We are developing the design for the Lyman-Ultraviolet (LUV) Imaging Spectrograph, LUVIS to propose for a small explorer (SMEX) mission. LUVIS will provide true long slit (six arc minute) imaging spectroscopic capability with large spectral resolving power, R. Minimizing the number of optical components to the required minimum of three enables large spectral throughput. The design uses a two-mirror Cassegrain Ritchey-Chretien Optical Telescope assembly (OTA), a single optic Rowland-like spectrometer, and a windowless 50 x 127 mm curved microchannel plate (MCP). The design is optimized over the 102 to 140 nm spectral range providing spectral imaging at R ~ 20K in a single exposure. Lyman-β enhanced Al + LiF mirror and grating coatings with the LiF protected with an atomic layer deposition (ALD) fluoride encapsulating overcoat provide high throughput over that spectral range. Line-of-sight (LOS) jitter control utilizes time-tag photon arrival to compensate field position jitter by re-registering pixel location in post-processing as well as tip/tilt active control of the secondary mirror of the OTA. This paper will describe the design as well as some of the key design trades that defined the design.
The Lyman-UV imaging spectrograph (LUVIS) is a NASA SMEX mission concept. Here, we describe the basic scientific requirements of LUVIS and instrumental requirements derived from the scientific requirements. Other papers in this conference by Woodruff et al. and Kendrick et al. describe the LUVIS instrument in detail.
The Lyman UV imaging spectrograph (LUVIS) accomplishes priority UV science contained in the budget of a SMEXclass mission. LUVIS consists of a 0.5-m f/24 Cassegrain optical telescope assembly feeding a UV/ far-UV spectrometer. LUVIS has a long 6 arcmin slit enabling spectral imaging and is optimized for 102-140 nm with a resolving power of 20,000 on a micro-channel plate detector with a CsI photocathode. The light gathering power is designed to reach galaxies with near-UV fluxes as low as 10-14 erg/s/cm2 /Å (and lower with long time exposures). The design approach encompasses a simple but elegant optical design, minimum number of reflective surfaces, limited mechanisms, and an orbit minimizing fuel requirements while offering operational advantages. All components are already at a high technology readiness level further reducing technical and cost risk to meet a SMEX budget with healthy cost reserves.
KEYWORDS: Ultraviolet radiation, Ultraviolet telescopes, Telescopes, Astrophysics, Large telescopes, Space telescopes, James Webb Space Telescope, Hubble Space Telescope, Space operations
We report on the status of CETUS, an all-UV, Probe-class mission concept to be evaluated by the Astro2020 Steering Panel. This report expands and updates the scientific uses of CETUS and CETUS technology as described earlier by Kendrick et al. (2019). The major updates derive form technological advances that promise to make CETUS a scientifically more powerful and long-lived space observatory than originally proposed. A long, useful lifetime will be needed to fulfill the future needs of the astronomical and planetary-science community.
The Cosmic Evolution Through UV Surveys (CETUS) concept has three UV instruments to achieve its science goals that work in the near ultraviolet (NUV) and far ultraviolet (FUV). The NUV multi-object spectrograph (MOS) and the NUV/FUV Camera operate simultaneously with their separate field of views. The key enabling technologies will be discussed including the micro-shutter array, detectors, and optical coatings. The NUV MOS can target up to 100 objects at a time which will allow over 100,000 galaxies to be observed during the mission lifetime. The UV Camera has the capability to image from the FUV to the NUV at the same time the MOS is operating at 180-350 nm. The UV Camera has a selection of bandpass filters, longpass filters, and two separate detectors to optimize observing in either the FUV or the NUV utilizing a sealed CsI solar blind micro-channel plate and a 4Kx4K CCD respectively. Both instruments have a tip/tilt/focus mechanism on one of their optics allowing independent focus correction and dithering of the image at the focal plane.
