This PDF file contains the front matter associated with SPIE Proceedings Volume 6687, including the Title Page, Copyright information, Table of Contents, Introduction, and the Conference Committee listing.

The need for JWST's metering structure to be stable over time while at cryogenic temperatures is derived from its scientific objectives. The operational scenario planned for JWST provides for the optical system to be adjusted on regular intervals based upon image quality measurements. There can only be a limited amount of optical degradation between the optical system adjustments in order to meet the scientific objectives. As the JWST primary mirror is segmented, the structure supporting the mirror segments must be very stable to preclude degradation of the optical quality. The design, development and, ultimately, the verification of that supporting structure's stability rely on the availability of analysis tools that are credibly capable of accurately estimating the response of a large structure in cryogenic environments to the nanometer level. Validating the accuracy of the analysis tools was a significant technology demonstration accomplishment. As the culmination of a series of development efforts, a thermal stability test was performed on the Backplane Stability Test Article (BSTA), demonstrating TRL-6 status for the design, analysis, and testing of Large Precision Cryogenic Structures. This paper describes the incremental development efforts and the test results that were generated as part of the BSTA testing and the associated TRL-6 demonstration.

The stability requirements for the James Webb Space Telescope (JWST) optical metering structure are driven by the science objectives of the mission. This structure, JWST Optical Telescope Element (OTE) primary mirror backplane, has to be stable over time at cryogenic temperatures. Successful development of the large, lightweight, deployable, cryogenic metering structure requires verification of structural deformations to nanometer level accuracy in representative test articles at cryogenic temperature. An instantaneous acquisition phase shifting speckle interferometer was designed and built to support the development of JWST Optical Telescope Element (OTE) primary mirror backplane. This paper discusses characterization of the Electronic Speckle Pattern Interferometer (SPS-DSPI) developed for JWST to verify its capabilities to measure structural deformations in large composite structures at cryogenic temperature. Interferometer performance during the Backplane Stability Test Article (BSTA) test that completed the TRL-6 (Technology Readiness Level-6) demonstration of Large Precision Cryogenic Structures will also be discussed.

This document describes the Cryogenic Damping Test (CDT) of the James Webb Space Telescope (JWST) Backplane Structural Test Article (BSTA). Contained in this report are descriptions of test configuration, highlights of data, review of methods used to extract modal parameters, and presentation of results and conclusions.

The one-meter Testbed Telescope (TBT) has been developed at Ball Aerospace to facilitate the design and implementation of the wavefront sensing and control (WFSC) capabilities of the James Webb Space Telescope (JWST). We have recently conducted an "end-to-end" demonstration of the flight commissioning process on the TBT. This demonstration started with the Primary Mirror (PM) segments and the Secondary Mirror (SM) in random positions, traceable to the worst-case flight deployment conditions. The commissioning process detected and corrected the deployment errors, resulting in diffraction-limited performance across the entire science FOV. This paper will describe the commissioning demonstration and the WFSC algorithms used at each step in the process.

The primary mirror of the James Webb Space Telescope (JWST) consists of 18 segments and is 6.6 meters in diameter. A sequence of commissioning steps is carried out at a single field point to align the segments. At that single field point, though, the segmented primary mirror can compensate for aberrations caused by misalignments of the remaining mirrors. The misalignments can be detected in the wavefronts of off-axis field points. The Multifield (MF) step in the commissioning process surveys five field points and uses a simple matrix multiplication to calculate corrected positions for the secondary and primary mirrors. A demonstration of the Multifield process was carried out on the JWST Testbed Telescope (TBT). The results show that the Multifield algorithm is capable of reducing the field dependency of the TBT to about 20 nm RMS, relative to the TBT design nominal field dependency.

NASA's Technology Readiness Level (TRL)-6 is documented for the James Webb Space Telescope (JWST) Wavefront Sensing and Control (WFSC) subsystem. The WFSC subsystem is needed to align the Optical Telescope Element (OTE) after all deployments have occurred, and achieves that requirement through a robust commissioning sequence consisting of unique commissioning algorithms, all of which are part of the WFSC algorithm suite. This paper identifies the technology need, algorithm heritage, describes the finished TRL-6 design platform, and summarizes the TRL-6 test results and compliance. Additionally, the performance requirements needed to satisfy JWST science goals as well as the criterion that relate to the TRL-6 Testbed Telescope (TBT) performance requirements are discussed.

We have developed microshutter array systems at NASA Goddard Space Flight Center for use as multi-object aperture arrays for a Near-Infrared Spectrometer (NIRSpec) instrument. The instrument will be carried on the James Webb Space Telescope (JWST), the next generation of space telescope, after the Hubble Space Telescope retires. The microshutter arrays (MSAs) are designed for the selective transmission of light from objected galaxies in space with high efficiency and high contrast. Arrays are close-packed silicon nitride membranes with a pixel size close to 100x200 μm. Individual shutters are patterned with a torsion flexure permitting shutters to open 90 degrees with minimized stress concentration. In order to enhance optical contrast, light shields are made on each shutter to prevent light leak. Shutters are actuated magnetically, latched and addressed electrostatically. The shutter arrays are fabricated using MEMS bulk-micromachining and packaged utilizing a novel single-sided indium flip-chip bonding technology. The MSA flight system consists of a mosaic of 2 x 2 format of four fully addressable 365 x 171 arrays. The system will be placed in the JWST optical path at the focal plane of NIRSpec detectors. MSAs that we fabricated passed a series of qualification tests for flight capabilities. We are in the process of making final flight-qualified MSA systems for the JWST mission.

