter Telescope (UC TMT), a compact 40-arcmin internal Cassegrain (f/1. 75 hyperbola to f/5. 0) broad-passband ( lambda 3300Angstrom to lambda 1. 0 mu ) corrector, suitable for imaging and multi-object spectroscopy at the UC TMT, three 60-arcmin external Cassegrain correctors for 300-inch f/1. 8 and f/2. 0 parabolic primary mirrors and a (300-inch) 40-arcmin external Cassegrain (f/1. 0 parabola to f/4. 0) broad-passband ( lambda 3300Angstrom to lambda 1. 0 mu ) corrector with ADC.
• Harms, R., Beaver, E., Burbidge, E., Hier, R., Allen, R., Angel, R., Bartko, F., Bohlin, R., Ford, H., Davidsen, A., Bowers, C., Pembroke, R., & Margon, B. (1984). CALIBRATION AND OPERATION OF THE FAINT OBJECT SPECTROGRAPH (FOS).. Proceedings of SPIE - The International Society for Optical Engineering, 445, 410-426.
• Hill, J., Angel, J. R., & Richardson, E. H. (1984). OPTICAL MATCHING FOR FIBER OPTIC SPECTROSCOPY.. Proceedings of SPIE - The International Society for Optical Engineering, 445, 85-92.
• McGraw, J. T., Stockman, H. S., Angel, J. R., Epps, H., & Williams, J. T. (1984). CCD/TRANSIT INSTRUMENT DEEP PHOTOMETRIC AND POLARIMETRIC SURVEY - A PROGRESS REPORT.. Optical Engineering, 23(2), 210-215.
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  Abstract: The authors have previously reported on the design and predicted performance of a new type of dedicated astronomical telescope to be used for a deep photometric survey for galactic and extragalactic variability and polarization. Data derived from this survey will be useful for astronomical investigations including the definition of a complete sample of quasi-stellar objects (QSOs), based on their variability and nonstellar colors, the detection of supernovae on the rising branch of their light curves, and the determination of the supernova production rate as a function of galaxy color, morphology, and red shift. The telescope to accomplish this survey is a transit instrument. It will incorporate a 1. 8 m primary mirror of high quality, fabricated as part of the program to develop the Space Telescope mirror technology, and, as its detectors, two RCA CCDs (512 multiplied by 320, 30 mu m pixels) used in the time-delay and integration (TDI) mode.
• Woolf, N. J., Angel, J. R., & McCarthy Jr., D. W. (1984). VERSATILE ARRAY.. Proceedings of SPIE - The International Society for Optical Engineering, 444, 78-84.
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  Abstract: The authors discuss a four element non-redundant array telescope-interferometer for ground use. The elements are 8 meter mirrors, and the maximum array spacing and two element spacing are 75m and 108m respectively. The array may be used as three separate telescopes, one 11. 3m and two 8m for work not requiring highest angular resolution. Discussed are the problems of making speckle measures to high enough precision for synthetic images to be produced. The authors conclude by showing that the high resolution presents opportunities to make types of observation that are neither possible with VLBA nor NNTT.
• Woolf, N. J., Angel, J. R., & Williams, J. T. (1984). OPTIONS FOR A NEW ARIZONA OBSERVATORY: CONFIGURATIONS, COSTS AND SITE.. Array, 831-843.
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  Abstract: This paper is concerned with the process of creating a major observatory at minimum cost. Its premises are that cost optimization starts with an appropriately modestly priced building block, a 7. 5m telescope. There are many possible configurations in which this building block can be used, ranging from a single telescope or a set of separate telescopes with at most electronic data addition, to an MMT in which the light of all telescopes can be phased when desired. In addition there are options for using elements in a non-redundant linear array or Michelson inteferometer. We provide the reader with a means of estimating costs of different facilities all on the same basis, and tied to the actual cost of producing the MMT. It is also important to place the observatory on a site which maximizes the output per unit cost of the facility. We discuss the selection process which has led to Mt. Graham in Arizona as being likely to satisfy this criterion. We give brief details of our preliminary knowledge about the site.
• Angel, J. R. (1983). NEW TECHNIQUES FOR FUSION BONDING AND REPLICATION FOR LARGE GLASS REFLECTORS.. Proceedings of SPIE - The International Society for Optical Engineering, 383, 52-56.
• Hill, J. M., & Angel, J. R. (1983). STEPS TOWARD 8m HONEYCOMB MIRROR BLANKS: II. EXPERIMENTS WITH WAFFLEPLATE AND HONEYCOMB CASTING.. Proceedings of SPIE - The International Society for Optical Engineering, 380, 100-110.
• Hill, J. M., Angel, J. R., & Scott, J. S. (1983). FIBER OPTICS FOR ASTRONOMICAL SPECTROSCOPY: THE MEDUSA SPECTROGRAPH.. Proceedings of SPIE - The International Society for Optical Engineering, 380, 354-363.
• Allen, R. G., & Angel, J. R. (1982). PERFORMANCE OF THE SPECTROPOLARIMETER FOR THE SPACE TELESCOPE FAINT OBJECT SPECTROGRAPH.. Proceedings of SPIE - The International Society for Optical Engineering, 331, 259-267.
• Angel, J. R. (1982). Very large ground-based telescopes for optical and IR astronomy. Nature, 295(5851), 651-657.
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  Abstract: Optical and IR astronomers are taking a hard look at their ground-based facilities and devising new ways of making more economic, bigger and better telescopes. Features of instruments of the 15-m class are likely to include servo control to compensate for atmospheric wavefront errors as well as structural deformation, large honeycomb mirror blanks and mirror surfaces produced by economical techniques developed for aspherics. © 1982 Nature Publishing Group.
• Angel, J. R., & Hill, J. M. (1982). MANUFACTURE OF LARGE GLASS HONEYCOMB MIRRORS.. Proceedings of SPIE - The International Society for Optical Engineering, 332, 298-306.
• Angel, J. R., & Parks, R. E. (1982). GENERATION OF OFF-AXIS ASPHERICS.. Proceedings of SPIE - The International Society for Optical Engineering, 332, 316-326.
• Angel, J. R., Woolf, N. J., & Epps, H. W. (1982). GOOD IMAGING WITH VERY FAST PARABOLOIDAL PRIMARIES: AN OPTICAL SOLUTION AND SOME APPLICATIONS.. Proceedings of SPIE - The International Society for Optical Engineering, 332, 134-140.
• Beckers, J. M., Poland, C., Ulich, B. L., Angel, J. R., Low, F. J., & Wyatt, W. (1982). PERFORMANCE OF THE MULTIPLE MIRROR TELESCOPE (MMT) - 6. MMT TELESCOPE COALIGNMENT SYSTEM.. Proceedings of SPIE - The International Society for Optical Engineering, 332, 42-49.
• Harms, R. J., Angel, J. R., Bartko, F., Beaver, E. A., Bohlin, R., Burbidge, E. M., Davidsen, A. F., Ford, H., Margon, B., McCoy, J., & Ripp, L. (1982). FAINT OBJECT SPECTROGRAPH (FOS) CALIBRATION.. Proceedings of SPIE - The International Society for Optical Engineering, 331, 268-278.
• Hill, J. M., Angel, J. R., Scott, J. S., Lindley, D., & Hintzen, P. (1982). MULTIPLE OBJECT FIBER OPTIC SPECTROSCOPY.. Proceedings of SPIE - The International Society for Optical Engineering, 331, 279-288.
• McGraw, J. T., Stockman, H. S., Angel, J. R., Epps, H., & Williams, J. T. (1982). CHARGE-COUPLED DEVICE (CCD)/TRANSIT INSTRUMENT (CTI) DEEP PHOTOMETRIC AND POLARIMETRIC SURVEY - A PROGRESS REPORT.. Proceedings of SPIE - The International Society for Optical Engineering, 331, 137-145.
• Weymann, R., Angel, J. R., Lebofsky, M., & Chaffee, F. (1982). PERFORMANCE OF THE MULTIPLE MIRROR TELESCOPE (MMT) - 9. DOING SCIENCE WITH THE MMT.. Proceedings of SPIE - The International Society for Optical Engineering, 332, 65-71.
• Woolf, N. J., Angel, J. R., Antebi, J., Carleton, N., & Barr, L. (1982). SCALING THE MULTIPLE MIRROR TELESCOPE (MMT) TO 15 METERS - SIMILARITIES AND DIFFERENCES.. Proceedings of SPIE - The International Society for Optical Engineering, 332, 79-88.