NASA has funded the Cosmic Evolution Through Ultraviolet Spectroscopy (CETUS) mission study in preparation for the Decadal Survey, ASTRO2020. CETUS is developed as a Probe Class Mission, a new NASA category for astrophysics cost capped at 1B USD. This enables larger and more sophisticated observatories than under NASA’s Explorer Programs, but less ambitious than under NASA Flagship Missions. The NASA CETUS Study has resulted in a wide-field-of-view (WFOV) telescope of 1.5m aperture, with the colleting area by solid angle product A*Ω substantially higher than that for HST. CETUS will include a wide field camera, a multi-object spectrograph of the same field, and also a point source spectrometer reaching down to 100nm wavelength.
As part of a study funded by NASA headquarters, we are developing a probe-class mission concept called the Cosmic Evolution through UV Spectroscopy (CETUS). CETUS includes a 1.5-m aperture diameter telescope with a large field of view (FOV). CETUS includes three scientific instruments: a far ultraviolet (FUV) and near ultraviolet (NUV) imaging camera (CAM); a NUV multiobject spectrograph (MOS); and a dual-channel point/slit spectrograph (PSS) in the Lyman ultraviolet (LUV), FUV, and NUV spectral regions. The large FOV three-mirror anastigmatic (TMA) optical telescope assembly (OTA) simultaneously feeds the three separate scientific instruments. That is, the instruments view separate portions of the TMA image plane, enabling parallel operation by the three instruments. The field viewed by the MOS, whose design is based on an Offner-type spectrographic configuration to provide wide FOV correction, is actively configured to select and isolate numerous field sources using a next-generation micro-shutter array. The two-channel CAM design is also based on an Offner-like configuration. The PSS performs high spectral resolution spectroscopy on unresolved objects over the NUV region with spectral resolving power, R ∼ 40,000, in an echelle mode. The PSS also performs long-slit imaging spectroscopy at R ∼ 20,000 in the LUV and FUV spectral regions with two aberration-corrected, blazed, holographic gratings used in a Rowland-like configuration. The optical system also includes two fine guidance sensors, and wavefront sensors that sample numerous locations over the full OTA FOV. In-flight wavelength calibration is performed by a wavelength calibration system, and flat-fielding is also performed, both using in-flight calibration sources. We describe the current optical design of CETUS and the major trade studies leading to the design.
The importance of high performance interference bandpass filters in the UV is growing recently. For the CETUS project a set of bandpass filters with a clear aperture of 70 mm is required centered at the wavelengths 215.5 nm / 256.5 nm / 297.5 nm / 338.5 nm / 379.5 nm with a FWHM of 41 nm and blocked as good as possible up to 1100 nm. We present a design study based on all-dielectric hard sputtered coatings on colorglass substrates for the wavelengths 297.5 nm / 338.5 nm / 379.5 nm. The colorglass substrates where chosen to suppress ghost images by reflection on the exit face and to improve the blocking in the required range. For the wavelengths 215.5 nm and 256.5 nm a conventionally evaporated design of metal-dielectric Fabry-Perot stacks was chosen on fused silica substrates. We comment on how system requirements are leading to filter specifications and show theoretical spectra of the chosen filter designs.
The Cosmic Evolution Through UV Spectroscopy (CETUS) concept1-3 enables parallel observations by the UV multiobject spectrometer (MOS) and near-UV/far-UV camera which operate simultaneously but independently with their separate field of views. The near-UV MOS can target up to 100 objects at a time without confusion with nearby sources or background zodiacal light. This multiplexing will allow over 100,000 galaxies to be observed over a typical mission lifetime. The MOS includes a next-generation micro-shutter array (NGMSA), an efficient aspheric Offner-like spectrometer design with a convex grating, and nanotube light traps for suppressing unwanted wavelengths. The NUV/FUV Camera has the capability to image in a range of sub-bands from 115-400 nm at the same time the MOS is operating at 180-350 nm. The UV camera has a similar Offner-like relay, selectable filters, and two separate detectors to optimize observing in either the far-UV (115-175 nm) or the near-UV (180-400 nm) utilizing a CsI Micro-Channel Plate detector (MCP) and a CCD respectively.
