With the discovery of galaxies that existed when the universe was very young, of planets not in our own solar system, and with the tantalizing evidence that he conditions for life may have existed within our solar system on planets or moons outside of the earth system, the pat year has seen an explosion of interest in astronomy. In particular, a new era of exploration and understanding seems imminent, where the connection between the existence for the conditions of life will be connected to the origin of galaxies, stars and planets within the Universe. Who knows where this quest for knowledge will take us.

As part of NASA's Origins theme, the Next Generation Space Telescope will investigate the origin of galaxies, stars, and planets, using IR observations with a cooled telescope. Located at L2 or farther from the Earth, it will be protected from near-Earth hazards and will be radiatively cooled to allow background limited observations. The scientific goals were described in the 'HST and Beyond report,' and the proposed NGST approach in 'Visiting a Time When Galaxies Were Young.' It is clear that an 8m class telescope in deep space would be a tremendous tool that would lead to surprising discoveries. It will also require revolutionary changes in technology and management approaches, science budget must be small compared with Hubble Space Telescope. A major challenge is to develop a scientific performance metric that represents a consensus on the importance of various engineering parameters, like accuracy, field of view, sensitivity, spatial and spectral resolution, temperature, vibration, stability, and so forth. Such a metric could be used for choosing instrument or telescope configurations, or for selecting or paying a contractor in the performance based contracting approach now in vogue. Ideally, it could also be used for optimizing the cost of the mission, ensuring that effort is proportionate to benefit. The scientific, mathematical, and social aspects of our approach will be reported.

The Next Generation Space Telescope (NGST) Design Reference Mission (DRM) represents a suite of potential astronomical programs and targets along with their expected physical properties, and desired observation modes. This broad science program is being used to drive the observatory design in a way as fundamental as traditional engineering parameters. Astronomers use the DRM to communicate their desires in a quantitative fashion to the engineers who will eventually construct the observatory. The DRM is also the primary tool used to measure the relative value of NGST mission architectures and technological readiness of the program. Specifically, the fraction of the DRM completed by a given observatory configuration in a given time is, to first order, a measure of the value of the design. Those designs which complete a higher fraction of the observations listed below are more capable than those complete lesser fractions.

The Next Generation Space Telescope (NGST) is currently being designed during a pre-phase A study in cooperation with the NASA Goddard Space Flight Center (GSFC) led team. Ball Aerospace and Technologies Corporation is leading one of two competitive GSFC funded pre-phase A studies of mission architectures for NGST. The NGST is the next major NASA astronomy mission following HST and SIRTF. NGST will be an observatory providing zodiacal light background limited 1-5 micron imagery and spectroscopy using a passively cooled 6 to 8 meter diameter telescope. The stretch goals specify imagery and spectroscopy in the 5 to 30 micron region and shortward of 1 micron. While emphasizing science return and maintaining simplicity, we have arrived at a configuration of a highly reliable mission architecture that achieves the desired science including many of the stretch science goals. This paper will describe the process and major trades we used to arrive at the system configuration, our current configuration and some of the key remaining trade-offs.

The Next Generation Space Telescope will extend astronomers vision of the early universe beyond the reach of the Hubble Space Telescope to much greater redshifts and luminosity distances. TRW is being funded by NASA to develop mission architectures that can achieve the NGST science goals, while still remaining within a rigid cost cap. This paper presents candidate NGST mission architectures and identifies the key enabling technologies which must be developed to realize them.

The Next Generation Space Telescope is intended to extend man's ability to observe the history of the universe to the time when galaxies were first forming. TRW is being funded by NASA to develop mission architectures that can achieve this and other science goals, while still remaining within a rigid cost cap. This paper presents the trade considerations for selecting the optimum NGST architecture. Mission architectures consist of combinations of technologies and mission design elements including different orbits and launch vehicles, aperture sizes, the wavelength bands, thermal control approaches, and observatory configurations. The systems engineering analyses involve defining and assessing the trade spaces for each possible architecture. For example, primary mirror approaches include single-piece, monolithic mirrors of about four meters diameter to much larger segmented mirrors employing advanced deployable technology. The performance of different configurations and mission architectures will be evaluated via end-to-end modeling, so that science utility will be balanced against system cost and risk for each technology. The trades presented suggest optimum directions for further NGST system studies.

The Next Generation Space Telescope (NGST) program is currently conducting a pre-phase A study to prove feasibility of design, show that the design can be implemented within cost guidelines, and meet the requirements of the science community. In an effort to achieve the science community's goals as outlined in the 'HST and Beyond' report, the NGST team has developed a Government 'yardstick' design of an 8-meter aperture segmented-mirror telescope which would be launched in an Atlas IIAS launch vehicle into the L2 orbit. This paper will discuss the design of the optical telescope assembly and the various issues and complications of designing lightweight optics to be placed in the environment that the NGST will encounter both during launch and during its mission at L2.