• Woolf, N. J., McCarthy, D. W., & Angel, J. R. (1982). PERFORMANCE OF THE MULTIPLE MIRROR TELESCOPE (MMT) - 7. IMAGE SHRINKING IN SUB-ARC SECOND SEEING AT THE MMT AND 2. 3m TELESCOPES.. Proceedings of SPIE - The International Society for Optical Engineering, 332, 50-56.
• Angel, J. R., Cromwell, R. H., & Magner, J. (1980). An Intensified Storage Vidicon Camera for Finding and Guiding at the Telescope. Advances in Electronics and Electron Physics, 52(C), 347-353.
• Cromwell, R. H., & Angel, J. R. (1980). Elimination of Corona and Related Problems with Astronomical Image Tubes. Advances in Electronics and Electron Physics, 52(C), 183-188.
• Hege, E. K., Angel, J. R., Weymann, R. J., & Hubbard, E. N. (1980). Morphology of the triple QSO PG1115 + 08. Nature, 287(5781), 416-417.
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  Abstract: Weymann et al.1 have reported that two faint unresolved objects located within 3arcs of the QSO PG1115 + 08 have also the same spectra. The emission lines of C III λl,909 and C IV λ1,549 at a redshift of 1.72 have central wavelengths and profiles that are indistinguishable between the objects. It is inferred that the three objects are likely to be images of the same QSO produced by intervening matter acting as a gravitational lens. Only one other example of such an effect is known, that is the double QSO 0957 + 56A,B. In this case, the two QSO images are of roughly equal magnitude ( V∼17) and separated by 6 arc s. A plate obtained in excellent seeing by Stockton2shows an intervening galaxy at z = 0.4 lying about 1 arc s from one of the QSO images. The present observations were undertaken to search for evidence of an intervening galaxy in PG1115 + 08, appearing either directly in a deep image, or as a difference in colour between components, and also to look for any structure in the three images. A colour difference could arise, as was the case for the double QSO, if a galaxy is nearly coincident with one of the QSO images. © 1980 Nature Publishing Group.
• McGraw, J. T., Angel, J. R., & Sargent, T. A. (1980). CHARGE-COUPLED DEVICE (CCD) TRANSIT-TELESCOPE SURVEY FOR GALACTIC AND EXTRAGALACTIC VARIABILITY AND POLARIZATION.. Proceedings of the Society of Photo-Optical Instrumentation Engineers, 264, 20-28.
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  Abstract: A new instrument and data handling scheme which will automatically carry out a photometric survey for variable objects to a limiting magnitude of about 20 is introduced. A polarimetric survey for objects which are 1% or more polarized, to a limiting magnitude of about 18 is described. The instrument consists of a dedicated 0. 76m, f/4. 7 Newtonian reflector which is used as a transit telescope, and two area-format CCDs which are placed in the focal plane and clocked at the sidereal rate. An effective integration time on a point on the sky of about one minute is achieved by this technique. The photometric advantages of ″averaging over″ all spatially dependent noise sources when CCDs are used in this way are discussed. A list-oriented real-time data compaction scheme is proposed to allow easy management of the vast amount of data generated by this instrument.
• Angel, J. R. (1979). MAGNET FOR USE WITH A MAGNETIC IMAGE INTENSIFIER AND A FOLDED SCHMIDT SPECTROGRAPH CAMERA.. Proceedings of the Society of Photo-Optical Instrumentation Engineers, 172, 377-379.
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  Abstract: An ideal fast camera for imaging linear spectra is the folded Schmidt design used, for example, in the Wampler-Robinson scanner at Lick Observatory (Epps 1975). This design, which can be as fast as f/1. 0, involves a folding flat directly adjacent to the tube photocathode, and the incoming light would be blocked by the solenoid in any conventional magnet design. More complex solenoid geometries are explored which give the very open access to the cathode while maintaining high field uniformity, and a design is presented which is practical in terms of power dissipation and weight, and is also screened against external transverse fields. The design may also have application in any optical system that requires large elements and clearance near the image tube cathode.
• Harms, R. J., Angel, R., Bartko, F., Beaver, E., Bloomquist, W., Bohlin, R., Burbidge, E. M., Davidsen, A. F., Flemming, J. C., Ford, H., & Margon, B. (1979). FAINT-OBJECT SPECTROGRAPH FOR SPACE TELESCOPE.. Proceedings of the Society of Photo-Optical Instrumentation Engineers, 183, 74-87.
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  Abstract: The Faint Object Spectrograph (FOS) is being designed and built for use with Space Telescope to provide digitized spectra of faint astronomical objects over the wavelength range from 115 to 700nm at resolving powers of 1000 and 100. A variety of concave gratings and prisms is employed to form nearly stigmatic spectra on either of two Digicon photon counting detectors which are optimized for two different but overlapping spectral ranges. The science goals are summarized, and the optical and detector designs and their predicted performance are discussed.
• Liebert, J., Angel, J. R., Hege, E. K., Martin, P. G., & Blair, W. P. (1979). The moving emission features in SS433 require a dynamical interpretation. Nature, 279(5712), 384-387.
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  Abstract: New spectrophotometry of SS433 shows that the variable-wavelength emission features discovered by Margon et al.are due to the simultaneous presence of material having a substantial redshift and a substantial blueshift. A magnetic interpretation for the features is also ruled out by polarimetric measurements. Implications for dynamical models are discussed. © 1979 Nature Publishing Group.
• Martin, P. G., Maza, J., & Angel, J. R. (1977). The polarisation of nova Vulpeculae [2]. Nature, 265(5592), 314-315.
• Worden, S. P., Stein, M. K., Schmidt, G. D., & Angel, J. R. (1977). The angular diameter of vesta from speckle interferometry. Icarus, 32(4), 450-457.
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  Abstract: It is shown that the autocorrelation function of a telescope diffraction-limited image is closely approximated by a subtraction of the mean cross correlation of pairs of speckle photographs from the mean autocorrelation of the same set of data. This fact is used to derive the angular diameter of the asteroid Vesta from a series of speckle interferometry data. The resultant apparent angular diameter of 0″.40 ± 0″.04 corresponds to an absolute diameter of 513 ± 51 km. © 1977.
• Gilbert, G. R., Angel, J. R., & Grandi, S. (1976). A Digital Television System for Astronomy. Advances in Electronics and Electron Physics, 40(PART B), 699-710.
• Stockman Jr., H. S., Angel, J. R., Woodgate, B. E., & Nidey, R. A. (1975). A large-area lithium-flouride Bragg spectrometer for stellar X-ray astronomy. Nuclear Instruments and Methods, 126(2), 217-220.
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  Abstract: A large-area Bragg spectrometer recently used to search for the Fe XXV X-ray emission lines in Sco X-1 is described. The instrument employs 3400 cm2 of LiF and has the capability of detecting a narrow line flux from Sco X-1 of approximately 10-2 photons (cm2 s)-1 at 6.7 keV. © 1975.
• Angel, J. R., Illing, R., & Martin, P. G. (1972). Physical sciences: Circular polarization of twilight. Nature, 238(5364), 389-390.
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  Abstract: LINEAR polarization of clear daytime sky and twilight is produced by molecular (Rayleigh) scattering and aerosol (Mie) scattering, which predominates in the infrared. Incident linearly polarized light scattered by aerosols is in general elliptically polarized, whereas no ellipticity can arise from the molecular component1. The principal component of clear daytime sky arises from single scattering of unpolarized sunlight and is not elliptically polarized. However, whenever multiple scattering by the aerosol component is important, the strong linear polarization produced on the first scattering will lead to elliptical polarization from the second and subsequent scatterings. Although in the daytime sky this mechanism should give only a small ellipticity2 (10-5-10-3) the possibility of a substantially larger effect at twilight arises because there is no direct illumination. We report here observations of circular polarization of twilight of order 10-3. © 1972 Nature Publishing Group.
• Landstreet, J. D., & Angel, J. R. (1971). Search for optical circular polarization in the crab nebula. Nature, 230(5289), 103-.
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  Abstract: IT has recently been suggested by Gunn and Ostriker1 and by Rees (paper presented at the IAU symposium 46 on the Crab Nebula, Manchester, August 1970) that the magnetic field in the Crab Nebula which causes relativistic particles to emit synchrotron radiation may be an oscillating 30 Hz electromagnetic field from the pulsar rather than a static field. Rees has shown that if this is the case, the nebula should show a component of circular polarization of order a few per cent in visible light. This is predicted to have opposite sense in the NW and SE regions of the nebula, supposing that the spin axis of the pulsar is in the direction of linear polarization. © 1971 Nature Publishing Group.