We are developing a NASA Headquarters selected Probe-class mission concept called the Cosmic Evolution Through UV Spectroscopy (CETUS) mission, which includes a 1.5-m aperture diameter large field-of-view (FOV) telescope optimized for UV imaging, multi-object spectroscopy, and point-source spectroscopy. The optical system includes a Three Mirror Anastigmatic (TMA) telescope that simultaneously feeds three separate scientific instruments: the near-UV (NUV) Multi-Object Spectrograph (MOS) with a next-generation Micro-Shutter Array (MSA); the two-channel camera covering the far-UV (FUV) and NUV spectrum; and the point-source spectrograph covering the FUV and NUV region with selectable R~ 40,000 echelle modes and R~ 2,000 first order modes. The optical system includes fine guidance sensors, wavefront sensing, and spectral and flat-field in-flight calibration sources. This paper will describe the current optical design of CETUS.
The ultraviolet multi-object spectrograph (MOS) for the Cosmic Evolution Through UV Spectroscopy (CETUS) concept1,2 is a slit-based instrument allowing multiple simultaneous observations over a wide field of view. It utilizes a next-generation micro-shutter array, an efficient aspheric Offner spectrometer design with a convex grating, and carbon nanotube light traps for suppressing unwanted wavelengths. The optical coatings are also designed to optimize the UV throughput while minimizing out-of-band signal at the detector. The UV MOS will be able to target up to 100 objects at a time without the issues of confusion with nearby sources or unwanted background like zodiacal stray light. With this multiplexing, the scientific yield of both Probe and Great Observatories will be greatly enhanced.
We report on the early phases of a NASA-sponsored study of CETUS (Cosmic Evolution Through Ultraviolet Spectroscopy), a Probe-class mission concept. By definition, the full lifecycle cost of a Probe mission is greater than $400M (i.e. Explorer missions) and less than $1.00B (“Flagship” missions). The animating idea behind our study is that CETUS can help answer fundamental questions about galaxy evolution by carrying out a massive UV imaging and spectroscopic survey of galaxies and combining its findings with data obtained by other survey telescopes of the 2020’s. The CETUS mission concept comprises a 1.5-m wide-field telescope and three scientific instruments: a near-UV multi-object slit spectrograph with a micro-shutter array as the slit device; a near-UV and far-UV camera with angular resolution of 0.42” (near-UV) or 0.55” (far-UV); and a near-UV or far-UV single-object spectrograph aimed at providing access to the UV after Hubble is gone. We describe the scientific rationale for CETUS and the telescope and instruments in their early design phase.
There is a range of lessons learned when taking an optical system from design through deployment in space. I will summarize some key lessons and observations related to both airborne and spaceborne optical systems with an emphasis on light weighted optics and space telescopes. Areas discussed encompass 1) the initial architecture and system trades as constrained by system error budgets, 2) manufacturing considerations, 3) testing implications, and 4) real and perceived cost impacts.
Traditional mirror manufacturing, particularly for astronomical purposes, requires substantial lead time, due to the nature of the materials and the grinding/polishing process. We propose a new technique for rapid, low-cost production of large, lightweight precision optics by fusing several technologies which in combination we call frozen membrane mirror technology (FMMT). FMMT combines well-understood subsystem technologies, including electrostatic control of membrane mirrors, adaptive optics, wavefront sensing and control, and inflatable structures technology to shorten production time. The basic technique is to control the surface of a reflective coated membrane mirror with electrostatic actuation and wavefront sensor feedback and freeze the membrane shape. We discuss the details of the concept and present results of early lab testing. We focus on the optical regime, but this technology has applicability from the microwave to x-ray spectral bands. Starting with a flexible membrane mirror, one can envision techniques for deployment of large apertures in space.
X-ray telescope architectures currently being examined for future missions such as concepts like the International X-ray
Observatory (IXO) are composed of thousands of extremely thin mirror elements (0.2 to 0.4 mm thick) arranged in
closely spaced arrays. The precise positioning, integration, and testing of those optical elements are some of the
fundamental challenges for fabrication of future X-ray telescopes. We will describe a novel pneumatic actuator and
initial testbed results for positioning a single mirror and subsequently an array of mirrors.
