Dual Anamorphic Reflector Telescope

A 1.2-meter prototype Dual Anamorphic Reflector Telescope (DART) system has been built and tested. The key design feature of the telescope is a pair of membrane mirrors stretched to single curvature parabolic cylindrical sections. The parabolic figure of the mirrors is controlled by a pair of edge rails at two opposing ends of the membrane. The flexible edge rails are adjusted to parabolic to very high accuracy and can potentially be easily refigured on-orbit. The prototype telescope is lightweight and has demonstrated excellent optical performance for the farIR. The design is readily scalable to larger apertures and for operation at shorter wavelengths. Design and test results are discussed.

Revolutionary optical space telescope concepts are described which utilize a set of new technologies that together promise to reduce the total on-orbit spacecraft weight, and cost, by 2-4 orders of magnitude compared to those using current space technologies, yet could be developed and orbited in the 2010-2015 time period. Astronomical telescopes and Earth imaging sensors with 25 - 250 meter diameter apertures are enabled. They use an adaptive piezoelectric primary membrane, unsupported and uninflated, shaped by an electron beam in response to an optical figure sensor; and a secondary correction stage consisting of a liquid crystal spatial light modulator to remove the residual errors, resulting in high quality optical imaging systems. The membrane is folded for launch, and only shaped after attaining final orbit. There is no inflation or other tensioning structure, nor is there any truss structure. All sensor and spacecraft elements are positioned by precision thrusting control in deep space, and in conjunction with a long passive tether if in GEO. The net result is a filled 25 meter diameter filled aperture imaging spacecraft weighing just 260 kg; or a 250 meter diameter sparse aperture imaging spacecraft weighing just 1,600 kg. These technologies enable implementation of apertures so large and lightweight as to simply not be feasible at affordable cost with current or inflatable technologies.

Eyeglass is a very large aperture (25 - 100 meter) space telescope consisting of two distinct spacecraft, separated in space by several kilometers. A diffractive lens provides the telescope's large aperture, and a separate, much smaller, space telescope serves as its mobile eyepiece. Use of a transmissive diffractive lens solves two basic problems associated with very large aperture space telescopes; it is inherently fieldable (lightweight and flat, hence packagable and deployable) and virtually eliminates the traditional, very tight, surface shape tolerances faced by reflecting apertures. The potential drawback to use of a diffractive primary (very narrow spectral bandwidth) is eliminated by corrective optics in the telescope's eyepiece. The Eyeglass can provide diffraction-limited imaging with either single-band, multiband, or continuous spectral coverage. Broadband diffractive telescopes have been built at LLNL and have demonstrated diffraction-limited performance over a 40% spectral bandwidth (0.48 - 0.72 μm). As one approach to package a large aperture for launch, a foldable lens has been built and demonstrated. A 75 cm aperture diffractive lens was constructed from 6 panels of 1 mm thick silica; it achieved diffraction-limited performance both before and after folding. This multiple panel, folding lens, approach is currently being scaled-up at LLNL. We are building a 5 meter aperture foldable lens, involving 72 panels of 700 μm thick glass sheets, diffractively patterned to operate as coherent f/50 lens.

The design strategy for environmental satellite constellations relies upon low orbiting satellites to provide global, high-resolution synoptic data and geostationary satellites to provide continuous observations of rapidly evolving local events. The latter category includes storm systems and, with the advent of cloud-track and water vapor winds, winds aloft. This division of labor is an architectural convenience for visual and infrared sensors but a necessity for microwave sensors. In fact, the typical ground resolution of microwave sensors on low orbiters is barely acceptable. Unfortunately, microwave sensors are preferable to visual and infrared systems for many applications including sea-surface temperature and wind measurement and the only viable method for remotely sensing sea surface salinity. Microwave sensors are also preferable for some atmospheric sounding applications because they are relatively insensitive to cloud cover. The most important cases where microwave sensors are preferred, those related to diagnosing the evolution of severe storm activity, require the highest spatial resolution and are best done with geostationary satellites. In particular, microwave sounding would be an ideal capability for a geostationary weather satellite.