• Angel, J. R. (1969). X-ray line emission from sco X-1. Nature, 224(5215), 160-161.
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  Abstract: THE emission of line radiation from hot optically thin plasmas is discussed by Tucker and Gould1, who find that line emission should be significant for plasma temperatures of 5 × 107 K. Calculations of line strengths were carried out by Tucker for three models of Sco X-1, assuming different compositions. As an example, assuming cosmic abundance, the predicted energy in line radiation from Fe+24 and Fe+25 is equal to 1/100 of the total radiated energy between 2 and 8 (ref. 2). In a high density plasma, however, the predictions must be modified to take account of electron scattering. © 1969 Nature Publishing Group.
• Angel, J. R., Novick, R., Bout, P. V., & Wolff, R. (1969). Search for x-ray polarization in sco X-1. Physical Review Letters, 22(16), 861-865.
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  Abstract: An x-ray polarimeter sensitive to x rays in the energy range from 6 to 18 keV, flown above the atmosphere on 27 July 1968, was used both to set an upper limit on the polarization of Sco X-1 and to check for spurious indications of polarization which might result from the anisotropy of the cosmic rays. Within the statistical limitations of the data, no evidence was found for spurious background polarization. © 1969 The American Physical Society.
• Angel, J. R., Sandars, P. G., & Tinker, M. H. (1967). Observation of a v × E effect in an electric dipole moment experiment using a reversible atomic beam machine. Physics Letters A, 25(2), 160-161.
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  Abstract: We have used a reversible atomic beam machine to eliminate the v × E effect and set a new limit de≤1× 10-22 e·cm on the electric dipole moment of the electron. © 1967.
• Angel, J. R., Sandars, P. G., & Woodgate, G. K. (1967). Direct measurement of an atomic quadrupole moment. The Journal of Chemical Physics, 47(4), 1552-1553.

Proceedings Publications

• Angel, R., Didato, N., Eads, R., & Kim, D. (2024). Design, performance, and field operation of twisting heliostats for 3,000-sun concentration and high temperature. In Nonimaging Optics: Efficient Design for Illumination and Concentration XIX 2024, 13132.
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  The idea is to combine the solar energy reflected from a field of many heliostats to obtain a single focus of high concentration and high power. We exploit a new type of “twisting” heliostat” in which, as the mount is moved to track the sun through the day, the reflector shape is twisted to maintain a focused image of the sun’s disc on the target over a wide range of angles of incidence. By combining the light reflected by many twisting heliostats into a single focus, we are not only able to accomplish but also maintain very high concentration and temperature through the day, with higher efficiency than has previously been possible with conventional heliostats having a fixed shape. A circular field of twisting heliostats is used to power a single intense focus atop a central tower. The light from all the heliostats is relayed to an upward facing receiver at the focus via a central Cassegrain secondary reflector located above the receiver. In a specific design targeting 1 MW of power, a 100 m diameter field contains 431 twisting heliostats, each with a 7 m2 reflector. The secondary reflector, 7.2 m in diameter, is located 24 m above the heliostat field, bringing sunlight with annual average power of 1 MW to a circular focus 0.8 m in diameter, at a concentration averaging 3,000 suns.
• Choi, H., Kim, D., Angel, R., Martin, H. M., Kang, H., & Huang, Y. (2024). Versatile Deflectometry Applications. In 2024 Conference on Lasers and Electro-Optics/Pacific Rim, CLEO-PR 2024.
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  Various applications in deflectometry optical testing are presented in this paper. The final accuracy of optics systems is determined by the accuracy of the metrology. While commercially available and standardized testing approaches exist, they may be challenging to adopt in different application environments due to strict optical layouts and limited dynamic range. In response, our group has been developing versatile deflectometry systems capable of addressing diverse optical testing demands. The paper summarizes various deflectometry types tailored to unique missions.
• Choi, S. O., Angel, R., Bender, C., Berkson, J., Bugueno, E., Chavez-Lopez, G., Dibelka, J., Didato, N., Ford, J., Foster, W., Garcia, N., Gilliam, K., Gray, P., Halverson, S., Huang, Y., Ketelsen, D., Kim, D., Monson, A., Oh, C. J., , Patrou, J., et al. (2024). The Large Fiber Array Spectroscopic Telescope: fiber feed fabrication and characterization. In Ground-Based and Airborne Instrumentation for Astronomy X 2024, 13096.
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  The Large Fiber Array Spectroscopic Telescope (LFAST) project seeks to construct large arrays of small, individual fiber-fed telescopes for very high resolution spectroscopy. We are currently developing a prototype of a 20× telescope to investigate the technical requirements for LFAST. For each unit telescope, the 0.76 m primary mirror operates at f/3.5, focusing light onto our fused silica fiber with an 18 µm core, which subtends 1.4” on the sky. This receiving fiber collects and transmits light to the entrance slit of the spectrograph. We are developing a reliable fiber fabrication recipe, including fiber-end termination and polishing, to ensure consistency, efficiency, and affordability in mass manufacturing of the thousands of fibers that the future LFAST arrays require. The 18 µm core size places our optical fiber in the “few-mode” regime, which is not widely used in astronomy. Since the properties of “few-mode” fibers are not yet well characterized, extensive testing is required to gain a comprehensive understanding of their behaviors, such as focal ratio degradation, throughput and modal scrambling. We are designing optical tests to study the optical properties of the LFAST custom fibers. In this paper, we present the fiber feed design and fabrication recipe of our prototype. We also outline our optical test procedures and report results on surface flatness of our fibers.
• Foster, W. B., Angel, R., Bender, C. F., Didato, N., Gilliam, K., Gray, P., Huang, Y., Ketelsen, D., Monson, A. J., Patrou, J., Sisco, M., & Wortley, R. W. (2024). The LFAST 0.76m primary mirrors: mass production, active control and on-sky performance. In Advances in Optical and Mechanical Technologies for Telescopes and Instrumentation VI 2024, 13100.
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  The Large Fiber Array Spectroscopic Telescope (LFAST) pursues large collecting aperture at low cost. Arrays of 0.76m, f/3.5 mirrors will focus light into fibers that are combined at a high-resolution spectrograph. The mirror substrates are fabricated from 25.4mm thick Schott Borofloat® discs in a one week slump and polish process that leaves less than 80nm rms wavefront error in medium and high spatial frequency modes. Low-order figure errors are corrected with a perimeter ring of thermoelectric controllers that induce expansion or contraction with top-to-bottom thermal gradients. In operation, temperature variations from nighttime cooling cause time-varying aberration modes. Using feedback from a stellar wavefront sensor, these aberrations can be compensated to focus starlight energy into a 1.4 arcsec fiber.
• Huang, Y., Angel, R., Kang, H., Choi, H., Rademacher, M., Tailor, H., Su, R., & Kim, D. (2024). Static metrology of the meter-scale deformable heliostat. In Nonimaging Optics: Efficient Design for Illumination and Concentration XIX 2024, 13132.
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  This paper describes a heliostat metrology system which is developed based on deflectometry, utilizing a static perforated panel instead of conventional dynamic monitor displays to provide incident rays. The developed method is named static screen deflectometry (SSD). This robust and scalable method is especially valuable for outdoor tests of large reflectors used in concentrating solar-thermal power (CSP) systems. The developed method has been successfully demonstrated on a 2.4 m × 3.3 m float glass deformable reflector bent to focus sunlight at 113 m distance throughout a day. From images obtained from a camera at 50m distance, the reflector surface was measured to an accuracy of less than 1 mrad rms slope error in the full test scope.
• Mauskopf, P., Mauskopf, P. D., Angel, R., Atwater, H. A., Atwater, H., Bazzani, E., Berggren, K., Berggren, K. K., Blase, P., Corvaja, R., Davoyan, A. R., Davoyan, A., Eubanks, T. M., Eubanks, T., Guglielmi, A., Hadfield, R., Hadfield, R. H., Hart, M., Hein, A., , Hein, A. M., et al. (2024). Technology Development for a Low-Mass Interstellar Communications System. In Free-Space Laser Communications XXXVI 2024, 12877.