In a setup similar to the self coherent camera, we have added a set of pinholes in the diffraction ring of the Lyot plane in a high-contrast stellar Lyot coronagraph. We describe a novel complex electric field reconstruction from image plane intensity measurements consisting of light in the coronagraph's dark hole interfering with light from the pinholes. The image plane field is modified by letting light through one pinhole at a time. In addition to estimation of the field at the science camera, this method allows for self-calibration of the probes by letting light through the pinholes in various permutations while blocking the main Lyot opening. We present results of estimation and calibration from the High Contrast Imaging Testbed along with a comparison to the pair-wise deformable mirror diversity based estimation technique. Tests are carried out in narrow-band light and over a composite 10% bandpass.
A 4-8m diameter telescope carrying a coronagraph instrument is a leading candidate for an anticipated flagship mission
to detect and characterize Earth-size exoplanets in the 2020s.1 Many candidate coronagraph instruments have been proposed,
and one is close to meeting some of the principal requirements for that mission. But the telescope and instrument
will need exquisite stability and precise control of the incoming wavefront to enable detection of faint companions (10-10of the star) at an angular separation of 2-4 Airy radii. In particular, wavefront errors cause speckles in the image, and
variations in those speckles can confound the exoplanet detection. This challenge is compounded by the background
light from zodiacal dust around our Sun and the target star, which limits the speed with which we can estimate and correct
the speckles. We are working on developing coherent speckle detection techniques that will allow rapid calibration
of speckles on the science detector, allowing subtraction in post-processing or correction with deformable mirrors. The
expected speed improvement allows a much quicker timeline for measurement & calibration, which reduces the required
telescope stability requirement and eases both the flight system design and the challenge of ground testing. We will describe
the experiments and summarize progress to date.
Herein we report on the development, sensing and control and our first results with the Vacuum Nuller Testbed to realize
a Visible Nulling Coronagraph (VNC) for exoplanet coronagraphy. The VNC is one of the few approaches that works
with filled, segmented and sparse or diluted-aperture telescope systems. It thus spans a range of potential future NASA
telescopes and could be flown as a separate instrument on such a future mission. NASA/Goddard Space Flight Center
(GSFC) has a well-established effort to develop VNC technologies, and has developed an incremental sequence of VNC
testbeds to advance this approach and the enabling technologies associated with it. We discuss the continued
development of the vacuum Visible Nulling Coronagraph testbed (VNT). The VNT is an ultra-stable vibration isolated
testbed that operates under closed-loop control within a vacuum chamber. It will be used to achieve an incremental
sequence of three visible-light nulling milestones with sequentially higher contrasts of 108, 109, and ideally 1010 at an
inner working angle of 2*λ/D. The VNT is based on a modified Mach-Zehnder nulling interferometer, with a "W"
configuration to accommodate a hex-packed MEMS based deformable mirror, a coherent fiber bundle and achromatic
phase shifters. We discuss the initial laboratory results, the optical configuration, critical technologies and the null
sensing and control approach.
Three of the recently completed NASA Astrophysics Strategic Mission Concept (ASMC) studies addressed the
feasibility of using a Visible Nulling Coronagraph (VNC) as the prime instrument for exoplanet science. The VNC
approach is one of the few approaches that works with filled, segmented and sparse or diluted aperture telescope systems
and thus spans the space of potential ASMC exoplanet missions. NASA/Goddard Space Flight Center (GSFC) has a
well-established effort to develop VNC technologies and has developed an incremental sequence of VNC testbeds to
advance the this approach and the technologies associated with it. Herein we report on the continued development of the
vacuum Visible Nulling Coronagraph testbed (VNT). The VNT is an ultra-stable vibration isolated testbed that operates
under high bandwidth closed-loop control within a vacuum chamber. It will be used to achieve an incremental sequence
of three visible light nulling milestones of sequentially higher contrasts of 108, 109 and 1010 at an inner working angle of
2*λ/D and ultimately culminate in spectrally broadband (>20%) high contrast imaging. Each of the milestones, one per
year, is traceable to one or more of the ASMC studies. The VNT uses a modified Mach-Zehnder nulling interferometer,
modified with a modified "W" configuration to accommodate a hex-packed MEMS based deformable mirror, a coherent
fiber bundle and achromatic phase shifters. Discussed will be the optical configuration laboratory results, critical
technologies and the null sensing and control approach.