The Space Infrared Interferometric Telescope (SPIRIT) was designed to accomplish three scientific objectives: (1) learn how planetary systems form from protostellar disks and how they acquire their inhomogeneous chemical composition; (2) characterize the family of extrasolar planetary systems by imaging the structure in debris disks to understand how and where planets of different types form; and (3) learn how high-redshift galaxies formed and merged to form the present-day population of galaxies. SPIRIT will accomplish these objectives through infrared observations with a two aperture interferometric instrument. This paper gives an overview of SPIRIT design and operation, and how the three design cycle concept study was completed. The error budget for several key performance values allocates tolerances to all contributing factors, and a performance model of the spacecraft plus instrument system demonstrates meeting those allocations with margin.

The Space Infrared Interferometric Telescope (SPIRIT) was designed to accomplish three scientific objectives: (1) learn how planetary systems form from protostellar disks and how they acquire their inhomogeneous chemical composition; (2) characterize the family of extrasolar planetary systems by imaging the structure in debris disks to understand how and where planets of different types form; and (3) learn how high-redshift galaxies formed and merged to form the present-day population of galaxies. SPIRIT will accomplish these objectives through infrared observations with a two aperture interferometric instrument. This paper gives an overview into the optical system design, including the design form, the metrology systems used for control, stray light, and optical testing.

The Space Infrared Interferometric Telescope (SPIRIT), a candidate NASA Origins Probe mission, is a cryogenic 6-36m variable-baseline imaging interferometer operating at 25 - 400 μm. SPIRIT utilizes dual, meter-class, telescopes which translate along opposed deployable booms. The collimated beams from the telescopes are combined in a central instrument module operating at 4K and lower. Mission-enabling mechanisms include the large, optical delay line scan mechanism, the afocal collector telescope trolley drives, and the boom deployment mechanisms. This paper provides an overview of the mechanical aspects of the conceptual design created to meet the challenging instrument requirements.

The Space Infrared Interferometric Telescope (SPIRIT) is envisioned to be a pair of one meter diameter primary light collectors on either side of a beam combiner, all cooled to 4 K or lower. During an observation, the collectors are required to move toward and away from the beam combiner to obtain information at various baselines to simulate a filled aperture. The thermal design of this mission as presented in this paper provides each light collector and the beam combiner with separate cryogenic systems. This allows the boom that attaches the combiner and collectors, the motors and many of the mechanisms to operate at room temperature, thus simplifying ground testing and reducing mission cost and complexity. Furthermore, the cryogenic systems consist of passive radiators and mechanical coolers - a cryogen-free approach. This paper gives a description of the requirements and resulting design for this architecture and some of the benefits and difficulties of this approach. A subscale thermal vacuum test of one of the collector thermal systems was performed. The thermal model and test agreed very well showing the viability of the thermal design and subscale cryo-thermal test approach.

SPIRIT is a spatial and spectral interferometer with an operating wavelength range 25 μm - 400 μm. As a double-Fourier interferometer, SPIRIT features sub-arcsecond spatial resolution and R≡λ/Δλ=3000 spectral resolution over a 1 arcmin field of view. Its three primary scientific objectives are to: (1) Learn how planetary systems form from protostellar disks, and how they acquire their chemical organization; (2) Characterize the family of extrasolar planetary systems by imaging the structure in debris disks to understand how and where planets form, and why some planets are ice giants and others are rocky; and (3) Learn how high-redshift galaxies formed and merged to form the present-day population of galaxies. The detector subsystem provides a set of far-infrared detector arrays in the SPIRIT instrument. These arrays are used for science purposes by detecting the faint interferometric signal. The resulting technology requirement is for a set of eight arrays operating at wavelengths of 25 μm - 400 μm, divided into two arrays (one for each interferometer output port) per octave of wavelength. At the short wavelength end, the arrays are 14×14 pixels, shrinking to 2×2 at the longest band. The per-pixel sensitivity requirement of 10^-19 W/√Hz, coupled with speed of τ_effective ~150 μs, make these relatively small arrays challenging. The operating temperature necessary to provide this sensitivity is around 50 mK. Over the majority of the SPIRIT wavelength range and sensitivity requirement, there are no commercial vendors of such detector arrays, and thus they will require a separate NASA-supported development.