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  We describe the requirements and associated technology development plan for the communications data link from low mass interstellar probes. This work is motivated by several proposed deep space and interstellar missions with an emphasis on the Breakthrough Starshot project. The Starshot project is an effort to send the first low mass interstellar probes to nearby star systems and transmit back scientific data acquired during system transit within the time scale of a human lifetime. The about 104 fold increase in distance to nearby stars compared to the outer planets of our solar system requires a new form of propulsion to reach speeds of approximately 20% of the speed of light. The proposed use of a low mass sailcraft places strong constraints on the mass and power for the Starshot communications system. We compare the communications systems in current and upcoming solar system probes, New Horizons and Psyche, against the requirements for Starshot and define Figures of Merit for the communications capability in terms of data downlink rate multiplied by distance squared per unit mass. We describe current and future technology developments required for the on-board transmitter (signal generation, signal distribution, and beamforming) and for the near-Earth communications receiver (low cost large aperture telescopes, high resolution spectrometers, and single photon counting detectors). We also describe a roadmap for technology development to meet the goals for future interstellar communications.
• Monson, A., Berkson, J., Bender, C., Angel, R., Gray, P., Gilliam, K., Schwab, C., & Foster, W. (2024). LFAST- Large Fiber Array Spectroscopic Telescope: Prime Focus Corrector Optics. In Ground-Based and Airborne Telescopes X 2024, 13094.
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  The Large Fiber Array Spectroscopic Telescope (LFAST) is designed to be a cost-efficient way to provide a large collecting area for spectroscopy by duplicating large numbers of small (0.76m f/3.33) spherical mirror 'unit' telescopes. Each telescope is equipped with a prime focus corrector (PFC) feeding a guider and an optical fiber; and all the fiber optics will feed into a large spectrograph. The design of the PFC is driven by the need to minimize costs while achieving acceptable performance. We are currently constructing a prototype 20-unit system for testing in the second half of 2024. For this 20-unit system we have begun coordinating with industry about scaling for mass fabrication. In this paper we will present status updates from the performance of this early version of the prime focus corrector.
• Vagher, A., Angel, R., Didato, N., & Rademacher, M. (2024). Performance of the first mechanically-coupled twisting heliostat: encircled energy vs angle of incidence, and closed loop tracking. In Nonimaging Optics: Efficient Design for Illumination and Concentration XIX 2024, 13132.
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  The first automatic, mechanically-coupled twisting heliostat, with 8 m2 reflector, was completed and tested on-sun in January 2024. It was set up on a target-oriented dual-axis mount, with the target-axis aimed at a target 113 m due west. The shape-twisting is purely automatic, made by a mechanical cam coupling to the cross-axis, which turns as the angle of incidence of the sun’s rays on the reflector change throughout the day. Three unsaturated images were recorded at different times of day of the sun on the target, reflected at angles of incidence of 5°, 49°, and 68°. The measured FWHM of the three images is very similar, up to 1.13 m or 10.0 mrad, compared to the ideal solar disc of 9.3 mrad. The encircled energy measured for all three images showed ≥ 85% within 1.34 m diameter (11.8 mrad). We report here also on a novel, closed-loop tracking camera for the target-axis mount, which employs a semitransparent beamsplitter fixed perpendicular to both the mirror surface and to the plane of incidence. During 2.5 hours of measurement of closed loop tracking, errors were ≤ 0.36 mrad rms in both tracking axes.
• Angel, R., Berkson, J., Didato, N., & Hart, M. (2023). A highly efficient, low-cost hybrid module, built on a full-size bifacial silicon module. In Joint Conference of the 18th International Conference on Concentrator Photovoltaic Systems, CPV 2022 and 13th World Conference on Thermophotovoltaic Generation of Electricity, TPV 2022, 2841.
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  By augmenting silicon PV cells with close to 50% efficient multi-junction cells, there is the potential to approximately double overall conversion efficiency of solar PV modules. In this paper we describe the design of a hybrid module built onto a commercial 1?m x 2?m bifacial PV module. This serves not only to harvest the energy of diffuse light, but also to carry and cool an array of small multijunction PV cells to harvest direct sunlight. The single-junction silicon PV cells of the bifacial are sealed between 2?mm thick glass windows, front and back. In our design, the MJ cells are attached to the bifacial module's front window via 5?mm square copper/ceramic substrates. Attached in front of the module is a glass sheet formed into an array of 34?mm square lenses which focus sunlight to the cells while also transmitting diffuse light to the module's silicon cells. An 8?mm diameter secondary ball lens is bonded in front of each 2?mm square MJ cell, yielding 290 x concentration, maintained at >95% geometric throughput for solar mis-pointing as far as 1° off-axis. To ensure good cooling of the MJ cell, despite its being mounted on glass, the copper wires used to carry electrical power to the sides of the bifacial module are configured to also carry heat away from the cells. The wires have 1?mm square cross section and are bonded along their full length to the glass, with thermally conductive adhesive. Thermal modeling shows that for DNI solar illumination of 1 kW/m2, the temperature drop between the MJ cells and the back convecting surface of the bifacial module is limited to 10°C. The lens array is imprinted into a full-size (1?m x 2?m), 6?mm thick sheet of glass, by a process in which the sheet is first heated to 850°C, then pressed between relatively cold molds. The focal length of these lenses is chosen to be 125?mm, slow enough at f/3.5 to limit total internal reflection losses of the diffuse light to ∼10%. To minimize shipping costs, the glass sheets and modified bifacial modules will be packed and shipped separately, for assembly on-site as complete hybrid modules, using reflective side walls. We envisage that dual axis tracking will be provided by augmentation of single axis E-W tracking now used for standard utility scale PV plants, with the addition of cross-axes to turn the modules also in the N-S direction.
• Angel, R., Eads, R., & Kautz, M. (2023). Accurate Measurement of Heliostat Reflector Shapes, Using Fully-Sampled Starlight Images. In 27th International Conference on Concentrating Solar Power and Chemical Energy Systems: Solar Power and Chemical Energy Systems, SolarPACES 2021, 2815.
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  In the method described here, a bright star is used at night to measure the shape of heliostat reflectors in the field, at different elevation and azimuthal settings. The heliostats are oriented to direct starlight to the heliostat array receiver, and are then fixed in orietation while their aberrated and overlapping star images are translated across the receiver by Earth’s rotation. At the receiver, each of a fixed line of cameras forms an image of the heliostats to be measured. The cameras use large aperture lenses and are closely spaced along a zig-zag line, turned to be perpendicular to the translation direction of the star. Exposures are repeated while the entire image star passes by, so as to fully sample the blurred star images in two dimensions. In this way, every point on the reflector surfaces of the viewed heliostats appears in one or another of the recorded images, as bright from reflected starlight. From this data the amplitude and direction of the surface slope at each of these points is computed from Snell’s law, using the known positions of the star at the time of each exposure, of each camera and of each point on each heliostat. The surface shapes are then obtained by integration of the surface slopes. The total time needed to obtain a complete data set depends on the amount of surface error. For example, if the heliostats have peak-to-valley slope errors (from shape and pointing) of 4 mrad, a complete measurement of many exposures may be obtained in 1 minute, independent of heliostat field size, while the Earth rotates 4 mrad. The method will be particularly valuable for measuring heliostats whose reflector shape is actively altered throughout the day, so as to always form sharp images of the solar disc. But also for segmented reflectors of fixed shape, it will yield the detailed shapes and canting of the facets. The method has the advantage of being a null measurement – if there is no shape error, the star image is a point, and all the surfaces of all the heliostats appear in a single exposure of one camera. The method exploits modern, large CMOS imagers with very low read noise, and very fast (f/1.1), large format lenses to measure many heliostats at once.
• Didato, N., Angel, R., & Rademacher, M. (2023). Shape-adjustable heliostats – designs for individuals and fields for > 3000 concentration. In Advances in Solar Energy: Heliostat Systems Design, Implementation, and Operation 2023, 12671.
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  Shape-adjustable heliostats can maintain a focused image of the solar disc on a receiver target throughout the day, as the angle of incidence (AOI) changes on the reflector. This requires the heliostat reflector to be deformed into a unique biconic concave shape dependent on AOI. The reflector comprises a single rectangular sheet of silvered float glass mounted to a steel frame. Its shape is set initially, by adjusting the height of mounting points, to the biconic shape required for imaging at 60 degrees AOI. Shapes for other AOIs are obtained by twisting the frame from its four corners and center. A finite element model was made of a heliostat designed to form a disc image on a 130 m distant receiver using a single sheet glass reflector, 1/8” thick x 130” x 96”, supported by 58 points on a rectangular tube frame structure weighing 120 kg. Analysis shows an overall RMS slope error 85% are achievable. Models of fields of closely packed heliostats of this type, on target axis mounts, demonstrate a geometrical throughput into the receiver of >73% of the total mirror area, after accounting for blocking, shadowing, and cosine loss. In one model, with 450 heliostats powering five compound parabolic concentrators at a receiver, a concentration of > 3,000 was obtained at powers > 1 MW, through much of the day.