Two-dimensional imaging with synthetic aperture ladar (SAL) has been demonstrated in the laboratory. The method is entirely analogous to scan-mode synthetic aperture radar (SAR), which was used on the Magellan mission to Venus, but with 10^4-5 times shorter wavelength has the potential for much better resolution. The laboratory experiment is described and the theoretical limits placed on SNR by the combination of photon statistics and laser speckle are stated. SAL's limitations of small ground footrpint size and SNR < 1 for single-look imagery can be alleviated by multiple images and mosaicking of scenes. Design equations are given that show what hardware capabilities are needed to implement a desired system. They show that a 10 μ SAL in orbit around Mars can give centimeter-class resolution with reasonable laser power (≤ 100 wt). Major engineering development hurdles must be overcome before any such system can be built.

We report on the development of composite mirror technology under the NASA Gossamer Spacecraft Initiative program. The objectives are to produce moderate aperture, extremely low areal density mirrors with smooth surfaces and good optical figure.

A computer ray-tracing rendered simulation to study the focusing behavior of a lens-like operating ring-array mirror (RAM) for extremely large telescopes is discussed.

A concept based on a two-mirror, three-reflection telescope has been investigated. Its anastigmatism and flat fielded properties, the compactness and optical performances over 2-2.5 arc deg field of view, make this optical system of high interest for the development of much larger telescopes than with Schmidt designs. The 2MTRT concept is a potential candidate for sky surveys with 2-3 meter class telescopes and particularily well adapted for UV space surveys. Preliminary developments have been carried out with the construction of a 30-cm prototype on Amoretti's design, providing encouraging results. At present, a 45-cm 2MTRT prototype has been realized for ground based sky survey of NEOs, based on active optics (MINITRUST), in order to overcome the difficulty of obtaining three aspherical surfaces. The primary and tertiary lie on the same double vase substrate, and have a rest profile. The hyperbolization is carried out in situ by air depressure. The secondary, in a tulip form substrate, has been hyperbolized by elastic relaxation. The project is planned for operation in 2003.

The space science community has identified a need for ultra-light weight, large aperture optical systems that are capable of producing high-resolution images of low contrast. Current mirror technologies are limited due either to not being scalable to larger sizes at reasonable masses, or to lack of surface finish, dimensional stability in a space environment or long fabrication times. This paper will discuss the development of thin-shell, nano-laminate mirror substrates that are capable of being electro-actively figured. This technology has the potential to substantially reduce the cost of space based optics by allowing replication of ultra-lightweight primary mirrors from a master precision tool. Precision master tools have been shown to be used multiple times with repeatable surface quality results with less than one week fabrication times for the primary optical mirror substrate. Current development has developed a series of 0.25 and 0.5 meter spherical nanolaminate mirrors that are less than 0.5 kg/m² areal density before electroactive components are mounted, and a target of less than 2.0 kg/m with control elements. This paper will provide an overview of nanolaminate materials for optical mirrors, modeling of their behavior under figure control and experiments conducted to validate precision control.

One contribution in reducing the costs of optics in space can be provided by production of ultralight mirrors. The decrease in the weight of the primary mirror of a telescope is anticipated to lead to the possibility of increasing the size of the telescopes, therefore increasing the amount and distance from which information is received. An electroplating process of ultralight replica mirrors from nickel sulfamate solution is described. Based on an experimental setup with cylindrical symmetry, flat mirrors with a diameter of 4 and 7 inches and thickness of 1 through 2 mm are made from a Ni-Co alloy. The composition of the resulting deposit is analyzed using Rutherford Backscattering Spectrometry (RBS) and Proton Induced X-ray Emission (PIXE). In order to isotopically resolve Ni and Co, 6 MeV nitrogen ions are used as projectiles in the RBS measurements. Solution parameters monitored during the deposition process using optical absorption spectrophotometry is correlated with the final concentration of Ni and Co in the deposit. Bath parameters like temperature; current density, agitation level and acidity are chosen at certain values and maintained constant from one sample to another throughout the deposition process. The purpose of the experiment is to obtain mirrors with near zero stress, and predetermined composition and hardness. This study is an intermediate step in obtaining through the same process, but with a larger scale setup, ultralight large aperture replica mirrors.