The Wide-Field Imaging Interferometry Testbed (WIIT) was designed to develop techniques for wide-field of view imaging interferometry, using "double-Fourier" methods. These techniques will be important for a wide range of future space-based interferometry missions. We have provided simple demonstrations of the methodology already, and continuing development of the testbed will lead to higher data rates, improved data quality, and refined algorithms for image reconstruction. At present, the testbed effort includes five lines of development; automation of the testbed, operation in an improved environment, acquisition of large high-quality datasets, development of image reconstruction algorithms, and analytical modeling of the testbed. We discuss the progress made towards the first four of these goals; the analytical modeling is discussed in a separate paper within this conference.

The Stellar Imager (SI) is a UV/optical, space-based interferometer designed to enable 0.1 milli-arcsecond (mas) spectral imaging of stellar surfaces and, via asteroseismology, stellar interiors and of the Universe in general. SI's science focuses on the role of magnetism in the Universe, particularly on magnetic activity on the surfaces of stars like the Sun. SI's prime goal is to enable long-term forecasting of solar activity and the space weather that it drives, in support of the Living with a Star program in the Exploration Era. SI will also revolutionize our understanding of the formation of planetary systems, of the habitability and climatology of distant planets, and of many magneto-hydrodynamically controlled processes in the Universe. SI is a "Flagship and Landmark Discovery Mission" in the 2005 Sun Solar System Connection (SSSC) Roadmap and a candidate for a "Pathways to Life Observatory" in the Exploration of the Universe Division (EUD) Roadmap (May, 2005). We discuss herein the science goals of the SI Mission, a mission architecture that could meet those goals, and the technologies needed to enable this mission. Additional information on SI can be found at: http://hires.gsfc.nasa.gov/si/.

Stellar Imager (SI) is a proposed NASA space-based UV imaging interferometer to resolve the stellar disks of nearby stars. SI would consist of 20 - 30 separate spacecraft flying in formation at the Earth-Sun L2 libration point. Onboard wavefront sensing and control is required to maintain alignment during science observations and after array reconfigurations. The Fizeau Interferometry Testbed (FIT), developed at the NASA/Goddard Space Flight Center, is being used to study wavefront sensing and control methodologies for Stellar Imager and other large, sparse aperture telescope systems. FIT initially consists of 7 articulated spherical mirrors in a Golay pattern, and is currently undergoing expansion to 18 elements. FIT currently uses in-focus whitelight sparse aperture PSFs and a direct solve phase retrieval algorithm to sense and control its wavefront. Ultimately it will use extended scene wavelength, with a sequential diversity algorithm that modulates a subset of aperture pistons to jointly estimate the wavefront and the reconstructed image from extended scenes. The recovered wavefront is decomposed into the eigenmodes of the control matrix and actuators are moved to minimize the wavefront piston, tip and tilt in closed-loop. We discuss the testbed, wavefront control methodology and ongoing work to increase its bandwidth from 1 per 11 seconds to a few 10's of Hertz and show ongoing results.

This paper presents the results of a Fresnel Interferometric Array testbed. This new concept of imager involves diffraction focussing by a thin foil, in which many thousands of punched subapertures form a pattern related to a Fresnel zone plate. This kind of array is intended for use in space, as a way to realizing lightweight large apertures for high angular resolution and high dynamic range observations. The chromaticity due to diffraction focussing is corrected by a small diffractive achromatizer placed close to the focal plane of the array. The laboratory test results presented here are obtained with an 8 centimeter side orthogonal array, yielding a 23 meter focal length at 600 nm wavelength. The primary array and the focal optics have been designed and assembled in our lab. This system forms an achromatic image. Test targets of various shapes, sizes, dynamic ranges and intensities have been imaged. We present the first images, the achieved dynamic range, and the angular resolution.

This paper describes computational results obtained with a high-fidelity optical model of the Wide-Field Imaging Interferometry Testbed (WIIT). The WIIT model includes imperfections inherent in the hardware testbed, such as deviations of the mirrors from their ideal shapes. Model interferograms (brightness in a detector pixel as a function of optical delay) are presented here for several representative test scenes "observed" with multiple interferometric baselines. The results match theoretical expectations and can be compared with real WIIT measurements to identify and characterize instrumental and environmental artifacts in our laboratory data, and to aid in the interpretation of those data.

Stellar Imager (SI) will be a Space-Based telescope consisting of 20 to 30 separated apertures. It is designed for UV/Optical imaging of stellar surfaces and asteroseismology. This report describes details of an alternative optical design for the beam combiner, dubbed the Spatial Frequency Remapper (SFR). It sacrifices the large field of view of the Fizeau combiner. In return, spectral resolution is obtained with a diffraction grating rather than an array of energy-resolving detectors. The SFR design works in principle and has been implemented with MIRC at CHARA for a small number of apertures. Here, we show the number of optical surfaces can be reduced and the concept scales gracefully to the large number of apertures needed for Stellar Imager. We also describe a potential application of this spatial frequency remapping to improved imaging with filled-aperture systems. For filled-aperture imaging, the SFR becomes the core of an improved aperture masking system. To date, aperture-masking has produced the best images with ground-based telescopes but at the expense of low sensitivity due to short exposures and the discarding of most of the light collected by the telescope. This design eliminates the light-loss problem previously claimed to be inherent in all aperture-masking designs. We also argue that at least in principle, the short-integration time limit can also be overcome. With these improvements, it becomes an ideal camera for TPF-C; since it can form speckle-free images in the presence of wavefront errors, it should significantly relax the stability requirements of the current designs.