• Douglas, E. S., Aldering, G., Allan, G. W., Anche, R., Angel, R., Ard, C. C., Chakrabarti, S., Close, L. M., Derby, K., Edelstein, J., Ford, J., Gersh-Range, J., Haffert, S. Y., Ingraham, P. J., Kang, H., Kelly, D. M., Kim, D., Lesser, M., Leisenring, J. M., , Lin, Y. C., et al. (2023). Approaches to lowering the cost of large space telescopes. In Astronomical Optics: Design, Manufacture, and Test of Space and Ground Systems IV 2023, 12677.
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  New development approaches, including launch vehicles and advances in sensors, computing, and software, have lowered the cost of entry into space, and have enabled a revolution in low-cost, high-risk Small Satellite (SmallSat) missions. To bring about a similar transformation in larger space telescopes, it is necessary to reconsider the full paradigm of space observatories. Here we will review the history of space telescope development and cost drivers, and describe an example conceptual design for a low cost 6.5 m optical telescope to enable new science when operated in space at room temperature. It uses a monolithic primary mirror of borosilicate glass, drawing on lessons and tools from decades of experience with ground-based observatories and instruments, as well as flagship space missions. It takes advantage, as do large launch vehicles, of increased computing power and space-worthy commercial electronics in low-cost active predictive control systems to maintain stability. We will describe an approach that incorporates science and trade study results that address driving requirements such as integration and testing costs, reliability, spacecraft jitter, and wavefront stability in this new risk-tolerant “LargeSat” context.
• Angel, R., Bender, C., Berkson, J., Didato, N., Ford, J., Gray, P., Jannuzi, B., Ketelsen, D., Kim, D., Chavez Lopez, G., Monson, A., Oh, C. J., Patrou, J., Rademacher, M., Schwab, C., Sisco, M., Wortley, R., & Young, A. (2022). LFAST, the Large Fiber Array Spectroscopic Telescope. In Ground-Based and Airborne Telescopes IX 2022, 12182.
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  The LFAST concept is to use thousands of small telescopes combined by fibers for high resolution (R=150,000) spectroscopy, in a way that will realize large cost savings and lead to an affordable aperture as large as 20,000 m2. Such large aperture is needed, for example, to make a comprehensive search for biosignatures in the atmospheres of transiting exoplanets. Each unit telescope of 0.76 m aperture (0.43 m2) will focus the image of a single star onto a small (17 μm core) fiber, subtending 1.32 arcsec. Our telescope design calls for a spherical mirror, with a 4-lens assembly at prime focus that corrects not only for spherical aberration, but also for atmospheric dispersion down to 30° elevation, from 390 nm-1700 nm, and for rapid image motion caused by seeing or wind jitter. A method for rapid production of such mirrors has been tested, in which a disc of borosilicate float glass is slumped over a high-precision polished mandrel to an accuracy that greatly reduces subsequent optical finishing time. A method for active thermal control of mirror figure using Peltier devices will be incorporated. The projected cost of each unit telescope, when mass produced by the thousand, would then be approximately $8,000. The telescopes will be mounted in the open in groups of 20 located 12 m apart. The mirrors will be arrayed on either side of a central, pedestal-mounted alt-az drive using commercial worm gear bearings. Protection against rain and dust will be provided by automated covers above and below the mirrors, and by pointing the mirrors down (-20° elevation). The first LFAST array, some 150 m in diameter, will comprise 132 mounts carrying a total of 2,640 mirrors and having 1,200 m2 in collecting area. The light from all the fibers is combined at the central spectrographs, with little increase in etendue, by a 5 x 528 array of adjacent hexagonal lenses. A telecentric lens is used to reimage the lens array at the entrance slits of two echelle spectrographs. Together, these two cover simultaneously the full 390 nm-1700 nm spectral range of the star being observed. The targeted cost for the installed LFAST telescope and fiber array is $60M.
• Angel, R., Eads, R., Didato, N., Rademacher, M., Emerson, N., & Davila, C. (2022). Actively shaped focusing heliostat. In 26th International Conference on Concentrating Solar Power and Chemical Energy Systems, SolarPACES 2020, 2445.
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  We describe a technology that uses a computer-driven, active servo control to change the shape of a heliostat reflector, in order to keep an image of the solar disc focused on a fixed distant target. The heliostat reflector is made with glass mirrors bent and rigidly attached to a support frame to form an initial specific concave toroidal shape. The different toroidal shapes needed throughout the day to maintain focus, despite the changing angle of the sun, are obtained by bending the frame by means of a truss of stiffening struts behind it. The struts are connected in pairs to a central back structural node, which incorporates linear actuators to change the truss geometry. For a hexagonal reflector, a total of three linear actuators suffice to adjust the amplitudes of the three lowest order orthogonal bending modes of the frame, and thereby to obtain all the different toroidal shapes needed for accurate imaging through the day. A recently constructed 1.6 m2 hexagonal prototype with three actuators has demonstrated this concept by producing sharp solar images throughout the day at a 40?m distant target. The measured flux concentration ranges from 90% to 98% into a square target measuring only 1.44 times the ideal disc diameter. Active heliostats of this type open new possibilities for nighttime solar electricity generation and solar industrial process heat, allowing fields of even relatively small numbers of heliostats to generate higher than current concentrations, as needed for high temperatures and more efficient energy conversion. A design for a scaled up hexagonal heliostat with 48 m2 reflector surface with eight petal mirror segments has been optimized using ANSYS. A 440?m diameter field of 776 of these heliostats would yield 25 MWth at 1,500x concentration by area, averaged over a cylindrical central receiver 2.75?m in height and diameter.
• Angel, R., Eads, R., Hartweg, B., Holman, Z., & Didato, N. (2022). Embossed sheet glass lens arrays for hybrid PV modules. In 17th International Conference on Concentrator Photovoltaic Systems, CPV 2021, 2550.
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  In hybrid PV-CPV modules the glass entrance window of a silicon module is replaced by an array of lenses that concentrate direct sunlight onto very small but twice-as-efficient multijunction PV cells while transmitting indirect daylight to the silicon. Hybrid modules have the potential for conversion efficiciency nearly double that of conventional silicon PV modules, and could become a major source of utility scale solar electricity generation. But for commercial success the added costs of the lens array, small CPV cells and dual rather than single axis tracking must be kept low. Here we describe a method for forming large lens arrays directly from glass as part of large-scale float glass manufacturing. A wide, continuous sheet of newly formed and still soft glass is embossed like shower glass, by passage between two rollers. These imprint cylindrical lenses on the upper and lower surfaces, arrayed at right angles to each other. The lenses act together to form point foci, where the multijunction cells are located on the silicon cells. The cost of lens arrays mass-produced in this way should be little more than that of the flat glass sheets of conventional modules, which in many cases are already roll-formed with textured surfaces to reduce reflection. We describe experiments made with commercially available embossed glass and cylindrical lenses.
• Bender, C. F., Angel, J. R., Berkson, J., Gray, P., Halverson, S., Kang, H., Kim, D., Monson, A., Oh, C. J., Rademacher, M., Schwab, C., Young, A., & Zaritsky, D. (2022). The Large Fiber Array Spectroscopic Telescope: Fiber Feed and Spectrometer Conceptual Design. In Ground-based and Airborne Instrumentation for Astronomy IX 2022, 12184.
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  The Large Fiber Array Spectroscopic Telescope, LFAST, will use optical fibers to combine light from thousands of small telescopes at centrally located high-resolution spectrometers. LFAST aims to use mass replication of small, self-contained telescope systems to provide ELT sized collecting area and spectroscopic capabilities at a drastically reduced price. However, fundamental constraints such as étendue, fiber modal noise, and focal-plane sampling that affect the size and complexity of spectrometers for single telescope ELTs also impact LFAST. We are carrying out a three year study to tackle these challenges. In this paper, we describe the conceptual designs for the fiber feed assemblies that carry light from the individual telescopes to a centralized location, and the high-resolution spectrometer that accepts this light.