All current concepts for the NGST are innovative designs which present unique systems-level challenges. The goals are to outperform existing observatories at a fraction of the current price/performance ratio. Standard practices for developing systems error budgets, such as the 'root-sum-of- squares' error tree, are insufficient for designs of this complexity. Simulation and optimization are the tools needed for this project; in particular tools that integrate controls, optics, thermal and structural analysis, and design optimization. This paper describes such an environment which allows sub-system performance specifications to be analyzed parametrically, and includes optimizing metrics that capture the science requirements. The resulting systems-level design trades are greatly facilitated, and significant cost savings can be realized. This modeling environment, built around a tightly integrated combination of commercial off-the-shelf and in-house- developed codes, provides the foundation for linear and non- linear analysis on both the time and frequency-domains, statistical analysis, and design optimization. It features an interactive user interface and integrated graphics that allow highly-effective, real-time work to be done by multidisciplinary design teams. For the NGST, it has been applied to issues such as pointing control, dynamic isolation of spacecraft disturbances, wavefront sensing and control, on-orbit thermal stability of the optics, and development of systems-level error budgets. In this paper, results are presented from parametric trade studies that assess requirements for pointing control, structural dynamics, reaction wheel dynamic disturbances, and vibration isolation. These studies attempt to define requirements bounds such that the resulting design is optimized at the systems level, without attempting to optimize each subsystem individually. The performance metrics are defined in terms of image quality, specifically centroiding error and RMS wavefront error, which directly links to science requirements.

We describe the instrument package concept that we have investigated as part of the Goddard Space Flight Center study for NGST. It is composed of highly integrated, high performance cameras and spectrometers covering the spectral region from 0.6 to 30 microns and with a large field of view.the suite has been configured to reduce cost and complexity with no sacrifice in scientific merit. A common optical bench minimizes interfaces, a guiding system integrated in the science module makes use of the science cameras with minimal penalty to science, and all near IR instruments are built around the same detector module.

NASA has embarked on the development of the NGST. The NGST is envisioned to be a large aperture deployable IR telescope with sensitivity 1000 times greater than any currently existing or planned IR telescope. The scientific goals of NGST include imaging the earliest galaxies and proto- galaxies which formed following the 'big bang'. Several studies have concluded that the mission is feasible within the proposed cost if a well-planned, aggressive technology development effort is implemented early in the development phase. This paper present an overview of the technology program NASA is pursuing to provide the necessary technology to enable an exciting, affordable NGST mission to launch early in the next century.

The Space Telescope Imaging Spectrograph (STIS) is a second- generation instrument for the Hubble Space Telescope (HST), designed to cover the 115-1000 nm wavelength range in a versatile array of spectroscopic and imaging modes that take advantage of the angular resolution, unobstructed wavelength coverage, and dark sky offered by the HST. STIS was successfully installed into HST in 1997 February and has since completed a year of orbital checkout, capabilities that it brings to HST, illustrate those capabilities with examples drawn from the first year of STIS observing, and describe at a top level the on-orbit performance of the STIS hardware. We also point the reader to related papers that describe particular aspects of the STIS design, performance, or scientific usage in more detail.

The Space Telescope Imaging Spectrograph (STIS), installed into the Hubble Space Telescope (HST) during the second servicing mission (SSM) in February 1197, has undergone the required servicing mission orbital verification (SMOV). The overall sensitivity of STIS is summarized for direct imagery in the visible with the charge coupled device (CCD), the near UV multi-anode microchannel-array (NUV MAMA) and the far UV MAMA (FUV MAMA) detectors and likewise for the spectroscopic modes. The FUV MAMA has exceedingly low background. The NUV MAMA has a higher, temperature-dependent background due to window phosphorescence. The principle gains of the CCD over WFPCs for limiting imaging sensitivity are: high quantum efficiency, wide bandpass, low dark current and low readout-noise. The CCD, like the WFPC2 CCDs, must ge annealed periodically to heat the hot pixels generated by radiation hits. Throughput of all modes has been stable at the 1 percent level or better except for the far UV, where sensitivity is dropping slowly across the order, but more rapidly below the Lyman alpha, and beyond 150 nm. This loss in sensitivity may be due to contamination similar to that which affected the first generation HST instruments. The thermal environment for STIS is warmer than specified in the HST Interface Control Document with the result that the back end of the STIS optical bench is not under positive thermal control. Temperature swings occur due to the spacecraft solar orientation and also due to power cycling of the MAMA low voltage power supplies that are turned off during orbits that encounter the South Atlantic Anomaly. Some motion of spectral and direct image formats occurs on the detector that is correlatable with changing aft bulkhead temperature and changes in external heatloads. The MAMA detectors are capable of time-tagging photon events within 125 microsecond resolution. The Crab Pulsar was used as a time standard and demonstrates the desired performance.

NICMOS, the near IR camera and multi-object spectrometer was placed in the Hubble Space Telescope in February 1997. Since then it has ben carrying out and extensive program of scientific research. This paper presents the current status and performance of the instrument along with a sample of the observations that have been carried out to this date.