NASA's Ares V cargo launch vehicle offers the potential to completely change the paradigm of future space science mission architectures. A major finding of the NASA Advanced Telescope and Observatory Capability Roadmap Study was that current launch vehicle mass and volume constraints severely limit future space science missions. And thus, that significant technology development is required to package increasingly larger collecting apertures into existing launch shrouds. The Ares V greatly relaxes these constraints. For example, while a Delta IV has the ability to launch approximate a 4.5 meter diameter payload with a mass of 13,000 kg to L2, the Ares V is projected to have the ability to launch an 8 to 12 meter diameter payload with a mass of 60,000 kg to L2 and 130,000 kg to Low Earth Orbit. This paper summarizes the Ares V payload launch capability and introduces how it might enable new classes of future space telescopes such as 6 to 8 meter class monolithic primary mirror observatories, 15 meter class segmented telescopes, 6 to 8 meter class x-ray telescopes or high-energy particle calorimeters.

The Single Aperture Far Infrared (SAFIR) observatory - a concept design for a 10m-class spaceborne far- infrared and submillimeter telescope, has been proposed for development, and given high priority by agency strategic planners. SAFIR will target star formation in the early universe, the chemistry of our interstellar medium, and the chemical processes that lead to planet formation. SAFIR is a telescope that, with passive cooling at Earth-Sun L2, achieves temperatures that allow background-limited broad-band operation in the far infrared. This observatory is baselined as being autonomous in deployment and operation, but consideration has been given to understanding the enabling opportunities presented by Exploration architecture. As this architecture has become better defined, these opportunities have become easier to understand.We present conceptual strategies that would use modestly enhanced Exploration architecture to service and maintain SAFIR, allowing extended duration, lower risk and hardware cost, and performance enhancements linked to the steep development curve for sensor technology. These efforts, which would rely on both human and robotic agents, presume routine operations at Earth-Sun L2, and servicing at an Earth-Moon L1 jobsite. The latter is understood to be easily accessible to a lunar-capable Exploration program. This study bridges the interface between Exploration technology and astronomical space observatory technology. Such an Exploration-enhanced version of SAFIR can be seen as a strawman for more ambitious far future work, in which much larger science instruments that cannot be packaged in a single launch vehicle are not only serviced and maintained in space, but also constructed there.

On-orbit servicing can provide significant benefits for scientific space programs through maintenance and upgrades of scientific spacecraft. The Hubble Space Telescope (HST) captured these benefits throughout its life because it was designed to be serviceable. However, serviceability has often been excluded from other telescope programs since the cost of serviceability could not be quantitatively justified. This paper develops a framework to determine the value of including serviceability in a space telescope. The framework incorporates three main principles: separation of cost and benefits, calculation of value through comparison of servicing to replacement, and the use of Monte-Carlo simulation and decision rule analysis to account for programmatic uncertainty and management flexibility. To demonstrate how the framework can be used in practice, a case study was performed with representative data from HST.

We present scientific rationale, concepts and technologies for far-IR (λ=35-600 μm) instrumentation for the cryogenic single-dish space telescopes envisioned for the next two decades. With the tremendous success of Spitzer, the stage is set for larger (3-10 meter) actively-cooled telescopes and several are under consideration including SPICA in Japan, and CALISTO/SAFIR in the US. The cold platforms offer the potential for far-IR observations limited only by the zodiacal dust emission and other diffuse astrophysical foregrounds. Optimal instrumentation for these missions includes large-format direct-detector arrays with sensitivity matched to the low photon backgrounds. This will require major improvements relative to the current state of the art, especially for wavelengths beyond the 38-micron silicon BIB cutoff, We review options and present progress with one approach: superconducting bolometers. We highlight in particular the scientific potential for moderate-resolution broadband spectroscopy. The large cold telescopes can provide line sensitivities below 10^-20 W m^-2, enabling the first routine survey spectroscopy of the redshift 0.5 to 5 galaxies that produced the cosmic far-IR background. These far-IR-bright dusty galaxies account for half of the photon energy released since stars and galaxies began forming, and the new far-IR spectroscopic capability will reveal their energy sources and chart their history. We describe concepts for the background-limited IR-Submillimeter Spectrograph (BLISS) designed for this purpose. BLISS is a suite of R~1000 spectrometer modules spanning the far-IR range, and is under study for SPICA; a similar but more capable instrument can be scaled for CALISTO/SAFIR.