We present the results of the Astrophysics Strategic Mission Concept Study for the New Worlds Observer (NWO). We show that the
use of starshades is the most effective and affordable path to mapping and understanding our neighboring planetary systems, to opening
the search for life outside our solar system, while serving the needs of the greater astronomy community. A starshade-based mission
can be implemented immediately with a near term program of technology demonstration.
KEYWORDS: Mirrors, Telescopes, Space telescopes, Phase modulation, Polishing, James Webb Space Telescope, Actuators, Silicon carbide, Glasses, Space mirrors
A number of ongoing astrophysical mission concept studies are based on large aperture spaceborne telescopes. As optics
get larger, both manufacturing and engineering trades come into consideration and must be balanced with the science
goals and requirements. One of the top-level telescope trades examines the impact of a large monolithic primary mirror
versus an array of smaller mirror segments to either fully or sparsely populate the same aperture. The first consideration
is the scientific impact. Should the scattered edge effects and diffraction of a segmented design be acceptable, it then
becomes a fabrication, test, and cost trade along with any associated risks. This paper will examine some of the key
factors that go into such a trade and looks at manufacturing breakpoints. Examples such as the 4-m aperture New World
Observer (NWO) and the 8-m aperture Advanced Technology Large Aperture Space Telescope (ATLAST) will be presented.
Optical testing of large mirrors for space telescopes can be challenging and complex. Demanding optical requirements
necessitate both precise mirror figure and accurate prediction of zero gravity shape. Mass and packaging constraints
require mirrors to be lightweighted and optically fast. Reliability and low mass imply simple mounting schemes, with
basic kinematic mounts preferable to active figure control or whiffle trees. Ground testing should introduce as little
uncertainty as possible, ideally employing flight mounts without offloaders. Testing mirrors with their optical axes
horizontal can result in less distortion than in the vertical orientation, though distortion will increase with mirror speed.
Finite element modeling and optimization tools help specify selective reinforcement of the mirror structure to minimize
wavefront errors in a one gravity test, while staying within mass budgets and meeting other requirements. While low
distortions are necessary, an important additional criterion is that designs are tolerant to imperfect positioning of the
mounts relative to the neutral surface of the mirror substrate. In this paper, we explore selective reinforcement of a 2-meter class, f/1.25 primary mirror for the proposed SNAP space telescope. We specify designs optimized for various
mount radial locations both with and without backup mount locations. Reinforced designs are predicted to have surface
distortions in the horizontal beam test low enough to perform optical testing on the ground, on flight mounts, and
without offloaders. Importantly, the required accuracy of mount locations is on the order of millimeters rather than
tenths of millimeters.
Optical design concepts for the telescope and instrumentation for NASA's New Worlds Observer program are presented. A four-meter multiple channel telescope is discussed, as well as a suite of science instrument concepts. Wide field instrumentation (imager and spectrograph) would be accommodated by a three-mirror-anastigmat
telescope design. Planet finding and characterization, and a UV instrument would use a separate channel that is picked off after the first two mirrors (primary and secondary). Guiding concepts are also discussed.
A number of upcoming astrophysical investigation concepts are based on large aperture spaceborne telescopes. The basic
science goals drive the required aperture to gather sufficient resolution and signal for reasonable integrations to complete
their planned design reference missions. In addition, certain fundamental requirements may dictate whether or not
a monolithic aperture is required or a segmented mirror array is acceptable. The operating temperature and required performance
(absolute and stability over time) are other important drivers. Based on such performance requirements a
number of mirror manufacturing trades can be performed to balance the technical performance, cost, and schedule.
We will discuss some of the overarching architectural and material trades along with particular manufacturing processes
(and their related step functions) that are integral to selecting primary mirror approaches. We will include examples
ranging from a few meters up to 16 meters which can be packaged into existing launch shrouds or in significantly expanded
future resources such as the Ares V.
An MWIR TDI (Time Delay and Integration) Imager and Spectrometer (MTIS) instrument for characterizing from orbit
the moons of Jupiter and Saturn is proposed. Novel to this instrument is the planned implementation of a digital TDI
detector array and an innovative imaging/spectroscopic architecture. Digital TDI enables a higher SNR for high spatial
resolution surface mapping of Titan and Enceladus and for improved spectral discrimination and resolution at Europa.