• Berkson, J., Angel, R., Bender, C., Young, A., & Gray, P. (2022). The Large Fiber Array Spectroscopic Telescope: optical design of the unit telescope. In Ground-Based and Airborne Telescopes IX 2022, 12182.
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  The concept for the Large Fiber Array Spectroscopic Telescope (LFAST) (Angel et al, these proceedings) is to collect the light from a target object using thousands of individual, small, low-cost telescopes, and bring it via optical fibers to a high resolution (R=150,000) spectrograph. Each mirror has a prime focus corrector feeding a 17 micron fiber at f/3.5, subtending a 1.3 arcsec diameter on the sky. Each LFAST unit has 20 separate 30 inch telescopes carried by a single alt-az mount to provide collecting area equivalent to a 3.5 m traditional aperture. Each mirror has a 4-element corrector provides subarcsecond imaging over an 8 arcmin field. The field is reflected by a mirror puck (which contains the receiving fiber) through relay optics to a CMOS camera for rapid guiding and wavefront measurement. The corrector optical design incorporates elements of common crown and flint glass to obtain achromaticity over a broad wavelength range of 380 nm-1700 nm. Large, slow lateral translations of the final 4th element correlated with primary mirror tilt correct for atmospheric dispersion, and small, rapid lateral translations correct for image motion without significantly disrupting atmospheric dispersion correction. We have explored both aspherical and spherical primary mirror designs and have chosen spherical, based on impacts to unit telescope cost.
• Davila-Peralta, C., Rademacher, M., Emerson, N., Chavez-Lopez, G., Sosa, P., Cabanillas, R., Peon-Anaya, R., Flores-Montijo, N., Didato, N., & Angel, R. (2020). Progress in track-mounted heliostat. In 2019 International Conference on Concentrating Solar Power and Chemical Energy Systems, SolarPACES 2019, 2303.
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  A novel concept is presented for a track-mounted heliostat for central receiver plants, intended to be manufacturable at low cost and to reach an installed cost of ~ $60 /m2 when implemented in high volume. Cost reduction is achieved by using steel in the structurally most efficient form of spaceframe structures. These spread the load broadly, halving the quantity of steel per square meter of reflecting area compared to a conventional post /fishbone heliostat design . The structure is turned in azimuth on wheels on a large diameter ground track, eliminating the need for an expensive, heavy slewing bearing. A preliminary finite-element design of a 22 m2 heliostat with spaceframe azimuth and elevation structures has a total steel mass of mass of 15 kg/m2 supporting 10 kg/m2 of glass reflectors and survives 145 km/h wind. Here we report of field tests of engineering performance of a first prototype of the lower, azimuth spaceframe mounted on a 7 m diameter concrete track. The moving structure with wheels and drive weighs 11 kg/m2 and has a measured lowest resonant frequency of 7.5 Hz, when loaded with a simulated elevation structure. The prototype incorporates inclinometers which measure alignment of the azimuth and elevation axes to better than 0.2 mrad. Absolute azimuth rotation angle measured via an incremental encoder on the drive motor encoder and metal reference bars in the concrete track was found to be accurate to 0.35 mrad. Combining the tip/tilt, azimuth rotation and servo errors in quadrature gives a total of 0.49 mrad in tracker orientation, providing encouragement that our full heliostat system target of 1 mrad RMS pointing accuracy (double the orientation accuracy) should be achievable.
• Gamil, A., Li, P., & Angel, R. (2020). Modeling of a small-scale solar central receiver with pressurized water for heat collection and storage – 612g. In 2020 AIChE Annual Meeting, 2020-.
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  This paper presents a new modeling and algorithm to analyze and optimize a Central Receiver System (CRS) for solar thermal energy collection and storage. The analysis includes associated modeling of the heliostat field and the heat transfer at the receiver to couple the energy in the two subsystems. Transient optical performance of the heliostat field and predicted heat flux at the receiver is provided. The modeling analyzed a case which is a medium temperature solar concentrating system with pressurized water as heat transfer fluid and thermal storage medium. A tubular external solar receiver is considered and the modeling determines the geometry to maintain a best thermal efficiency. The transient optical performance and heat flux mapping are conducted using Monte Carlo Ray Tracing (MCRT) technique. The new receiver modeling and approach of optimization design was verified through analysis of two reference solar thermal power plants with molten salt heat transfer fluid (i.e. SolarTwo and GemaSolar Plants) with a satisfactory agreement to the reported design parameters. The computations and algorithm was implemented in a use-friendly interface via Visual Basic. The modeling and algorithm provides an efficient tool for solar thermal power plant design in industry.
• Eads, R., Hyatt, J., & Angel, R. (2019). Optical design for a Fly's eye CPV system with large, onaxis dish solar concentrator. In 15th International Conference on Concentrator Photovoltaic Systems, CPV 2019, 2149.
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  Here we describe an optical concentrator system for CPV which uses a large dish reflector to concentrate sunlight and a small receiver with fly's eye lens array to apportion the focused sunlight equally between many multi-junction cells. The small receiver is fan cooled. This architecture is aimed at a reduction in cost of complete CPV generating systems, which include dual axis tracking. In many CPV systems the light is evenly divided by identical Fresnel lenses facing the sun, and the cells are typically housed along with the lenses in modules, backed by large, passively cooled aluminum plates. The fly's eye system advantages are that the weight, complexity and cost of the inherently large scale optical collection is reduced to just that of the 4 mm thick glass mirrors, and the small receiver packages are easier and less expensive to manufacture than large modules. A specific fly's eye generator design using two large off-axis primary reflectors and nonobscuring receivers below the collection area has been described recently by Hyatt et al. [1], and the principles have been demonstrated in a prototype. Here we describe an alternative on-axis design optimized for greater uniformity of the light distributed to the cells, and excellent tolerance to mispointing. Ray tracing analysis shows deviations in uniformity of the full spectrum of sunlight distributed to the many cells of no more than 1% for sun mispointing angles of up to 0.5° and 80% of the maximum flux reaches the CPV cells when mispointing to +/- 0.9°. In this design, the central receiver blocks 4% of the entering sunlight, however, mechanical construction is simplified with a corresponding reduction in weight and cost. Also, not all the light blocked by the receiver is wasted, since it will be used to power the receiver's cooling fan via a built-in small PV panel, eliminating parasitic power loss.
• Hyatt, J., Eads, R., Didato, N., & Angel, R. (2019). Laser beam projection for starshot launch. In 6th International Conference on Adaptive Optics for Extremely Large Telescopes, AO4ELT 2019.
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  The Breakthrough Starshot Initiative aims to send a nanocraft to the Proxima Centauri system. Light sails 4 m in diameter will be accelerated by a 100 GW laser beam projected from a segmented aperture of several kilometer diameter. Adaptive optics to maintain a coherent beam require wavefront and phase sensing across the entire aperture. One projection system considered here comprises millions of 1 m2, 20 kW block of lasers each passing through beam expanding and beam directing units with a steering flat to project a 2 m x 2 m square diffraction limited beam. These units are packed close together for ~70% filled aperture. The beam expanders allow for mechanical and servicing access between the laser blocks. The off-axis Mersenne beam expanders both project the outgoing laser beam and receive incoming light from a beacon or the spacecraft for wavefront detection. A dichroic mirror separates the outgoing and incoming light. A wavefront sensor measures the atmospheric aberration across each 2 m aperture, and an adaptive deformable mirror is used to correct the projected beam to the diffraction limit over each 2 m aperture. Our approach is compatible with fixed frequency or tunable lasers. For fixed frequency lasers, a dichroic coating that transmits 99.99% at 1060 nm and reflects 90% of the redshifted return beam from the spacecraft accelerated beyond 70 km/s for P polarization. The 2m square beams will be steered over 2.5° to aim the beam during the 10 minute launch interval. Atmospheric wavefront tilts will be measured and the beam wavefront will be adaptively corrected by modulating laser phases. A preliminary trade study will compare performance and material and mass production costs for these two options.
• Didato, N., Sodari, F., Stalcup, T., Strittmatter, P., & Angel, R. (2018). REhnu dish based CPV: Performance and reliability improvements based on a year of field experience. In 14th International Conference on Concentrator Photovoltaic Systems, CPV 2018, 2012.