We present a design for a cryogenically cooled large aperture telescope for far-infrared astronomy in the wavength range 30 μm to 300 μm. The Cryogenic Aperture Large Infrared Space Telescope Observatory, or CALISTO, is based on an off-axis Gregorian telesocope having a 4 m by 6 m primary reflector. This can be launched using an Atlas V 511, with the only optical deployment required being a simple hinged rotation of the secondary reflector. The off-axis design, which includes a cold stop, offers exceptionally good performance in terms of high efficiency and minimum coupling of radiation incident from angles far off the direction of maximum response. This means that strong astronomical sources, such as the Milky Way and zodiacal dust in the plane of the solar system, add very little to the background. The entire optical system is cooled to 4 K to make its emission less than even this low level of astronomical emission. Assuming that detector technology can be improved to the point where detector noise is less than that of the astronomical background, we anticipate unprecedented low values of system noise equivalent power, in the vicinity of 10^-19 WHz^-0.5, through CALISTO's operating range. This will enable a variety of new astronomical investigations ranging from studies of objects in the outer solar system to tracing the evolution of galaxies in the universe throughout cosmic time.

We have proposed the development of a low-cost space telescope, Destiny, as a concept for the NASA/DOE Joint Dark Energy Mission. Destiny is a 1.65m space telescope, featuring a near-infrared (0.85-1.7m) survey camera/spectrometer with a large flat-field Field Of View (FOV). Destiny will probe the properties of dark energy by obtaining a Hubble diagram based on Type Ia supernovae (SN) and a large-scale mass power spectrum derived from weak lensing distortions of field galaxies as a function of redshift.

We compare and contrast the Korsch (1972) full-field three-mirror anastigmat telescope (TMA) to the Korsch (1977) annular-field TMA. Both TMAs offer flat fields with comparably good aberration correction and comparably good telephoto advantage. Both offer good accessibility of the focal plane. The advantages of the FFTMA are its extremely uniform focal length over its field, its nearly telecentric final focus, and the fact that there is no hole in the center of its field. The advantages of the AFTMA are its complete accessible cold stop (essential if a warm telescope is to be used to image the sky at near-IR wavelengths) and its low sensitivity to mirror location error. Either alternative can deliver diffraction-limited visible-wavelength images over a one degree diameter field with a two meter aperture.

Pleiades is the last generation of French satellite for earth observation. For this space program, SESO has been awarded the contract (fully completed end 2006), for the manufacturing of the whole set of telescope mirrors (EM, QM and FM, primary mirror with 700mm CA). These works did also include the mechanical design, manufacturing and mounting of the attachment flexures (MFDs) between the mirrors and the telescope main structure. This presentation will be focused on the different steps of manufacturing and control of these mirrors, as well as a presentation of the existing SESO facilities and capabilities to produce such kind of aspherical components/sub-assemblies.

Presented are unique concepts for space telescopes and optics, based on carbon fiber reinforced polymer (CFRP) thin-shelled mirror technology. Thin-shell CFRP mirrors have been proven for IR and longer wavelengths and to a large extent, visible wavelength optics. The unique structural/mechanical and lightweight characteristics of thin shells open the design possibilities for advanced space telescopes with active/adaptive mirrors. Low weight and general ease of manufacturing of CFRP structures can result in reduced part-count and inexpensive lightweight telescopes for space applications. Three advanced mirror concepts will be presented in this paper, 1) Advanced stowage of thin-shell mirrors for segmented telescopes, 2) advanced deformable mirror concepts, and 3) simple and inexpensive fabrication concepts using simplified molding tools for space telescope mirrors. Also presented will be empirical data of CFRP thin-shell mirrors and composite structures produced supporting their use for space telescope applications.

The next generation of space telescopes will be required to meet very challenging science goals. In order to achieve these goals, the size of the primary mirror will need to be increased. However, since current telescopes are reaching their limits in terms of size and mass, new designs will require advanced technologies such as lightweight mirrors and active optical control. Traditional shape control of the primary mirror relies on feedback from a wavefront sensor located in the optical path. However, a wavefront sensor reduces the amount of light available for image formation. Therefore, to view very dim objects, it will be necessary to use a different type of sensor. In this work, a quasi-static shape control algorithm is developed to correct errors in the mirror due to thermal disturbances using only sensors embedded in the mirror. Control algorithms are presented for both embedded strain gages and temperature sensors. Finite element models of both a simple flat plate mirror and a rib-stiffened mirror are generated and analyzed using Nastran. The flat plate model, with surface-parallel actuation is used to compare the two algorithms. Following this, the parametric model for a rib-stiffened mirror is used to analyze the effects of the shape control algorithm as the mirror geometry is changed. It is shown that correction of a mirror can be achieved using these embedded sensors.