The need for extremely large aperture telescopes drives the requirement for new materials and novel approaches to mirror production. Many lightweight mirror concepts are currently being persued, some with promise for extending their ability to facilitate 100-meter and larger space telescope primaries. These concepts include some rather unorthodox materials in unique configurations. Past experience in producing extremely thin CFRP composite mirrors, using unidirectional CFRP prepreg tape, has led us to a more novel CFPR material, which could further reduce the mass and cost of their predecessors. We present a carbon-based, ultra-lightweight fleece material, which have been shown to exhibit high specularity and extremely low areal density, 200 grams/m², at 2-plies, in contrast to more typical unidirectional CFPR material.

SiC ceramics offer unique thermomechanical and optical characteristics. However, their practical application in optics is very limited. A novel approach was developed which combines carbon composites, carbon foam, Chemical Vapor Reaction (CVR)-Si and Chemical Vapor Deposition (CVD)-Si. This unique approach provided for a breakthrough in optical structures the elimination of the print-through at the rib section.

We propose thin silicon wafers as the building blocks of highly segmented space telescope primary mirrors. Using embedded MEMS actuators operating at high bandwidth control, this technology can achieve diffraction-limited image quality in the 3-300 micron wavelength range. The use of silicon wafers as cryogenic mirror segments is carried forward considering a point design of a future FAIR-class NASA ORIGINS mission. We recognize four major economic factors that justify a massive paradigm shift in the fabrication of ultralightweight space telescopes: The precise process control and repeatability of silicon wafer manufacturing dramatically reduces the huge labor investment in mirror figuring experienced with Hubble Space Telescope. Once developed, the incremental cost of additional space telescopes based upon proven silicon manufacturing techniques can be very small. We estimate the marginal cost of a 30m mirror when deploying a constellation can be as low as $36 million (Year 2002 dollars). Federal R&D funding in the area of microelectromechanical devices and advanced 3-D silicon processing is certain to have far greater economic return than similar investments in other technologies, such as optical membrane technology. The $300B per year silicon processing industry will continue to drive increased MEMS functionality, higher product yields, and lower cost. These advances will continue for decades. The intention here is to present the case for the economic advantage of silicon as a highly functional optical substrate that can be fabricated using unparalleled industry experience with precision process control. We maintain that many architectures superior to the ASSiST concept presented here are possible, and hope that this effort prompts future thinking of the silicon wafer telescope paradigm

This paper presents an overview of the development and capabilities of a space-traceable testbed developed for investigation of research issues related to deployable space telescopes. The Air Force Research Laboratory (AFRL) is developing the Deployable Optical Telescope (DOT), which upon completion will be a fully-deployable, sub-scale, space-traceable ground testbed for development and demonstration of critical technologies for the next-generation of space-optics systems. The paper begins with an overview of the DOT project’s technology goals, including the specific performance objectives of the various technologies that are being incorporated into the DOT testbed. The paper presents an overview of the DOT design, including the central integrating structure, deployable primary mirror petals, deployable secondary tower, deployment mechanisms, lightweight mirror segments, metrology, and control systems. The paper concludes with a report on the current status of DOT activities as well as a view of the future research that is planned for the project.

COROT is a mission of the CNES space agency, to be launched in 2005 in a near Polar orbit. It is devoted to star seismology and to exoplanetary transit search. Five star fields chosen close to the galactic plane will be observed during the mission with a high photometric accuracy (relative). Each observation run will last 150 days monitoring continuously more than 6000 stars. This paper presents a new method designed to perform optimal aperture photometry on board in high density fields. We describe the way the photometric windows or patterns are defined and centered on the CCD around each target star, with the expected performances. Each pattern depends on the specific 2D profile of the point spread function (PSF) but also on the pointing jitter and on the tiny deformations of the telescopes. These patterns will be stored on board in order to define for each target star the optimal pattern which will produce the integrated flux to be measured. This method allows a significant increase of the sampling rate to approximately one measure per star each 8 mn).