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  REhnu has now built up a year of experience operating two M-8 CPV generators with dish-receiver architecture described in Stalcup et al. 2017 [1]. The M-8 generators use 8 primary collector mirrors made of back-silvered low-iron glass, 1.65 m square with a 1.5 m focal length. Small sealed receivers at each dish focus house 36 Solar Junction 3J cells operating at 950× concentration. Over the year of operation, a good measure of output power vs DNI and atmospheric conditions has been obtained. The efficiency varies through the day depending on air mass. The system is currently best matched to the solar spectrum at air mass 2. The daily peak CSTC efficiency averages 31%, with seasonal variation of ±1% through the year, peaking at 32% in August. Several improvements were made over the year to ensure reliability and to improve optical throughput. A dry air purging system has mitigated damage to the cells from moisture. Material upgrades to secondary reflectors in the receiver have resolved a runaway failure mode that went previously undetected due to inadequate testing time.
• Hyatt, J., Davila, C., Didato, N., Peon, R., Rademacher, M., Reshidko, D., Sodari, F., Strittmatter, P., Vincent, G., Wheelwright, B., Zammit, C., & Angel, R. (2018). CPV generator with dish reflector and fly's eye receiver. In 14th International Conference on Concentrator Photovoltaic Systems, CPV 2018, 2012.
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  We describe a CPV generator in which an off-axis paraboloidal dish reflector powers a small receiver near the focus, housing many individually illuminated multijunction cells. The receiver entrance window doubles as a field lens that forms a reduced scale image of the reflector, at concentration of ∼30×. The image has a sharp boundary, and its position is stable against tracking errors. A fly's eye lens array divides the image into equal portions, and further concentrates it to ∼500× onto the cells. This approach is in contrast to nearly all previous PV and CPV, where sunlight is equal apportioned (for simple series electrical connection) directly on entering the system. In our approach, small multijunction cells are packaged into a small receiver module that will be less expensive (per watt) to manufacture than large conventional PV or CPV modules, and can be economically upgraded for 40 year lifetime. Our concept differs from REhnu's dish/receiver design [1] in its lack of obscuration and simpler cooling, using forced air convection rather than pumped liquid coolant, this made possible by the lower heat density at the cell array. In preliminary on-sun system data with a 2.4 m2 prototype powering 5.2 mm cells at 500× concentration, we demonstrate good tolerance to mispointing (90% at 0.5°off-axis), good air cooling (cell mounting plate at 19°C above ambient) and uniform division of light between the cells (scatter of 3.3% rms).
• Kingsley, J. S., Angel, R., Davison, W., Neff, D., Teran, J., Assenmacher, B., Peyton, K., Martin, H. M., Oh, C., Kim, D., Pearce, E., Rascon, M., Connors, T., Alfred, D., Jannuzi, B. T., Zaritsky, D., Christensen, E., Males, J., Hinz, P., , Seaman, R., et al. (2018). An inexpensive turnkey 6.5m observatory with customizing options. In Ground-Based and Airborne Telescopes VII 2018, 10700.
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  A turnkey observatory with 6.5-m telescope has been developed for a broad range of science applications. The observatory includes the telescope, mount and enclosure, installed on site and ready for operation. The telescope's primary mirror is an f/1.25 honeycomb sandwich of borosilicate glass, similar to that of the MMT and Magellan telescopes. The baseline optical design is for a Gregorian Nasmyth focus at f/11. A Gregorian adaptive optics secondary that provides a wide-field focus corrected for ground layer turbulence (0.25 arcsecond images over a 4 arcminute field) as well as a narrow-field diffraction-limited focus is optional. Another option is a corrected f/5 focus with a 1° field. The observatory, built by partners from academia and industry with extensive experience, can be delivered within five years at a fixed price.
• Davila-Peralta, C. .., Hyatt, J., Alfred, D., Struble, M., Sodari, F., & Angel, R. (2017, sep). Dish-based CPV-T for rooftop generation. In American Institute of Physics Conference Series, 1881.
• Stalcup, T., Angel, R., Geary, A., Sodari, F., Rademacher, M., Whiteside, A., Will, J., Didato, N., & Strittmatter, P. (2017, sep). On-grid performance of REhnu's 8-mirror CPV-T tracker. In American Institute of Physics Conference Series, 1881.
• Angel, R., & Hyatt, J. (2016). Track and tilt collection for central receiver CSP. In ASME 2016 10th International Conference on Energy Sustainability, ES 2016, collocated with the ASME 2016 Power Conference and the ASME 2016 14th International Conference on Fuel Cell Science, Engineering and Technology, 1.
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  Here we describe a new type of point-focus solar collector for CSP called "track and tilt". It bridges the gap between dish and heliostat arrays collectors, having the high optical efficiency of a dish but with larger aperture (> 1000 m2) focused to a tower mounted receiver in fixed gravity orientation. It is well matched to the next generation of high efficiency cavity receivers transferring heat to a storage medium at temperatures exceeding 700C. The collector uses silvered glass reflectors mounted on a rotating, rigid structure in the form of a 120 degree conical arc. In operation, this large structure rotates in azimuth on a track around the central receiver tower, keeping the gravity load on the structure constant. The central receiver is rotated about a vertical axis so as to face the reflector arc. The reflectors are concave, all with the same focal length, and are individually tilted to follow solar elevation to focus sunlight onto the towermounted receiver.
• Martin, H. M., Angel, J. R., Angeli, G., Burge, J. H., Gressler, W., Kim, D. W., Kingsley, J. S., Law, K., Liang, M., Neill, D., Sebag, J., Strittmatter, P. A., Tuell, M. T., West, S. C., Woolf, N. J., & Xin, B. (2016, July). Manufacture and final tests of the LSST monolithic primary/tertiary mirror. In Advances in Optical and Mechanical Technologies for Telescopes and Instrumentation II, Proc. SPIE 9912.
• Stalcup, T., Angel, R., Strittmatter, P., Whiteside, A., Geary, A., Sodari, F., Rademacher, M., Didato, N., & Ayala, S. (2016). REhnu dish-based CPV: Module performance and planned 100 kW plant. In 12th International Conference on Concentrator Photovoltaic Systems, CPV 2016, 1766.
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  A REhnu CPV module uses a 2.6 m2, back-silvered glass reflector to focus sunlight into a 150 mm diameter receiver housing 36 multijunction cells. Current modules use commercially available 8.8 mm cells operated at 950x concentration with a cell efficiency of 41% for an AM1.5 solar spectrum. Optics in the receiver format the sunlight to illuminate the cells which are mounted slightly apart on four flat circuit boards. Active cooling is provided by liquid circulated to a radiator which can easily be configured to also provide thermal energy in the form of hot fluid at an adjustable temperature up to 80°C. Modules mounted in pairs on a dual axis tracker have been tested in the field. Module conversion efficiency, corrected to 25°C cell temperature (CSTC), is found to peak at 31.2% for air mass 2.75. The I-V curve shows that the concentrated sunlight is distributed between the cells with a uniformity of ±7%. Steps are now being taken to improve uniformity, to reduce infrared losses caused by iron absorption in the reflector glass and by the receiver's antireflection coatings, and to upgrade to 42% efficient cells. Overall efficiency is projected to then increase to 35%. In hybrid mode (electrical + thermal) the total efficiency approaches 80%. REhnu's basic generator unit to be replicated for large scale installations has eight modules in a 2 x 4 array on a dual axis tracker. The first of these 6 kW units with mirrors of very low iron absorption glass has now been installed in the field, and 16 more units are under construction for a 100 kW, grid-connected plant at the Solar Zone of the University of Arizona Tech Park.
• Angel, R. (2015). High concentration solar photovoltaic systems using glass dish collectors. In Conference on Lasers and Electro-Optics, CLEO 2015, 2015-.
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  Glass solar reflectors, similar to those widely used to concentrate sunlight in solar thermal plants, form the basis for REhnu's HCPV generators. Multijunction cells are housed in small, upgradeable units at each mirror focus. © 2015 OSA.
• Wheelwright, B., & Angel, R. (2015). Freeform Illumination Design with Cartograms. In Freeform Optics, Freeform 2015.
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  Freeform illumination optics allow highly prescribed irradiance control. Calculating a suitable source-to-target map which results in a continuous surface remains a computational challenge. We produce source-to-target maps using a modified cartogram generation technique.
• Angel, R., Angel, J. R., & Coughenour, B. M. (2014). XRX-Köhler Optical Design and System Optimization. In Classical Optics 2014.
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  We present multiple optical design embodiments of a dish-based concentration photovoltaic (CPV) system utilizing Kohler illumination. Spectral optimization, opto-mechanical tolerancing, prototype development, and on-sun field validation are presented showing good agreement with the illumination model.