The MTIS imaging/spectroscopic architecture combines a high spatial resolution coarse wavelength resolution imaging
spectrometer with a hyperspectral sensor to spectrally decompose a portion of the data adjacent to the data sampled in
the imaging spectrometer. The MTIS instrument thus maps with high spatial resolution a planetary object while
spectrally decomposing enough of the data that identification of the constituent materials is highly likely. Additionally,
digital TDI systems have the ability to enable the rejection of radiation induced spikes in high radiation environments
(Europa) and the ability to image in low light levels (Titan and Enceladus). The ability to image moving objects that
might be missed utilizing a conventional TDI system is an added advantage and is particularly important for characterizing atmospheric effects and separating atmospheric and surface components. This can be accomplished with on-orbit processing or collecting and returning individual non co-added frames.
From hitting a comet to long-term observations to find and characterize extrasolar planets, the spacecraft platform pointing accuracy and stability are fundamental. We describe the pointing requirements for Deep Impact, Kepler, and future extrasolar planet missions such as EPIC, and the approach to allow stable long-term measurements. The guidance, navigation, and control system consists of a suite of systems which can include star trackers, gyros, fine guidance sensors, reaction wheels, fast steering mirrors, and active and passive isolation features. One-fifth to one-twentieth of a pixel attitude determination may be needed with stabilities an order of magnitude tighter for observations that may last thousands of seconds. 1.5 milliarcsecond 3-sigma pointing stability can be achieved for the observatory enabling precision measurements by the scientific payloads.
Precise testbeds are required to investigate the physics and engineering aspects of suppressing extrasolar starlight
sufficiently to discern faint companion planets. In addition, testbeds that can simultaneously produce star and planet
stimuli will be necessary ground support equipment for evaluating instruments designed for imaging and characterizing
extrasolar planets. Integral to this is the ability to represent the broad spectral bands and relative geometry of stars and
planets. We have built upon the Terrestrial Planet Finder Coronagraph (TPF-C) requirements as well as those of
programs like Extrasolar Planet Imaging Coronagraph (EPIC) and Eclipse to develop a star/planet simulator (SPS) that,
in conjunction with other testbed modules, can facilitate the pursuit of pertinent questions. The star/planet simulator
developed has a broadband visible light source that illuminates independently adjustable star and planet sources (angular
separation and orientation, relative magnitude). It is capable of providing either collimated or direct imaged light to
proposed instruments and can be configured to produce the source stimuli in a vacuum environment. We will describe
the physical set-up, measurements, and initial observations as well as the plans for combining with a coronagraphic
testbed.
KEYWORDS: Space telescopes, Observatories, Space operations, Integrated modeling, Sensors, Telescopes, Control systems, Systems modeling, Mirrors, Optical alignment
As envisioned space-based telescopes, observatories, and constellations of sensors grow in size and complexity, the ability to perform complete ground verification becomes increasingly difficult or impossible. Integrated system modeling offers one bridge in analyzing the expected optical performance and metrology of extended platforms in space to an accuracy exceeding the optical testing that can be performed in 1-g. In addition, some aspects of the final integration and system performance testing will eventually progress to on-orbit operations in the not-to-distant future as the infrastructure for lunar and Mars manned exploration proceeds. Specifically, the possibility of an Earth-moon L1 Gateway or a similar "shipyard" in space opens up the potential for some final optical characterizations being performed in space while additional human or robotically assisted alignments and integrations can be performed prior to final deployment to distant operational destinations such as at the Earth-Sun L2. Programs like Laser Interferometer Space Antenna (LISA) and Terrestrial Planet Finder (TPF) are examples of missions where sole reliance on ground optical testing will be extremely difficult, impossible, or inconclusive. Spitzer is a recent example where modeling was a key component of predicting temperature environment and corresponding performance. The future will require a greater reliance on modeling and, where warranted, optical testing and final alignment utilizing on-orbit test facilities. In fact, the case can be made that system modeling will need to be embraced more strongly if space-based assembly and test are to be realized. The necessary analytical tools, verification ground testbeds, and confirming flight experiments are crucial along with the planning that will take full advantage of the flexibility of final system verification at a Gateway prior to a low energy transfer to the observatory's final deployed operating orbit.