The Near IR Camera and Multi-Object Spectrometer (NICMOS), installed into the Hubble Space Telescope (HST) in February 1997, incorporates a coronagraphic imaging capability. The coronagraph is comprised of two optical elements. The camera 2 field divider mirror, upon which the HST f/24 input beam is imaged, includes a 170 micrometers diameter hole which contains approximately 93 percent of the encircled energy from a stellar Point Spread Function (PSF) at a wavelength of 1.6 micrometers . The coronagraphic hole lowers both the diffracted energy in the surrounding region by reducing the high spatial frequency components of the occulted core of the PSF< and down stream scattering. The geometrical radius of this occulting spot, when re-imaged through the camera 2 f/45 optics, is approximately 4 pixels at the detector focal plane. An oversized cold pupil-plane mask, with radial structures co-aligned with the HST secondary mirror spider, acts over the whole 19.1 inch by 19.2 field to further reduce the diffracted energy in the direction of the spider vanes. The absolute performance levels of the coronagraph were ascertained during the servicing mission observatory verification program. Using a differential imaging strategy we expect to achieve statistically significant detectors of sub-stellar companions at 1.6 micrometers with a (Delta) H of approximately 10 and separations as close as 0.5 inch. The NICMOS environments of nearby stars programs is exploiting this capability in systematic surveys of nearby, and young stars searching for brown dwarfs and giant planets, and protoplanetary disks around main-sequence stars.

The Advanced Camera for the Hubble Space Telescope has three cameras. The first, the Wide Field Camera, will be a high- throughput, wide field, 4096 X 4096 pixel CCD optical and I-band camera that is half-critically sampled at 500 nm. The second, the High Resolution Camera (HRC), is a 1024 X 1024 pixel CCD camera that is critically sampled at 500 nm. The HRC has a 26 inch X 29 inch field of view and 29 percent throughput at 250 nm. The HRC optical path includes a coronagraph that will improve the HST contrast near bright objects by a factor of approximately 10 at 900 nm. The third camera, the solar-blind camera, is a far-UV, pulse-counting array that has a relatively high throughput over a 26 inch X 29 inch field of view. The advanced camera for surveys will increase HST's capability for surveys and discovery by a factor of approximately 10 at 800 nm.

The advanced camera for surveys, designed to be installed in 1999 during the third servicing mission of the Hubble Space Telescope, is a high performance axial bay camera.

The Cosmic Origins Spectrograph is a new instrument for the Hubble Space Telescope that will be installed in 2002. It is designed for high throughput, medium resolution spectroscopy of point sources, allowing the efficient observation of numerous faint extragalactic and galactic UV sources. The primary science objectives of the mission are the study of the origins of large scale structure in the universe, the formation, and evolution of galaxies, and the origin of stellar and planetary systems and the cold interstellar medium.

The 'far IR and submillimeter telescope', (FIRST), is the fourth European Space Agency cornerstone mission in the current 'Horizons 2000' science program. FIRST will perform photometry and spectroscopy in approximately the 80-670 micrometers range in the far IR and submillimeter part of the spectrum.

We present a conceptual design of a future Japanese IR astronomical satellite: the HIII/L2 mission. We propose a 'warm launch' cooled telescope; the telescope is to be launched at ambient temperature and is to be cooled in orbit to 4.5K by a modest cryogenic cooler with the help of radiative cooling. Since liquid helium and hence a heavy vacuum vessel are not longer required, the warm launch design reduces the weight of the satellite dramatically. We propose to launch this satellite into a halo orbit around S- E L2, one of the Sun-Earth Lagrangian liberation points. The S-E L2 is an ideal orbit for IR astronomy, since (1) radiative cooling can become very effective, and (2) by the Japanese H-IIA launching vehicle. This mission focuses on high-resolution mid- to far-IR observations with unprecedented sensitivity, since the large aperture reduces confusion noise and the cooled optics suppresses instrumental background radiation. The HII/L2 mission is an ideal observatory-type platform to make follow-up observations to the ASTRO-F/IRIS survey mission. The target launch year is 2010.

The IR Imaging Survey (IRIS) is the second IR astronomy mission of the Institute of Space and Astronomical Science (ISAS). The IRIS is a 70 cm cooled telescope dedicated for IR sky survey. This project has been approved as ISAS's 21st science mission 'ASTRO-F', and prototype model development has been ongoing since 1997. The IRIS will be launched with ISAS's launch vehicle M-V, into a sun-synchronous polar orbit with an altitude of 750 km. The IRIS telescope has a 70 cm aperture and is cooled to 6K using Stirling-cycle coolers and liquid helium. The primary and secondary mirrors are light-weight mirrors make of silicon carbide. Two focal- plane instruments are installed. One is the far-IR surveyor (FIS) which will survey the entire sky in the wavelength range from 50 to 200 micron with angular resolutions of 30- 50 arcsec. The other focal-plane instrument is the IR camera (IRC). It employs large-format detector arrays and will take deep images of selected sky regions in the near and mid IR range. The field of view of the IRC is 10 arcmin and the spatial resolution is approximately 2 arcsec. The IRIS has much higher sensitivity than that of the IRAS survey. The detection limits are 1-100 micro Jy in the near-MID IR and 10-100 mJy in the far IR. With the IRIS survey, great progress is expected in the research on evolution of galaxies, formation of stars and planets, dark matter and brown dwarfs. The IRIS is now scheduled to be launched in early 2003.

This paper describes the design of the space IR telescope Facility (SIRTF) as the project enters the detailed design phase. SIRTF is the fourth of NASA's Great Observatories, and is scheduled for launch in December 2001. SIRTF provides background limited imaging and spectroscopy covering the spectral range from 3 to 180 micrometers , complementing the capabilities of the other great observatories - the Hubble Space Telescope (HST), the Advanced X-ray Astrophysics Facility, and the Compton Gamma Ray Observatory. SIRTF will be the first mission to combine the high sensitivity achievable forma cryogenic space telescope with the imaging and spectroscopic power of the new generation of IR detector arrays. The scientific capabilities of this combination are so great that SIRTF was designated the highest priority major mission for all of US astronomy in the 1990s.