Provided the MTF has no holes, data from an unfilled aperture can be processed to form an image. The filled aperture always has higher signal-to-noise, but if that system is too large to build, then lower signal-to-noise may be better than no signal whatsoever. For constant collecting area, it is better to have fewer, larger telescopes and move them around to fill the MTF, but for many applications, e.g., the NASA/GSFC Stellar Imager, the change in the target during the reconfiguration time is a more significant limit than the signal-to-noise ratio. This drives the optical design towards a larger number of smaller apertures. Reconfiguring has higher signal to noise than snapshot imaging because different portions of the MTF are filled by different photons, thereby allowing better filtering. With this insight, we see that the signal-to-noise can also be improved by filling different portions of the MTF with different wavelengths. Furthermore, the use of multiple narrow bandwidths avoids the artifacts inherent in broadband synthesis. We have demonstrated that this works, although artifacts can form if the scene changes with wavelength. We will present results from algorithms developed for removing artifacts and for exploiting spectral diversity

This paper presents the development of the control system architecture for vibration mitigation and autonomous phasing of sub-apertures on a sparse-array test bed. The paper begins with a brief description of the telescope system under consideration, including the actuation system providing 3 degree-of-freedom rigid body correction to each sub-aperture, and the metrology system, comprised of a white-light-based low-bandwidth absolute position sensing system and a high-bandwidth, laser-based relative position sensing system. The control problem posed by the telescope is described, including a discussion of the performance requirements the control system must meet, which include asymptotic set point tracking, broadband and tonal disturbance rejection, and tracking of non-stationary objectives. The use of system identification techniques in development of an accurate model of the input-output dynamics of the system is presented. The overall control system architecture including discussions on aspects such as tolerance of sensor dropouts, and the design of these control systems based on the identified model is presented. The paper presents the results of the application of this control system approach to the experimental system, demonstrating performance of the controlled system.

This paper describes a novel real-time algorithm for optically phasing sub-apertures of a sparse-array telescope system based on recursive estimation of the sub-aperture placement that maximizes a fringe-contrast metric. The sub-apertures are phased in pairs using broad spectral band flood-illumination of the sparse-array, while blocking reflections from all but two sub-apertures. The resulting Young’s geometry at the pupil produces an interference pattern that is characterized to determine spatial-frequency filters that are utilized to generate a contrast metric from the fringe patterns. This contrast metric is shown to generate a near-Gaussian variation as a function of optical path-length difference (OPD), with the maximum contrast occurring at zero OPD. The functional relationship between fringe contrast and sub-aperture position based on a common-path, laser-based relative-piston measurement system is developed into an estimator for maximization of fringe contrast (and therefore phasing of sub-apertures). The recursive algorithm produces real-time estimates of the zero OPD value of the relative position that improves as additional data is acquired.

The future telescopes indeed for exploring deep space should possess a large-clear-aperture low-weight primary mirror. Optical quality of such a mirror will apparently be non-ideal and, therefore, such a telescope will have to be supplied with a system of image correction. We consider a computer-based telescope comprising its primary mirror (PM) of a non-ideal optical quality, a laser system illuminating the PM to inquire the distortions of the specular surface, secondary optics, CCD matrices, and a computer with a proper software. This telescope allow one, in principle, to obtain images of deep-space objects with a high angular resolution in the time-delayed (rather than real-time) mode because of computer processing of the information. In the paper, we also discuss advantages and disadvantages of the analog (nonlinear optical) and digital (phase-diversity) image correction techniques.

The use of propellant to maintain the relative orientation of multiple spacecraft in a sparse aperture telescope such as NASA's Terrestrial Planet Finder (TPF) poses several issues. These include fuel depletion, optical contamination, plume impingement, thermal emission, and vibration excitation. An alternative is to eliminate the need for propellant, except for orbit transfer, and replace it with electromagnetic control. Relative separation, relative attitude, and inertial rotation of the array can all be controlled by creating electromagnetic dipoles on each spacecraft and varying their strengths and orientations. This paper addresses some of the control issues that arise when using electromagnets to control formation geometry.