SNAP is a proposed space-based experiment designed to study dark energy and alternate explanations of the acceleration of the universe's expansion by performing a series of complementary systematic-controlled astrophysical measurements. The principal mission activities are the construction of an accurate Type Ia supernova Hubble diagram (the supernova program) and conducting a wide-area weak gravitational lensing (WL) survey. WL measurements require highly constant point spread function (PSF) second moments (ellipticity), and the aim of this study is to expand on the 2005 Sholl, et al. preliminary work, specifically via use of the Ball Aerospace integrated modeling tool, EOSyM (End-to-end Optical System Model). This modeling environment combines thermal, structural and optical effects, including alignment errors, manufacturing residuals and diffraction, in an integrated model of the telescope. Thermo-mechanically induced motions and deformations of the mirrors are modeled as well as other disturbances, and corresponding ellipticity variations of the PSF are quantified for typical operational scenarios. In this study, the effects of seasonal variations in solar flux, transients introduced when pointing the body-fixed Ka-band antenna toward Earth, 90° roll maneuvers (planned every three months of operations) and structure dimensional changes associated with composites desorption are quantified and introduced into the optical system. Uncertainty in the telescope ellipticity distribution may be reduced by examination of foreground stars within the field of view. Reference is made to ongoing work on the use of foreground stars in quantifying the PSF.

Primary mirror with Φ 1m and f 3.5m is the most important optical part in the space Main Optical Telescope (MOT). Since its required surface error is less than λ/40(rms.), where λ is about 0.6μm, the mirror deformation induced by space heat and gravity must be within 0.015μm, it's necessary to make thermal calculation and structural analysis to improve its structure. In this paper, the MOT structure and its finite element model is described. The mechanical properties are then analyzed in order to verify whether this structure can meet the optical requirements of sufficient strength, stiffness, and thermal stability. Mechanical analysis is carried out with MSC.Nastran software under 3 different load cases: gravity influence on-ground, dynamic impact during launching, weightlessness and heat environment in-orbit. Space thermal analyses are also done to simulate the space environment. The coupled deformation of heat and structure is finally analyzed. Calculation results show that different support ways and support forces will be the keys to determine the surface precision of primary mirror. The structure can meet the optical demands, but the thermal deformation can not, especially in an asymmetric temperature distribution, which should be tested and controlled by some strict methods.

Future space telescopes require larger apertures to continue to improve performance. However, balancing the large, high performance optics with the desire for lightweight systems proves quite challenging. One way to achieve both goals is to utilize active, on-orbit wavefront control. A promising method of wavefront control implementation is surface-parallel piezo-electric actuation. The primary mirror backplane is ribbed to provide increased stiffness even at very low areal densities, with piezo-electric actuators embedded at the top of each rib. When the piezo-electrics expand or contract, they bend the surface of the mirror and can be used to directly correct for dynamic distortions of the wavefront. In addition, rigid-body petal control can be used to allow for the possibility of systems with segmented primary mirrors. This paper examines the implementation of both the piezoelectric deformable mirror and petal wavefront controllers, along with their implications on both optical performance and stability robustness. The systems analyzed in this paper are integrated models of the entire space telescope system, considering the transmission of disturbances and vibrations from the reaction wheels in the bus through the structure, isolators, and bipods to the aperture. The deformable mirror control is performed using a Linear Quadratic Gaussian (LQG) controller, while the mirror segment control is performed using a positive position feedback (PPF) controller. For all cases, the wavefront error is the primary optical performance metric and is calculated using the Zernikes of the primary mirror. The major deterrents to the use of control are complexity and the loss of stability robustness. The integrated model allows for the calculation of all metrics together to enable the examination of the potential benefits of implementing dynamic wavefront control.

We have developed a new, adaptive cross-correlation (ACC) algorithm to estimate with high accuracy the shift as large as several pixels in two extended-scene images captured by a Shack-Hartmann wavefront sensor (SH-WFS). It determines the positions of all extended-scene image cells relative to a reference cell using an FFT-based iterative image-shifting algorithm. It works with both point-source spot images as well as extended scene images. We have also set up a testbed for extended-scene SH-WFS, and tested the ACC algorithm with the measured data of both point-source and extended-scene images. In this paper we describe our algorithm and present our experimental results.

Image-based wavefront sensing algorithms are being used to characterize the optical performance for a variety of current and planned astronomical telescopes. Phase retrieval recovers the optical wavefront that correlates to a series of diversity-defocused point-spread functions (PSFs), where multiple frames can be acquired at each defocus setting. Multiple frames of data can be co-added in different ways; two extremes are in "image-plane space," to average the frames for each defocused PSF and use phase retrieval once on the averaged images, or in "pupil-plane space," to use phase retrieval on each PSF frame individually and average the resulting wavefronts. The choice of co-add methodology is particularly noteworthy for segmented-mirror telescopes that are subject to noise that causes uncorrelated motions between groups of segments. Using models and data from the James Webb Space Telescope (JWST) Testbed Telescope (TBT), we show how different sources of noise (uncorrelated segment jitter, turbulence, and common-mode noise) and different parts of the optical wavefront, segment and global aberrations, contribute to choosing the co-add method. Of particular interest, segment piston is more accurately recovered in "image-plane space" co-adding, while segment tip/tilt is recovered in "pupil-plane space" co-adding.