The joint US and German SOFIA project to develop and operate a 2.5 meter IR airborne telescope in a Boeing 747-SP is now in its second year. The Universities Space Research Association, teamed with Raytheon E-Systems and United Airlines, is developing and will operate SOFIA. The 2.5 meter telescope will be designed and built by a consortium of German companies led by MAN. Work on the aircraft and the primary mirror has started. First science flights will begin in 2001 with 20 percent of the observing time assigned to German investigators. The observatory is expected to operate for over 20 years. The sensitivity, characteristics and science instrument complement are discussed.

The Stratospheric Observatory for Infrared Astronomy (SOFIA) is a joint NASA/DLR program in airborne infrared astronomy. The observatory consists of a 2.5 meter telescope mourned in a modified B747SP aircraft The telescope is being provided by a consortium of German contractors. Raytheon Systems Company (RSC), a subcontractor to Universities Space Research Association (USRA), will modify the B747SP, provide the Mission Controls and Communications System (MCCS), and provide the overall observatory integration at its Waco, Texas location. This paper will address the unique aspects of providing an FAA certified observatory piatform for a 44,000 pound telescope operating at altitudes between 41,000 and 45,000 feet. This paper will also provide an overview of the MCCS, both hardware and software functionality, and describe the tasks of integrating and verifying the TA and Aircraft operations. Finally, the paper will describe the operational limitations imposed on SOFIA by the reality of maintaining and flying a B747SP aircraft for the 20 year lifetime ofthe observatory.

The optical system of the airborne SOFIA telescope consists of a Cassegrain telescope with an effective aperture of 2.5 m and a so-called Nasmyth focus providing a lateral focus exit with access from the cabin side. The central optical part of the SOFIA Telescope is the 2.7m primary mirror made from a solid block of Zerodur as a monolithic element. The mirror will be lightweighted by making it to a dedicated 'double-arch' shape and by milling hexagonal holes from the backside. The lightweighting factor will be approximately 80 percent yielding a mirror mass of 850 kg only. The secondary mirror has a high-performance chopper actuator enabling an efficient background suppression especially for far IR observations. The tertiary mirror is implemented as a dichroic beamsplitter transmitting the visible part of the incoming radiation. This part is fed by an additional mirror to one of the tracking images, the focal plane imager, which allows a high-precision pointing of the star field under observation in the sub-arcsecond range. The two other imagers, the wide field imager and the fine field imager, are boresighted to the main telescope and will be used for the acquisition of the star field as well as for pointing and tracking of the telescope. The paper presents the current status of the development of the optical system including the imagers.

The US Air Force Research Lab (AFRL) has integrated several technology development efforts together to form a cohesive approach for enabling deployable optical systems in the future. Aperture size dominates the cost/architecture trades for space based laser systems for missile defense and tactical imaging system pursuing broad area coverage with local access. Larger apertures allow both systems to consider higher orbits, offering greater fields of regard. However, large monolithic apertures quickly run into launch vehicle faring volumetric and throw mass constraints. Several technologies may enable space deployable of optical segments to form a large primary mirror at a reduced mass, circumventing the launch vehicle constraints. However, to produce an optically phased wavefront, a combination of technologies, deployment mechanisms, lightweight structures and mirrors, mirror mount isolators and actuators, adaptive optics, and processing techniques, must be applied in concert. While this paper concentrates on the hardware development activities under the UltraLITE program, namely the Precision Deployable Optical Structure ground demonstration and the brassboard Deployable Space Telescope, it will also briefly cover and provide references to related technology programs on-going at the AFRL.

The Far UV Spectroscopic Explorer (FUSE) satellite will make high spectral resolving power measurements of astrophysical objects in the 905-1187 angstrom wavelength region from low- earth orbit. Its high effective area and low background will permit observations of solar system, galactic, and extragalactic targets that have been too faint for previous instruments at this high resolution. Integration and test of the FUSE instrument is currently underway in preparation for launch in late 1998. We describe the current status of the FUSE satellite, including details of the optical and mechanical measurements made during component and subsystem- level testing. In addition, we make an estimate of instrument on-orbit performance from data obtained during instrument integration and test.

A large area telescope of diameter 30 meters may be constructed as a set of multiple parallel coelostats, with flat mirrors feeding into a single telescope forming multiple images. These images may then be recombined into a single image. The flat mirror array must be separated by over a kilometer from the combining telescope. In space this requires that the two portions should preferably maintain a fixed direction and similar spacing between them during an exposure. A low consumable method suitable for a Lagrangiun halo orbit would be to use solar radiation pressure for controlled balance relative to the gravitational field for station keeping, and for attitude control. Arrangements of controllable solar sails are discussed and shown to have sufficient thrust.