In order to better understand the technological difficulties involved in designing and building a sparse aperture array, the challenge of building a white light Golay-3 telescope was undertaken. The MIT Adaptive Reconnaissance Golay-3 Optical Satellite (ARGOS) project exploits wide-angle Fizeau interferometer technology with an emphasis on modularity in the optics and spacecraft subsystems. Unique design procedures encompassing the nature of coherent wavefront sensing, control and combining as well as various system engineering aspects to achieve cost effectiveness, are developed. To demonstrate a complete spacecraft in a 1-g environment, the ARGOS system is mounted on a frictionless air-bearing, and has the ability to track fast orbiting satellites like the ISS or the planets. Wavefront sensing techniques are explored to mitigate initial misalignment and to feed back real-time aberrations into the optical control loop. This paper presents the results and the lessons learned from the conceive, design and implementation phases of ARGOS. A preliminary assess-ment shows that the beam combining problem is the most challenging aspect of sparse optical arrays. The need for optical control is paramount due to tight beam combining tolerances. The wavefront sensing/control requirements appear to be a major technology and cost driver.

Sparse aperture arrays use rotation as a method of filling the u-v plane for interferometric image construction. Tethers and electromagnetic coupling are two methods that have been proposed to hold these arrays of spacecraft in formation without the use of on-board propellant. Tethers and electromagnetic coupling can also be used to tailor the aperture velocity profile and re-target the array without any propellant usage. In the case of the electromagnetic coupling, complete control of all relative degrees of freedom within an array can be achieved. This paper will describe these two methods and demonstrate how they can be used to control the angular momentum of the arrays while deployed. The dynamics of each method are explained with examples to demonstrate performance.

The Submillimeter Probe of the Evolution of Cosmic Structure (SPECS) is a proposed mission being considered by NASA to take high resolution sky images at the submillimeter wavelengths. At these wavelengths, we can observe highly red-shifted light that has been traveling towards the Earth over a great deal of time, allowing observation of the developing universe in the distant past. To achieve Hubble class resolution, the system must synthesize an aperture that is 1000 meters in diameter, making interferometry the only currently reasonable approach to design. This paper addresses a high level system trade of a general class of rotating tethered architectures that reel apertures in and out radially to fill the u-v plane. A metric of cost per image is used to identify how aperture area and number of apertures should be selected, and considerations of how spectral information is acquired is used to generate design ideas for reducing the overall system mass.

The SuperNova/Acceleration Probe (SNAP) mission will require a two-meter class telescope delivering diffraction limited images spanning a one degree field in the visible and near infrared wavelength regime. This requirement, equivalent to nearly one billion pixel resolution, places stringent demands on its optical system in terms of field flatness, image quality, and freedom from chromatic aberration. We discuss the advantages of annular-field three-mirror anastigmat (TMA) telescopes for applications such as SNAP, and describe the features of the specific optical configuration that we have baselined for the SNAP mission. We discuss the mechanical design and choice of materials for the telescope. Then we present detailed ray traces and diffraction calculations for our baseline optical design. We briefly discuss stray light and tolerance issues, and present a preliminary wavefront error budget for the SNAP Telescope. We conclude by describing some of tasks to be carried out during the upcoming SNAP research and development phase.

Superconduting tunnel junctions (STJ) have been developed to detect X-ray ~ visivle photons for application to astrophysics, particle physics, material physics, and so on. STJ are applicable as photon detectors with good energy resolution and a high photon-counting rate. STJ also have good efficiency because of their high absorption efficiency below 1 keV photon energy. That is advantageous in the observation of the faint objects with which the photon number is limited like astronomical objects and planetary plasma observation. STJ have potentials to open new windows of telescope. On the other hand, the progress of multilayers makes it possible to fabricated a normal incidence telescope (NIT) with high angular resolution and wide field of view up to 500 eV photon energyThe combination of the improved optical elements (multilayer) and STJ will enable us to design a new optical system in the near future. We demonstrate the design combined Visible - X-ray Wide-Band Space Telescope (WBST).