The optical telescope for a spaceborne coronagraph to detect terrestrial to Jovian-sized planets has unusually stringent phase and amplitude requirements - far exceeding a "conventional" telescope like Hubble or the James Webb Space Telescope. The key engineering requirements will be summarized based on probable mission science objectives and an engineering solution with a monolithic primary mirror on the order of 6 meters by 4 meters. We will also present an optical design for a sub-scale coronagraphic simulator as a logical and essential step in examining the system sensitivities. Testbed simulations will include F, G, and K stars and companion planets ranging in size from earth-like up to Jovian-like.
The 1.4-meter semi-rigid, beryllium Advanced Mirror System Demonstrator (AMSD) mirror completed initial cryogenic testing at Marshall’s X-ray Calibration Facility (XRCF) in August of 2003. Results of this testing show the mirror to have very low cryogenic surface deformation and possess exceptional figure stability. Additionally, the mirror substrate exhibits virtually no change in surface figure over the James Webb Space Telescope (JWST) operational temperature range of 30 to 62 Kelvin. The lightweighted, semi-rigid mirror architecture approach demonstrated here is a precursor to the mirror technology being applied to the JWST observatory. Testing at ambient and cryogenic temperatures included the radius of curvature actuation system and the rigid body displacement system. These two systems incorporated the use of 4 actuators to allow the mirror to change piston, tilt, and radius of curvature. Presented here are the results of the figure change, alignment change, and radius change as a function of temperature. Also shown will be the actuator influence functions at both ambient and cryogenic temperatures.
The telescope for a Terrestrial Planet Finder (TPF) coronagraph has exceedingly stringent phase and amplitude requirements, especially for the large, monolithic primary mirror (possibly as large as 4 meters by 10 meters). The pertinent derived engineering requirements will be summarized based on a described set of science objectives to simulate solar type stars and their companion earth-size planets. We will also present an optical design for a sub-scale coronagraphic testbed as an essential step in examining the system sensitivities. The major subassemblies of the testbed include: 1) a star/planet simulator that affords variation in contrast, adjustable relative separation and angular orientation and 2) a relay optical system representative of a TPF 3-mirror telescope that allows the imposition of known optical perturbations over the desired wavefront spatial frequencies. We will compare these TPF testbed mirror wavefront requirements with levels recently achieved on the Advanced Mirror System Demonstrator and planned for the James Webb Space Telescope (JWST).
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.
The 1.4-m semi-rigid, beryllium Advanced Mirror System Demonstrator (AMSD) mirror has been lightweighted by over 90% (achieving 10 kg/m2 areal density) and optically ground and polished. The mounting structures have been completed and key attachments integrated prior to final polishing. The displacement actuators have been fabricated and tested at ambient and cryogenic temperatures. The integrated assembly represents an off-axis, aspheric, flight panel of a spaceborne mirror array whose radius of curvature (RoC) can be matched with its companion segments and whose position can be separately phased in a rigid body fashion. The results of the initial ambient testing and the cryogenic test set-up of the mirror assembly will be presented including mirror surface characterization and the correction afforded by radius of curvature actuation. Cryogenic testing at MSFC was completed in August 2003. The lightweighted, semi-rigid mirror architecture approach demonstrated here is a precursor to the mirror technology being applied to the James Webb Space Telescope (JWST).
One possible implementation of an optical coronagraphic approach to finding exo-solar planets incorporates a large, monolithic primary mirror (PM) that is approximately 4 meters by 10 meters in size. The optical requirements on a mirror that is part of a suppression system to achieve at least 1010 rejection are extremely challenging, and a series of pathfinder demonstrations and testbeds are warranted.
We examine the optical manufacturing and tolerancing requirements on the mirror itself as a function of spatial frequency where in certain regimes we desire better than 1/1000th of a wave surface accuracy. An atypical requirement is also imposed on the optical coatings where the uniformity of reflectance is desired to be a few parts in 10,000. In addition, we present an optical design for a sub-scale coronagraphic testbed as an essential step in examining the system sensitivities.