A concept of 'Space Factory' on the International Space Station Alpha (ISSA) is described. Following the four great observatories deployed by the Space Transportation System (STS), the next generation of great observatories would require a very large, 10-meter class optical telescope. A telescope of this size will require careful assembly and tuning by astronauts on orbit before deployment. Once built, it could visualize the universe to the earliest galaxies, and could explore the earth-like planet in other star- system. The 'Space Factory' is conceived by including four or five frontier astrophysics programs. Less demanding experiments could precede the construction of the most demanding optical telescope. Space SUBARU is a 10 meter- diameter optical telescope with a diffraction limited optics. Space-Submillimeter-and-IR-Telescope is a 20 meter- diameter sub-millimeter telescope. A 10-meter-cube telescope is for observing gamma-rays from 1 GeV to 10 TeV. The Multiple-OWL is an earth's night-sky-watcher for the highest energy cosmic rays. Space SUBARU envisages a plan of orbital construction, fine-tuning and deployment of large scale astrophysical instruments into the desired free-flying orbit. It incorporates physical aids of the robotics and extra-vehicle activities of astronauts.

The detection of exo-Zodiacal discs is an important step in the NASA program for the detection of exo-solar planets, in particular, the detection of earth-like planets. We show that by incorporating a nulling interferometer NGST would be very well suited to study exo-zodiacal disks in nearby stars. Over 400 stars could be surveyed and at least 40 could be resolved such that structural parameters on the 2-3 AU scale could be measured. This can be done within the existing optical NGST design but demands a wavefront RMS error less than (lambda) /100 at 10 micron in order to assure a wavefront cancellation of 1000. In addition, a new concept for an off-axis NGST design is discussed.

We report on the design of a single 1.5-2.0-meter orbiting telescope capable of direct imaging of Jupiter-class planets around solar-type stars at a distance of up to 10 parsecs. Unlike previous designs, our telescope would operate in the visible spectrum, measuring reflected light to maximize angular resolution. The telescope would achieve a contrast of up to 10 orders of magnitude by utilizing a combination of superpolished low-scattering mirror surfaces, Gregorian telescope design with field and Lyot stops, and low-loss apodization to control diffraction. Calculations indicate that the system should reliably detect Jupiter-class planets around solar-class stars at angular separations of 0.5 arcsec. Such a single-telescope system would fit within the constraints of a Discovery-level NASA program and would serve as a pathfinder for large space-based interferometric systems.

With the detection of giant extrasolar planets and the quest for life on Mars, there is heightened interset in finding earth-class planets, those that are less than ten earth masses and might be life supporting. A space-based photometer has the ability to detect the periodic transits of earth-class planets for a wide variety of spectral types of stars. From the data and known type of host star, the orbital semi-major axis, size and characteristic temperature of each planet can be calculated. The frequency of planet formation with respect to spectral type and occurrence for both singular and multiple-stellar systems can be determined. A description is presented of a one-meter aperture photometer with a twelve-degree field of view and a focal plane of 21 CCDs. The photometer woudl continuously and simultaneously monitor 160,000 stars of visual magnitude <EQ 14. Its one-sigma system sensitivity for a transit of a 12th magnitude solar-like star by a planet of one-earth radius would be one part in 50,000. It is anticipated that about 480 earth-class planets would be detected along with 140 giant planets in transit and 1400 giant planets by reflected light. Densities could be derived for about seven case where the planet is seen in transit and radial velocities are measurable.

Natural occultations have long been used to study sources on small angular scales, while coronographs have been used to study high contrast sources. We propose launching the Improved Resolution and Image Selection (IRIS) Satellite, a large steerable occulting satellite. IRIS will have several advantages over standard occulting bodies. IRIS woudl block over 99.8 percent of the visible light from an occulted point source. Because the occultation occurs outside both the telescope and the atmosphere, seeing and optical imperfections do not degrade this performance. If placed in Earth orbit, integration times of 160-1600 s can be achieved from most major telescope sites for objects in over 90 percent of the sky. Alternately, IRIS could be combined with a 2-4 meter space telescope at the Earth-Sun L2 point to yield very long integration times. Applications for IRIS include direct imaging of planets around nearby stars, and resolution of micro-lensed images of LMC and Galactic bulge stars into distinct image pairs. Resolution of microlensed stars would greatly improve our understanding of the massive compact halo objects comprising 20-90 percent of the mass of our galaxy. Direct imaging of planets, would enhance our understanding of star formation, formation of planetary systems, and perhaps ultimately help evaluate the probability of extraterrestrial life.

We have investigated the design of a small astrometric and photometric survey instrument in the Hipparcos tradition. Such a mission will support a rich and diverse ensemble of scientific investigations. The design objectives, which have been met in this study, are to be able to measure 10⁷ stars over the full sky, with an accuracy of 0.05 mas for mag < 9 and 20 mas for mag 15. A scanning survey instrument that uses CCD detectors is able to measure many stars simultaneously. As compared to a pointed astrometric instrument of comparable size, the survey instrument generally has much higher measurement throughput, but on average, less scientific interest per target. An instrument for astrometry, unlike those for imaging, can be compact and yet scientifically productive.