Perhaps the most compelling piece of science and exploration now under discussion for future space missions is the direct study of planets circling other stars. Indirect means have established planets as common in the universe but have given us a limited view of their actual characteristics. Direct observation holds the potential to map entire planetary systems, view newly forming planets, find Earth-like planets and perform photometry to search for major surface features. Direct observations will also enable spectroscopy of exoplanets and the search for evidence of simple life in the universe. Recent advances in the design of external occulters - starshades that block the light from the star while passing exoplanet light - have lowered their cost and improved their performance to the point where we can now envision a New Worlds Observer that is both buildable and affordable with today's technology. We will summarize recent studies of such missions and show they provide a very attractive alternative near term mission.

We describe the TPF-O science program composed of interleaved imagery and spectroscopy of planets in the habitable zones of nearby stars. We give the rationale for the science program and argue that TPF-O offers the best approach to achieving the original goals set for the Terrestrial Planet Finder.

The Terrestrial Planet Finder-Occulter (TPF-O) is a proposed mission to find and characterize planets around nearby stars. It uses a telescope and an external occulter to suppress the starlight so that the planets close to the star can be observed. We have constructed Design Reference Missions (DRMs) that show that the TPF-O architecture can achieve the science requirements. A 4.0 meter telescope and occulter system should be able to find Earth-like planets in the equivalent search space of 42.7 continuous habitable zones (CHZ) and characterize the planets including detection of water (at 1000 ppm) and oxygen (at 21%) in the planet's atmosphere. With a smaller telescope (2.4 meter) and occulter, we can still probe 21.9 CHZs and detect water and oxygen in many of the planets detected.

This article was originally published online on 20 September 2007. The following errors were discovered by the authors after publication: missing author (Park J. McGraw) and missing references.

The New Worlds Observer (NWO) mission uses a large external occulter, or "starshade," to block the light from nearby stars and cast a deep shadow over the entrance aperture of a space telescope, enabling it to detect and characterize Exo-Solar Planets. Since these planets are intrinsically faint (30th to 32nd magnitude), the telescope must have a large aperture (2.4 to 4 meters) and the starshade must be large enough (25 to 50 meters) to create a shadow that is deep enough (108 to 1010 starlight suppression) and large enough (5 to 10 meters in diameter) to envelop the telescope. The telescope must also be far enough from the starshade (30,000 to 80,000 kilometers) that planets close to the star (50 to 65 milli-arc-seconds) are not occulted. Since the starshade's performance is inversely proportional to the wavelength of the starlight, the telescope must operate in the visible and near infrared. The telescope should also have a significant capability for general astrophysics observations, since it will have more than half its time available for other observations while the starshade is moving from one target to the next. This paper describes our conceptual design for the NWO telescope, including its instrument suite and operations concept. We note that in addition to comparative planetology studies and the detection and characterization of terrestrial planets, the telescope could provide a UV/Optical observing capability for the general astronomical community in the post-HST era.

The Terrestrial Planet Finder - Occulter (TPF-O) mission has two Spacecraft (SC) buses, one for a formation-flying occulter and the other for a space telescope. These buses supply the utilities (support structures, propulsion, attitude control, power, communications, etc) required by the payloads: a deployable shade for the occulter and a telescope with instruments for the space telescope. Significant requirements for the occulter SC bus are to provide the large delta V required for the slewing maneuvers of the occulter and communications for formation flying. The TPF-O telescope SC bus shares some key features of the one for the Hubble Space Telescope (HST) in that both support space telescopes designed to observe in the visible to near infrared range of wavelengths with comparable primary mirror apertures (2.4 m for HST, 2.4 - 4.0 m for TPF-O). Significant differences from HST are that 1) the TPF-O telescope is expected to have a Wide Field Camera (WFC) that will have a Field of View (FOV) large enough to provide fine guidance, 2) TPF-O is designed to operate in an orbit around the Sun-Earth Lagrange 2 (SEL2) point which requires TPF-O (unlike HST) to have a propulsion system, and 3) the velocity required for reaching SEL2 and the limited capabilities of affordable launch vehicles require both TPF-O elements to have compact, low-mass designs. Additionally, it is possible that TPF-O may utilize a modular design derived from that of HST to allow robotic servicing in the SEL2 orbit.

A multitude of coronagraphic techniques for the space-based direct detection and characterization of exo-solar terrestrial planets are actively being pursued by the astronomical community. Typical coronagraphs have internal shaped focal plane and/or pupil plane occulting masks which block and/or diffract starlight thereby increasing the planet's contrast with respect to its parent star. Past studies have shown that any internal technique is limited by the ability to sense and control amplitude, phase (wavefront) and polarization to exquisite levels - necessitating stressing optical requirements. An alternative and promising technique is to place a starshade, i.e. external occulter, at some distance in front of the telescope. This starshade suppresses most of the starlight before entering the telescope - relaxing optical requirements to that of a more conventional telescope. While an old technique it has been recently been advanced by the recognition that circularly symmetric graded apodizers can be well approximated by shaped binary occulting masks. Indeed optimal shapes have been designed that can achieve smaller inner working angles than conventional coronagraphs and yet have high effective throughput allowing smaller aperture telescopes to achieve the same coronagraphic resolution and similar sensitivity as larger ones. Herein we report on our ongoing modeling, simulation and optimization of external occulters and show sensitivity results with respect to number and shape errors of petals, spectral passband, accuracy of Fresnel propagation, and show results for both filled and segmented aperture telescopes and discuss acquisition and sensing of the occulter's location relative to the telescope.