A series of developmental as well as flight mirrors have been in process over the last few years for IR cryogenic telescope applications such as the Space Infrared Telescope Facility (SIRTF) and the Next Generation Space Telescope (NGST) and for visible ambient systems such as Space Based Laser (SBL). We will discuss the performance of the 0.85-m SIRTF primary mirror (26.6 kg/m2 areal density) and the 0.5-m Subscale Beryllium Mirror Demonstrator (SBMD) beryllium mirror (9.8 kg/m2 areal density) as well as the current status of the 1.4-m Ball semi-rigid, beryllium Advanced Mirror System Demonstrator (AMSD). The AMSD mirror itself has an areal density of 10.4 kg/m2 and is currently in polishing. The entire AMSD assembly including composite reaction structure, flexures, and actuators, has an areal density less than 15 kb/m2.
Cyrogenic test results of the SIRTF and SBMD mirrors will be presented along with test data on the AMSD actuators. The SBMD mirror wsa cryofigured based on ambient and cryo testing to achieve a wavefront quality of 19 nm rms at 35 K. In addition, the effects of optically coating SBMD with a protected gold multi-layer system will be shown - demonstrating that a lightweight mirror can be coated without adverse print-through due to coating stress at ambient or cryo operating temperatures.
Ball Aerospace & Technologies Corp. is currently under contract to design, build, and test a state-of-the-art lightweight beryllium mirror for cryogenic space applications. This Advanced Mirror System Demonstrator (AMSD) has been designed for lightweight, deployable, spaceborne mirror applications. The major components are currently being fabricated and will comprise a lightweighted mirror assembly including a composite reaction structure. The 1.4-m point-to-point hexagon, semi-rigid beryllium mirror will be integrated with the reaction structure, actuators, and flexures to achieve a mirror system capable of ambient and cryogenic (20 to 55K) operation. The mirror prescription is an off-axis asphere of a parent with a 10-m radius of curvature. Presented here is the current status and a summary of the planned optical fabrication and testing. This work is being performed under a contract to Marshall Space Flight Center (MSFC) in Huntsville, AL and is co-sponsored by the USAF and the NRO.
The Subscale Beryllium Mirror Demonstrator (SBMD) has been fabricated and tested, successfully demonstrating some of the necessary enabling technologies for the Next Generation Space Telescope (NGST) and other lightweight cryogenic space mirror applications. The SBMD is a 0.532-meter diameter concave spherical mirror with a 20-meter radius of curvature fabricated from a single billet of consolidated spherical powder beryllium. The mirror is lightweighted by 90% through the use of open back triangular cells and a thin facesheet. The mirror is mounted to a rigid backplane with titanium bipod flexures. Surface figure requirements at 35K of 1/4 wave p-v (full aperture) and 1/10 wave p-v (1-10 cm spatial frequency) required initial vacuum cryogenic characterization of the mirror. Cryogenic deformation and repeatability were characterized using the Optical Testing System (OTS) at the X-Ray Calibration Facility (XRCF) at Marshall Space Flight Center (MSFC). The mirror underwent cryofiguring to optimize performance and was subsequently tested to verify final performance requirements of surface figure, radius of curvature, and microroughness. Presented here are the final results of the SBMD program, showing that all requirements have been met.
Ball Aerospace is currently under contract to Marshall Space Flight Center (MSFC) in Huntsville, AL to design, build, and test a state-of-the-art lightweight beryllium mirror for cryogenic space applications, the Next Generation Space Telescope Sub-scale Beryllium Mirror Demonstrator (SBMD). The mirror is manufactured from spherical powder beryllium and optimized for cryogenic use. This 0.53-meter diameter lightweight mirror (< 12 kg/m2) has been tested at MSFC at ambient and cryogenic temperatures down to 23 K, cryofigured for optimal performance at 35 K, and subsequently retested at cryogenic temperatures. In addition, Ball has a separate contract with MSFC for an Advanced Mirror system Demonstrator (AMSD) to fabricate and test an ultra-lightweight mirror system which extends the semi-rigid SBMD mirror design to a 1.4-meter point-to-point beryllium hexagon mirror, flexures, rigid body and radius of curvature actuators, and reaction structure. This paper will describe the SBMD mirror performance and its cryogenic testing and present an overview of the AMSD semi-rigid beryllium mirror.
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