An advanced Microwave Limb Sounder (MLS), now in concept development for a potential future mission, is a space-borne heterodyne instrument to measure pressure, temperature, and atmospheric constituents from thermal emission between 120 and 2400 GHz. Previous MLS instruments used pencil-beam antennas sized to resolve ~1 vertical scale height. Current atmospheric models need better horizontal resolution than orbit spacing provides. To meet these needs, a new antenna concept combines the wide scan range of the parabolic torus with unblocked offset Cassegrain optics. The resulting system is diffraction-limited in the vertical plane but extremely astigmatic, with beamwidths 0.13×2.5°. Nadir axis symmetry ensures that this Beam Aspect Ratio (BAR) is invariant over ±33 degrees of azimuth. The antenna can feed either an array of receivers or multiplexed low-noise receivers whose FOVs are swept by a small scanning mirror. We describe 3 stages of antenna design: First, using a paraxial-optics method, we choose conic profiles given vertical resolution orbit geometry, then develop the surfaces by nadir axis rotation, matching axisymmetric feeds to the BAR. A ray-trace program validates the design and generates alignment and deformation tolerances. Finally, a physical optics analysis verifies reflector surface currents and radiation patterns.

I discuss the guidelines to the optical design for (sub)millimeter telescopes employing a spherical primary and large-format bolometer arrays. Although various optical solutions for ground visible/IR telescopes using a spherical primary are discussed in the literature, these configurations are usually not acceptable for submillimeter-wave telescopes on orbital or sub-orbital platforms. I thus analyse alternative optical solutions that might be used on spherical-primary antennas, employing either on-axis or (cooled) off-axis optical elements to correct the spherical aberration introduced by the primary, that might be also suitable for ground-based telescopes. These configurations are discussed in relation to optical performance and various operational design constraints, using as a baseline spherical-primary telescope BLAST, the "Balloon-borne Large-Aperture Sub-millimeter Telescope", currently under construction. Modern (sub)millimeter telescopes also require optimized auxiliary optics to efficiently couple the telescope to the receiver over large fields of view. I will thus discuss how to analyze and optimize the design using both the Strehl ratio and the coupling efficiency to evaluate the quality of the off-axis wavefronts and the loss in the power coupling with the horn, using a customized optimization algorithm.

Traditionally a telescope system consists of a large collecting element, usually called the primary, located at the entrance pupil and some smaller elements to relay or convey the light to an image plane. As telescope systems become larger and larger, in order to achieve higher resolution and collect more light, a point is reached where the size of the required elements exceeds the current state of the art in fabrication and support. For telescopes larger than this, the entrance pupil must either be divided into manageable segments, or the entrance pupil is divided into an array of separate telescopes. A multiple telescope array consists of afocal collector telescopes distributed in the entrance pupil, relay optics to bring the light to the center and control tilt and piston errors, and a focal combiner telescope to form the image. Sparse telescope arrays have been designed for various applications. This paper addresses the issues and design constraints leading to a multiple telescope array with a high fill factor.

Ground-based surveillance of deep space has traditionally been the purview of optical telescope systems. Unlike their imaging counterparts, space surveillance telescopes emphasize wide field of view (FOV) over resolution, permitted the most rapid survey of the entire sky. At the same time there is a constant push to detect ever fainter objects, such as small pieces of space debris or small, distant asteroids. Unfortunately increased sensitivity requires very large aperture diameters, which when combined with the requirement for wide FOV results in very fast f/# telescopes. How far this set of requirements can be expanded is typically limited by large, complex, and costly corrector optics to flatten the wavefront. An alternative approach is to design the telescope to a curved focal plane. This is an approach that was once taken with film, but it has not been feasible with electronic focal plane arrays (FPA). A major break-through in FPA design may open up a new range of telescope design options. A new array fabrication technique now provides the ability to produce FPAs with a specified degree of curvature while preserving required electro-optical characteristics. This paper presents a design for a new space surveillance telescope utilizing these curved FPAs.

Advances in systems engineering, applied sciences, and manufacturing technologies have enabled the development of large ground based and spaced based astronomical instruments having a large Field of View (FOV) to capture a large portion of the universe in a single image. A larger FOV can be accomplished using light weighted optical elements, improved support structures, and the development of mosaic Focal Plane Assemblies (mFPA). A mFPA designed for astronomy can use multiple Charged Coupled Devices (CCD) mounted onto a single camera baseplate integrated at the instrument plane of focus. Examples of current, or proposed, missions utilizing mFPA technology include FAME, GEST, Kepler, GAIA, LSST, and SNAP. The development of a mFPA mandates tighter control on the design trades, component development, CCD characterization, component integration, and performance verification testing. This paper addresses the capability Lockheed Martin Space Systems Company's (LMSSC) Advanced Technology Center (ATC) has developed to perform CCD characterization, mFPA assembly and alignment, and mFPA system level testing.