The UV Imaging Telescope is a part of the proposed Indian Multiwavelength Astronomy Satellite mission. The initiate for an astronomy mission has ben taken by several institution in India, after the successful demonstration of satellite and launch technologies. A base level proposal has been prepared that envisages two major payloads on a platform that can be launched into a low inclination orbit wit an altitude between 500 to 700 km. Apart from the UV telescope, the other major payload would be an x-ray proportional counter with total area in the range 4000-5000 squ cm. The goal for the mission lifetime is five years. The primary objective of the UV Imaging Telescope is an all-sky survey in two bands with the wavelength region 120 to 300 nm. Initially, alternate telescope configurations are being considered: the aperture of the telescope will be 50 to 60 cm and there will be two parallel channels for observations, one for the range 120-190 nm and the other longwards of 190 nm. The goal for spatial resolution on the sky over a full field of 2 degrees is 3 to 4 seconds of arc, depending on the detector. It is expected that the three axis stabilized platform will be able to achieve a pointing stability of better than 2 seconds of arc. The detectors of choice are photocathode, MCP and readout anode combinations for the two wavebands; the use of CCD detectors has, however not been ruled out at this stage. Each channel will have a filter changing mechanism, with a provision for up to 4 full size filter and up to 12 small filters. Though there are some science drivers for grism field spectroscopy, the design aspects of this are not specified at this stage. The all-sky survey is expected to reach a UV limiting magnitude of 20 depending on the detector(s) chosen. After the all sky survey, deeper surveys over selected regions of the sky are planned. Scheduling of guest observer targeted programs will also be done, interspersed with programs for the x-ray payload. The launch of the mission is expected between the years 2002 and 2004.

A telescope to follow the HST does not necessarily have to be a lot bigger to make a dramatic advance in the observing capability. Provided it is cooled to allow exploration the 2-4 micrometers window of very dark sky background, and has several times the 0.85 m aperture of SIRTF, it would have already unique capability to resolve and study the crowded fields of the most strongly red-shifted, distant galaxies. Such a telescope could test many of the advanced technologies needed for the mirror elements in future deployed telescope or interferometers of very large size.

The next generation of space telescopes will require primary mirrors that push beyond the current state of technology of mirror fabrication. These mirrors are large, up to 8 meters in diameter, have low mass per unit area, less than 15 kg/m² and must maintain diffraction limited performance at cryogenic temperatures. To meet these requirements, have developed an active mirror that has a thin membrane as the optical surface, which is attached to a stiff lightweight support structure through a set of screw-type actuators. This system allows periodic adjustments with the actuators to maintain the surface figure as measured from star light. The optical surface accuracy and stability are maintained by the active system, so the support structure does not have to be optically stable and can be made using light weight carbon fiber laminates to economically provide stiffness. The key technologies for implementing this technology are now in place. We have performed two critical demonstrations using 2-mm glass membranes--diffraction limited optical performance of a 0.5-m diameter mirror and launch survival of a 1-m diameter mirror. We have also built and tested a prototype actuator that achieves 25 nm resolution at cryogenic temperatures. We are now building a 2-m mirror as a prototype for the Next Generation Space Telescope. This mirror will have mass of only 40 kg, including support structure, actuators and control electronics. It will be actively controlled and interferometrically measured at 35 K.

Fabrication of lightweight mirrors from low expansion glasses can be achieved using several techniques. Simple machining can reduce the weight of a typical solid mirror blank by up to 50 percent. Even more weight reduction can be realized through a sandwich type assembly of a lightweighted central core and two faceplates. The central core can be fabricated by cutting out cells form a solid blank using traditional fixed abrasive diamond grinding or abrasive waterjetting technology, or by fusion bonding thin struts into a honeycomb structure. The resultant lightweight core can then be bonded to mirror faceplates using either a fusion or frit bonding process. Each mirror fabrication approach offers its own advantages and disadvantages in term so weigh reduction, design flexibility, manufacturing time, and cost. These factor will be discussed, along with Corning's current size capability for each techniques.

A very low mass cryogenic telescope of moderate size is needed for the Space IR Telescope Facility (SIRTF). We evaluated multiple concepts for the JPL's SIRTF precursor, the 0.85 m IR telescope technology testbed with an emphasis on simultaneously achieving excellent image quality, minimum mass, and design simplicity. We selected an all-beryllium approach over one employing either silicon carbide or fused quartz mirrors. Based upon recent advances in beryllium powder metallurgy, including techniques for the reduction of residual stress, we are demonstrating that the telescope when cooled to 5 kelvins is capable of simultaneously meeting both the 6.5-micrometers diffraction-limited image quality requirement and the 30-kg mass goal. The design employs very few components and uses a single arch mirror to minimize telescope mass and simplify cryogenic mirror design, analysis, and testing. The telescope's inherently stiff metering tower combines the functions of secondary mirror support and stray light baffling. We describe the design and the trades by which we arrived at a final configuration.

An 85 cm aperture beryllium mirror was fabricated as part of the IR Telescope Technology Testbed (ITTT), a facility to which the SIRTF flight telescope will be traceable. The ITTT was developed to demonstrate that diffraction-limited performance at a wavelength of 6.5 micrometers is attainable from an ultra-lightweight meter-class beryllium telescope operating at a temperature of 5.5K. Cryo-null figuring was employed to meet the requirements for the shape of the primary mirror at its operating temperature over an aperture of 79cm. The results of this process will be presented, including the repeatability of the surface through cryogenic temperature cycling. Modeling of system performance using the residual figure error will be described. Image-based methods were used to characterize a turned up edge that is too steep to be measured with an interferometer.