New Worlds Observer (NWO) is a formation flying mission that combines a starshade with a telescope to study Earthlike exoplanets around neighboring stars. The general architecture consists of a telescope and detector that share one spacecraft platform pointed toward a nearby solar system. Planets in the solar system are revealed by blocking the bright star with a starshade, on its own spacecraft, positioned between the telescope and its target. Questions arise regarding the type of precision, tolerances, and diffraction control required when considering the practicality of such an endeavor. We address the generalities here by presenting an overview of requirements necessary for this type of system. Basic tolerances are described at both the mission and starshade level.

A new mission concept for direct imaging of exo-solar planets called New Worlds Observer (NWO) has been proposed. It involves flying a meter-class space telescope in formation with a newly-conceived, specially-shaped, deployable star-occulting shade several meters across at a separation of some tens of thousands of kilometers. The telescope would make its observations from behind the starshade in a volume of high suppression of incident irradiance from the star around which planets orbit. For an efficacious mission, the required level of irradiance suppression by the starshade is of order 0.1 to 10 parts per billion in broadband light. We discuss an experiment to accurately measure the irradiance suppression ratio at the null position behind candidate starshade forms to these levels. We also present results of broadband measurements which demonstrated suppression levels of less than 100 parts per billion in air using the Sun as a light source. A simulated spatial irradiance distribution surrounding the null from an analytical model developed for starshades is compared with a photograph of actual irradiance captured in situ behind a candidate starshade.

The concept of flying an occulting shade in formation with an orbiting space telescope to enable astronomical imaging of faint targets while blocking out background noise primarily from starlight near distant Earth-like planets has been studied in various forms over the past decade. Recent analysis has shown that this approach may offer comparable performance to that provided by a space-based coronagraph with reduced engineering and technological challenges as well as overall mission and development costs. This paper will present a design of the formation flying architecture (FFA) for such a collection system that has potential to meet the scientific requirements of the National Aeronautics and Space Administration's (NASA's) Terrestrial Planet Finder mission. The elements of the FFA include the relative navigation, intersatellite communication, formation control, and the spacecraft guidance, navigation, and control (GN&C) systems. The relative navigation system consists of the sensors and algorithms to provide necessary range, bearing or line-of-sight, and relative attitude between the telescope and occulter. Various sensor and filtering (estimation) approaches will be introduced. A formation control and GN&C approach will be defined that provides the proper alignment and range between the spacecraft, occulter, and target to meet scientific objectives. The state of technology will be defined and related to several formation flying and rendezvous spacecraft demonstration missions that have flown.

The objective of the Synthetic Imaging Formation Flying Testbed (SIFFT) is to develop and demonstrate algorithms for autonomous centimeter-level precision formation flying. Preliminary tests have been conducted on SIFFT at the Flat Floor facility at NASA's Marshall Space Flight Center (MSFC). The goal of the testing at MSFC was to demonstrate formation reconfiguration of three "apertures" by rotation and expansion. Results were very successful and demonstrate the ability to position and reconfigure separate apertures. The final configuration was with three satellites floating in an equilateral triangle. The two Follower satellites expand the formation with respect to the Master satellite, which executes a 10° rotation. Testing was performed successfully under various initial conditions: initial Follower rotation, initial Follower drift, and initial significant position error of each Follower. Results show roughly 10cm steady state error and ±5cm precision. Formation capturing technique, where satellites search for each other without prior knowledge of the position of the other satellites, were also developed and demonstrated both on the 2D flat table and in the 3D International Space Station environment. Future work includes using a minimum set of beacons for estimation and implementing a search algorithm so satellites can acquire each other from any initial orientation.

The Synchronized Position Hold Engage and Reorient Experimental Satellites (SPHERES), developed by the MIT Space Systems Laboratory, enable the maturation of control, estimation, and autonomy algorithms for distributed satellite systems, including the relative control of spacecraft required for satellite formation flight. Three free-flyer microsatellites are currently on board the International Space Station (ISS). By operating under crew supervision and by using replenishable consumables, SPHERES creates a risk-tolerant environment where new high-risk yet high-payoff algorithms can be demonstrated in a microgravity environment. Through multiple test sessions aboard the ISS, the SPHERES team has incrementally demonstrated the ability to perform formation flight maneuvers with two and three satellite formations. The test sessions aboard the Space Station include evaluation of coordinated maneuvers which will be applicable to interferometric spacecraft formation missions. The satellites are deployed as a formation and required to rotate around a common center about a given axis, mimicking an interferometer. Various trajectories are then implemented to point the synthetic aperture in a different orientation by changing the common axis of revolution. Observation-time optimizing synchronization strategies and fuel balancing/fuel optimizing trajectories are discussed, compared and evaluated according to resulting mission duration and potential scientific output.