We are investigating a novel superconducting detector and readout method that could lead to photon counting, energy resolving focal plane arrays. This concept is intrinsically different from STJ and TES detectors, and in principle could deliver large pixel counts, high sensitivity, and Fano-limited spectral resolution in the optical/UV/X-ray bands. The readout uses the monotonic relation between the kinetic surface inductance L_s of a superconductor and the density of quasiparticles n, which holds even at temperatures far below T_c. This allows a sensitive readout of the number of excess quasiparticles in the detector by monitoring the transmission phase of a resonant circuit. The most intriguing aspect of this concept is that passive frequency multiplexing could be used to read out ~10⁴ detectors with a single HEMT amplifier. Single x-ray events have been observed in prototype detectors.

A multidisciplinary analysis is demonstrated for the NEXUS space telescope precursor mission. This mission was originally designed as an in-space technology testbed for the Next Generation Space Telescope (NGST). One of the main challenges is to achieve a very tight pointing accuracy with a sub-pixel line-of-sight (LOS) jitter budget and a root-mean-square (RMS) wavefront error smaller than λ/50 despite the presence of electronic and mechanical disturbances sources. The analysis starts with the assessment of the performance for an initial design, which turns out not to meet the requirements. Twentyfive design parameters from structures, optics, dynamics and controls are then computed in a sensitivity and isoperformance analysis, in search of better designs. Isoperformance allows finding an acceptable design that is well “balanced” and does not place undue burden on a single subsystem. An error budget analysis shows the contributions of individual disturbance sources. This paper might be helpful in analyzing similar, innovative space telescope systems in the future.

A novel concept for large aperture lightweight deformable mirrors is presented. This new concept is based on using a flexure-hinged truss substrate as opposed to monolithic substrates used in all past and current deformable mirror technologies. With its ability to deform in tangential as well as normal directions, this new concept solves the problem of coefficient of thermal expansion (CTE) mismatch between face sheet and substrate. It takes advantage of the newly emerging face sheet technologies such as nanolaminates that produce extremely lightweight optical quality face sheets and require very small forces to deform them. It also provides rigidity to the thin face sheet mirror, necessary for the uniqueness of response to actuator commands and for tolerance to disturbances. Any stresses in substrate structure are mainly limited to those induced by the forces in the face sheet itself, which are small in the new lightweight face sheets. The dynamic range of deformation is limited only by the actuator stroke, and not by the stresses induced in the substrate. Therefore it drives the actuator design to small force large stroke actuators, as opposed to the current designs that use large force small stroke moment actuators.

Previous research has demonstrated the feasibility of manufacturing polymer membranes with surfaces suitable for use as optical elements on scales up to 1.5 meters. These membranes have optical surface finishes characterized by a roughness of 1.2 nanometers (rms) and mid spatial frequency figure errors (caused by thickness variations) of approximately 350 nanometers-adequate for many optical applications. With optical quality membranes fabrication demonstrated, the next technical challenges that must be met before large-aperture, ultra-light membrane mirrors can be practically achieved are to develop (1) light-weight deployable support structures, (2) the ability to control the global figure of large optical quality membranes, and (3) an improved understanding of the effects of membrane material properties (e.g., material in-homogeneities, coatings, and boundary conditions) on global figure. The work reported herein further characterizes several key system properties and their effects on optical aberrations. This analysis helps establish technical requirements for membrane optical systems and provides additional insight required to optimize deployable support structures capable of providing passive figure control for membrane optical elements. The results are also used to investigate the need for an electrostatic control system that can actively control the figure of a large membrane mirror.