The objective of this paper is to report the recent developments in lightweight mirror technology that have occurred at Composite Optics, Inc. The developments occurred as a result of the activities being conducted in support the Next Generation Space Telescope, microwave limb sounder, and small business innovative research programs. Our sponsors on these programs are the Marshall and Goddard Space Flight Centers and the JPL. The requirements, design approach, actual performance, and the technology status for each program are summarized in the following sections. The emergence of composite designs provides exciting potential for nontraditional, accurate, lightweight, stable, stiff, and high strength composite mirrors. This evolving technology promises significant improvement in reducing weight, cost and cycle time for future IR, visible, and ex- ray systems. Customers currently embracing composite mirror technology for radiometric use are already reaping substantial system performance benefits. Other customers interested in LIDAR, IR, visible, and grazing incidence x- ray applications are eagerly awaiting successful completion of current technology development and demonstration efforts.

An intensive investigation has been carried out to study the surface profiles obtained as a result of the large deformations of pressurized membranes. The study shows that the inflated membrane shapes may have the requisite surface accuracy for use in future large space apertures. Both analytical and experimental work have been carried out. On the analytical side, the classical work of Hencky on flat circular membranes was extended to eliminate the limitations it imposed; namely a lateral non-follower pressure with no pre-stress. The result is a computer program for the solution of the pressurized circular membrane problem. The reliability of the computer program is demonstrated via verification against FAIM, a nonlinear finite element solver developed primarily for the analysis of inflated membrane shapes. The experimental work includes observations made by Veal on the (W-shaped) deviations between the membrane deflected shape and the predicted profile. More recent measurements have been made of the deformations of pressurized flat circular and parabolic membranes using photogrammetric techniques. The surface error quantification analyses include the effect of material properties, geometric properties, loading uncertainties, and boundary conditions. These effects are very easily handled by the special FEM code FAIM which had recently been enhanced to predict the on-orbit dynamics, RF, and solar concentration characteristics of inflatable parabolic antennas/reflectors such as the IAE that flew off the space shuttle Endeavour in May 1996. The results of measurements have been compared with analyses and their ramifications on precision-shape, large-aperture parabolic space reflectors are discussed. Results show that very large space apertures with surface slope error accuracies on the order to space reflectors are discussed. Results show that very large space apertures with surface slope error accuracies on the order of 1 milliradian or less are feasible. Surface shape accuracies of less than 1 mm RMS have been attained on ground measurements.

The Next Generation Space Telescope will depart from the traditional means of providing high optical quality and stability, namely use of massive structures. Instead, a benign orbital environment will provide stability for a large, flexible, lightweight deployed structure, and active wavefront controls will compensate misalignments and figure errors induced during launch and cool-down on orbit. This paper presents a baseline architecture for NGST wavefront controls, including initial capture and alignment, segment phasing, wavefront sensing and deformable mirror control. Simulations and analyses illustrate expected scientific performance with respect to figure error, misalignments, and thermal deformation.

Holographic correction of low-quality telescopes is an inexpensive method of obtaining large aperture devices suitable for lidar, imaging, and directed energy weaponry. We present an analysis of two different methods for producing diffraction-limited telescopes from the holographic correction of spherical mirrors. These evaluations are essential for choosing the optimal design for a given telescope application. Included in our discussion are the results from preliminary experiments into the various designs. The aim of the project is construct the first ever holographically corrected astronomical telescope for both ground and space-based operations.

Inflatable optics have the potential for large reduction in launch mass and volume, but they involve significant challenges to achieve the wavefront accuracy required for diffraction-limited operation in the visible and near IR. Current studies identify two major subsets of this topic: 1) inflation-deployed structures with a monolithic, but rolled, hyper-thin primary mirror and 2) an inflatable structure and inflatable membrane primary mirror. We address the current state of the art, the challenges involved, and a program development plan.

Both top-down and bottom-up cost algorithms have been generated for deployable and/or adaptive spaceborne optical systems. These models are anchored in real hardware, and can be used to estimate the cost of deployable optics at various levels of system decomposition. Recently, we have exercised these models to generate NGST cost estimates.

The hypothesis is tested: space telescopes with apertures larger than a few meters will have lower mass and cost and better optical performance if the primary mirror is aggressively segmented. Optical performance variations are considered from several factors including the gap between regular hexagonal mirror segments, the relative ability of different size to be manufactured with low wavefront error, and expected mirror deformations. A mass variation is derived to relate diameter and thickness of the mirror segments to satisfy mirror deflections and thermally induced stress. Mass estimation includes support structures, actuators, cabling, electronics, hinges, and latches. Cost is evaluated from several models previously proposed to address multiple mirror systems. The analyses conclude that there is a relatively-small optimum segment size that is independent of the dimensions of the overall array but which does depend upon the state of technology. It is further shown that a significant mass penalty will be incurred for segments that are either smaller or larger than the optimum size. Minimum mirror thickness is constrained, but engineering design principles for structural deflections and model frequencies otherwise dictate the design.

The High Resolution Camera (HRC) is a microchannel plate based, photon counting, imaging detector for the Advanced X- ray Astrophysics Facility (AXAF) that will be placed in a high earth orbit scheduled for launch in December, 1998. An end-to-end calibration of the HRC and the AXAF High Resolution Mirror Assembly was carried out at the Marshall Space Flight Center's X-ray Calibration Facility. This activity was preceded by various subsystems level calibrations of the detector components, but only through complete end-to-end testing was it possible to fully study the instrument and identify areas for improvement. As a result, several modifications were made to the HRC. These were followed by a series of flat field calibrations used to 'correct' the end-to-end results for flight.