• Angel, R., Cuerden, B., Whiteside, A., Kinsey, G., Riley, D., Kurtz, S., Araki, K., & Baudrit, M. (2014, 2014). Lightweight Dual-axis Tracker Designs For Dish-based HCPV. In 10TH INTERNATIONAL CONFERENCE ON CONCENTRATOR PHOTOVOLTAIC SYSTEMS (CPV-10), 1616, 220-223.
• Angel, R., Stalcup, T., Wheelwright, B., Warner, S., Hammer, K., Frenkel, M., Plesniak, A., & Pfefferkorn, C. (2014, 2014). Shaping solar concentrator mirrors by radiative heating. In HIGH AND LOW CONCENTRATOR SYSTEMS FOR SOLAR ENERGY APPLICATIONS IX, 9175.
• Coughenour, B., & Angel, R. (2014). XRX-Kóhler optical design and illumination optimization. In International Optical Design Conference 2014, IODC 2014, 9293.
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  This paper describes the optical and illumination design of a CPV solar energy system. The challenges of creating a highly efficient yet low-cost system architecture come from many sources, but are primarily limited by the photoelectron conversion efficiency of the cells and the illumination performance of the system for on-axis and off-axis pointing scenarios. Furthermore, the need for high solar spectral throughput, evenly concentrated sunlight, and tolerance to offaxis pointing places strict illumination requirements on the optical design. To be commercially viable, the cost associated with all components must be minimized so that when taken together, the absolute installed cost of the system in kWh is lower than any other solar energy method. We present two low-cost optical design embodiments of a dishbased concentration photovoltaic (CPV) system that utilize Kóhler illumination to achieve good illumination uniformity across an array of solar cells. Further optimization for active shadowing compensation and compound electrical I-V curve modeling for the solar cell array is performed that allows realistic off-axis performance scenarios to be modeled with the correct power response sensitivity.
• Wheelwright, B., Angel, R. P., Coughenour, B., Hammer, K., Plesniak, A., & Pfefferkorn, C. (2014, 2014). Freeform solar concentrator with a highly asymmetric acceptance cone. In High and Low Concentrator Systems for Solar Energy Applications IX, 9175.
• Wheelwright, B., Angel, R., Coughenour, B., Figueiro, M., Lerner, S., Muschaweck, J., & Rogers, J. (2014, 2014). Tracking-integrated optics: applications in solar concentration. In INTERNATIONAL OPTICAL DESIGN CONFERENCE 2014, 9293.
• Wheelwright, B., Angel, R., Coughenour, B., Hammer, K., Geary, A., Stalcup, T., Kinsey, G., Riley, D., Kurtz, S., Araki, K., & Baudrit, M. (2014, 2014). Field Weighting Model For Tracking-Integrated Optics. In 10TH INTERNATIONAL CONFERENCE ON CONCENTRATOR PHOTOVOLTAIC SYSTEMS (CPV-10), 1616, 237-241.
• Coughenour, B., Wheelwright, B., & Angel, J. R. (2012). Illumination characterization of glass and metal secondary concentrator elements in a photovoltaic solar concentration system. In High and Low Concentrator Systems for Solar Electric Applications VII, 8468.
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  Using illumination modeling, we provide a comparison of glass Total Internal Reflection Concentrators (TIRC) and metal Hollow Reflective Concentrators (HRC) used as secondary concentrator elements in a dish-based highconcentration photovoltaic (CPV) system. Comparisons of optical efficiency, flux uniformity, off-axis acceptance angle, and cost are vital to choosing an ideal secondary concentrator element for a CPV system employing multi-junction (MJ) cells. In many CPV systems, a free-form optic or sharp-cornered rectangular TIRC composed of glass is used to increase the geometrical flux concentration at the surface of the MJ cell, and may also serve as homogenizers to mix the light to increase flux uniformity. We have demonstrated in on-sun testing that an electroformed metal HRC can be used in place of a glass TIRC of the same geometry, eliminating the need for polymeric bonding to the MJ cell surface, and providing a side-contact surface pathway for active cooling. Although geometrically equivalent, we show that glass TIRC's achieve superior off-axis performance (higher etendue from surface refraction) and are generally acknowledged to have less degradation than optics with over-coated silver, yet metal HRC's employing over-coated silver are superior in spectral absorption characteristics under high solar flux (no losses from glass absorption or Fresnel surface reflections) and don't require accurate glass pressing into many shapes. To better understand the trade-offs between optical efficiency, off-axis performance, mechanical tolerances, cost and reliability, metal (HRC) and glass (TIRC) tapered funnels are analyzed at the surface of equal irradiance in a Kohler-Illumination concentrator system, and a trade study is presented. © 2012 SPIE.
• Stalcup, T., Coughenour, B., Wheelwright, B., Connors, T., Lesser, D., Elliott, J., Schaefer, J., Angel, J. R., & Davison, W. B. (2012). On-sun performance of an improved dish-based HCPV system. In SPIE Proceedings, 8468.
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  The University of Arizona has developed a new dish-based High Concentration Photovoltaic (HCPV) system which is in the process of being commercialized by REhnu, Inc. The basic unit uses a paraboloidal glass reflector 3.1 m x 3.1 m square to bring sunlight to a high power point focus at a concentration of ~20,000x. A unique optical system at the focus reformats the concentrated sunlight so as to uniformly illuminate 36 triple junction cells at 1200x geometric concentration1. The relay optics and cells are integrated with an active cooling system in a self-contained Power Conversion Unit (PCU) suspended above the dish reflector. Only electrical connections are made to the PCU as the active cooling system within is completely sealed. Eight of these reflector/PCU units can be mounted on a single two axis tracking structure2. Our 1st generation prototype reflector/PCU unit consistently generated 2.2 kW of power normalized to 1kW/m2 DNI in over 200 hours of on-sun testing in 20113. Here, we present on-sun performance results for our 2nd generation prototype reflector/PCU unit, which has been in operation since June 2012. This improved system consistently generates 2.7 kW of power normalized to 1kW/m2 DNI and has logged over 100 hours of on-sun testing. This system is currently operating at28% DC net system efficiency with an operating cell temperature of only 20°C above ambient. Having proven this system concept, work on our 3rd generation prototype is underway with a focus on manufacturability, lower cost, and DC efficiency target of 32% or better.
• Stephens, K., & Angel, J. R. (2012). Comparison of collection and land use efficiency for various solar concentrating field geometries. In High and Low Concentrator Systems for Solar Electric Applications VII, 8468.
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  In this paper we analyze the effects of neighbor shadowing of tracking solar photovoltaic arrays when they are set out in solar farms for large scale generation. Closer tracker spacing yields more power per unit area of land, but less power per tracking unit because of shadowing. A model has been developed to quantify and compare efficiencies for different tracker aspect ratios and field layouts, on an hourly, daily and annual basis. The model accounts for atmospheric absorption as well as neighbor shadowing at low solar altitude angles. We have focused on the case of CPV arrays which are oriented normal to the rays from the sun. The field layout is best characterized by the ratio of total array area to land area (the ground cover ratio or GCR). We explore as a function of GCR both the fraction of all the direct sunlight energy that is intercepted by the arrays (the irradiance collection efficiency) and the energy lost by each array because of shadowing. Examples are worked out for rectangular arrays on dual axis trackers at 33° latitude. We find that for a ground cover ratio of 30% the annual irradiance collection efficiency is 50%, almost independent of the layout pattern or the array aspect ratio. For a ground cover ratio of 40%, the irradiance collection efficiency rises to 65%. The corresponding shadowing losses do depend on aspect ratio, thus for 30% GCR the annual average of shadowing loss is 7.2% for 3:1 aspect ratio, rising to 7.8% for 2:1 aspect ratio. High GCR is not realizable for higher aspect ratios, which lead to large swing radius, but for 2:1 aspect ratio 40% GCR results in shadowing loss of 11.5%. One conclusion is that a solar farm with arrays of 2:1 aspect ratio set out with 40% GCR is good compromise when land is scarce: 64% of all the direct sunlight energy incident on the land is harvested by the arrays, with only 11.5% shadowing loss. We have compared these efficiencies with those for trough CSP systems, which also harvest direct sunlight but with reflectors turning about a single, horizontal N-S axis. For given GCR, the shadowing loss is slightly less (0.5%) than for the above dual-axis arrays, however the irradiance collection efficiency is worse in winter, leading to a lower annual average for a given GCR. For example, at 40% GCR, a single-axis system realizes a 56% irradiance collection efficiency compared to 64% for the dual axis systems. © 2012 SPIE.