Large, lightweight telescopes in space will enable future earth science, space science, and reconnaissance. The state of the art in space telescope is the Hubble Space Telescope launched in 1990 with its 2.4 m primary mirror. Missions within the decade such as the Next Generation Space Telescope will push this aperture diameter to over 6.5 m. But truly revolutionary observation in many wavelengths will require increasingly large and lightweight apertures. Although these telescopes of the future will have low areal mass density, the deployed aperture structures must capture and hold a surface figure to a fraction of a wavelength in the presence of thermal, slew, and vibration disturbances. Active control of surface figure is a key technology for the success of gossamer space structures. For structures with thousands of actuators distributed in the surface, the control hardware and computations should be distributed as well. This paper discusses how an efficient control of a membrane reflector shape can be achieved using embedded actuators distributed over the membrane surface. Advanced algorithms using only local information about errors and actuation for collocated and neighboring positions in each of the distributed computational elements allow achieving required control performance. Electrostatic actuators implemented on compliant plastic substrates, represent a highly attractive proposition thanks to their very low areal density. Control, sensing, and communication is distributed and integrated in the adaptive membrane to provide the imaging surface quality of a thick stiff mirror at an infinitesimal fraction of the mass. An adaptive membrane with built-in distributed actuators, sensors, and computational elements can be made scalable to a very large size.

Next generation space telescopes with apertures >10m will require novel technologies to permit lightweight primaries to operate at the diffraction limit in the optical regime. One solution is to construct a telescope from a lightweight, membrane primary, which is holographically corrected for surface distortions, in situ. We have demonstrated the correction of >10,000 waves of error in a 1-m diameter primary having an areal mass of just 17 grams per square meter.

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.

Gossamer mirrors have the potential to reach 100 meter baselines in space because of their very light weight. We explore a type of system that uses an array of flat gossamer mirrors as a primary mirror. Using wavefront reconstruction, we can easily estimate the fields of view for these systems. We report the fields of view as a function of the free parameters for these systems.

Two areas of great current interest are wide-sky surveys of the afterglows from distant high-energy events and higher-resoution wide-area surveys of Mars from orbit and of Jovian moons from fly-by. Large space-based telescopes would create significant capability improvements in either area. To be affordable such telescopes must be extremely light-weight relative to current technology. To be feasible such telescopes must be packaged within small volumes (relative to their deployed size) and deployed within relaxed positional tolerances. Sparse apertures reduce primary weight by factors of 5-20 but pose control and phasing challenges and increase susceptibility to radiometric noise. What is needed is a concept which reduces weight and packaged volume by a factor of 100 or more while relaxing the primary control and phasing requirements and incorporating spectral and directional flexibility.

Aluminum foam core optics can be lightweight, cryo-stable, and low cost. The optimal design of a lightweight mirror is a sandwich with very thin, closely spaced support ribs. Open cell foams, used in sandwich mirrors, approach this optimum design. The availability of high quality aluminum foam and a bare aluminum polishing process have allowed high performance foam core optics made entirely of aluminum to be produced. The long history of aluminum space structures makes all aluminum optical systems attractive for many applications. We report on fabrication and testing of foam core and solid aluminum mirrors. Mirrors with integral mounts were designed for minimum surface error induced by self-weight deflection, thermal gradients, and mounting stresses. Previous work demonstrated the superiority of foam sandwich mirror construction over isogrid lightweighting, and finite element modeling to optimize the mirror design. Recent progress includes: (1) delivery of a lightweight aluminum foam core scan mirror for the Compact Visible-Infrared Radiometer, (2) cryo-stability tests on lightweight foam core spherical mirrors, and (3) an interferometric test of the 'align warm, use cold' concept using a simulated instrument, the Offner Relay. The 'align warm, use cold' concept eliminates the iterative process of misalignment compensation for CTE mismatch as well as figure changing due to CTE mismatch.

Imaging constraints are applied in lens design software macros for automatic generation of Ritchey-Chretien, Three-Mirror-Anastigmat (TMA), and plane symmetric configurations of spherical, conic, and general aspheric mirrors. These tools provide rapid development of telescope and instrument designs by reducing the available degrees of freedom confronting the designer while maintaining the desired set of image properties (e.g. focal length and aberration control). A brief example of a Ritchey-Chretien telescope is presented.