The Multiband Imaging Photometer for SIRTF (MIPS) provides the space IR telescope facility (SIRTF) with imaging, photometry, and total power measurement capability in broad spectral bands centered at 24, 70, and 160 micrometers , and with low resolution spectroscopy between 50 and 95 micrometers . The optical train directs the light from three zones in the telescope focal plane to three detector arrays: 128 by 128 Si:As BIB, 32 by 32 Ge:Ga, and 2 by 20 stressed Ge:Ga. A single axis scan mirror is placed at a pupil to allows rapid motion of the field of view as required to modulate above the 1/f noise in the germanium detectors. The scan mirror also directs the light into the different optical paths of the instrument and makes possible an efficient mapping mode in which the telescope line of sight is scanned continuously while the scan mirror freezes the image motion on the detector arrays. The instrument is designed with pixel sizes that oversample the telescope Airy pattern to operate at the diffraction limit and, through image processing, to allow superresolution beyond the traditional Rayleigh criterion. The instrument performance and interface requirements, the design concept, and the mechanical, optical, thermal, electrical, software, and radiometric aspects of MIPS are discussed in this paper. Solutions are shown to the challenge of operating the instrument below 3K, with focal plane cooling requirements done to 1.5K. The optical concept allows the versatile operations described above with only a single mechanism and includes extensive self-test and on- board calibration capabilities. In addition, we discuss the approach to cryogenic end-to-end testing and calibration prior to delivery of the instrument for integration into SIRTF.

Segmented apertures are playing an increasingly important role in telescope and beam director design as aperture sizes increase an d as active optics are employed to overcome wavefront distortions due to atmospheric effects and to mirror and structural deformations. This paper provides a comprehensive look at the effects of phase errors and of the contributions of edges and gaps so as to provide guidance for controlling optical losses. In particular, while diffraction from the segments is smeared by turbulence with ground-based systems, it persists with space paths, and the resulting multiple images of a single source can be misinterpreted as dim objects. The effects have been modeled in the past, but finite numerical integration scales can confuse the issue by producing mathematical artifacts. Closed-form solutions have therefore been generated for particular cases to facilitate interpretation, and guidelines have been developed from them to assist prediction with more general situations.

Sounding rocket observations of G191-B2B are planned for 1999 January over the bandpass 256-304 angstrom with a high resolution spectrograph. The optical system implements two diffraction gratings with broadband multilayer coatings in a Wadsworth mount. Since the spectral resolution of the experiment will be signal limited, preliminary reflectivity test on test gratings have been performed to determine how high the resolution might be.

We describe a new technique of space based spectral imagery for a spinning platform. The technique uses tomographic inversion to produce a 2D spectral image using a novel imaging spectrograph. The spectrograph uses a single optical element with a large field of view. Our technique delivers high throughput due to continuous observation of the scene at all wavelengths. The challenge of spectral imaging is to obtain 3D information from a time series of 2D data. In our technique, we obtain spectral information along one detector dimension, while two dimensions of spatial information are combined into the second dimension of the detector for each time step. With the spin axis of the spacecraft located at the center of the scene, we recover the 2D of spatial information from a series of these individual 'snapshots'. We will report on the results obtained on May 8, 1997 by a sounding rocket experiment using this technique in the 80- 140nm wavelength range, on the Scorpio constellation.

The wide-field camera in the advanced camera for surveys (ACS) produces very large 4096 by 4096 pixel images. We will have on-board image compression in order to reduce both the storage requirements at the telescope an the time to transmit the data to the ground. This is the first time on- board compression has been included in a Hubble Space Telescope instrument. We have developed a new lossless image compression algorithm that is designed to compress the CCD data by factors of 2 to 3.5 with the minimum possible computational load on the ACS computer. The new algorithm takes differences of adjacent pixels and then compresses the difference in pairs, producing output codes of 1, 2 or 4 bytes for each pair. The pair-coding algorithm gives slightly inferior compression to the Rice algorithm but is more than three times faster than Rice on our computer. The Rice algorithm was unfortunately too slow for us to use, but the pair-coding algorithm is fast enough to handle high data rates even on our 16-MHz 80386 computer. This paper will describe the detail of the compression algorithm and its implementation in the ACS flight software. Important implementation problems include the unpredictable data volume after compression and the need to compress four independent data streams during readout from the Wide Field Camera CCDs. We will also describe the compression performance of the new algorithm on various types of astronomical images.

The Next Generation Space Telescope will provide at least ten times the collecting area of the Hubble Space Telescope in a package that fits into the shroud of an expendable launch vehicle. The resulting large, flexible structure provides a challenge to the design of a pointing control system for which the requirements are at the milli-arcsecond level. This paper describes a design concept in which pointing stability is achieved by means of a nested-loop design involving an inertial attitude control system (ACS) and a fast steering mirror (FSM). A key to the integrated control design is that the ACS controllers has a bandwidth well below known structural modes and the FSM uses a rotationally balanced mechanism which should not interact with the flexible modes that are within its control bandwidth. The ACS controller provides stable pointing of the spacecraft bus with star trackers and gyros. This low bandwidth loop uses nearly co-located sensors and actuators to slew and acquire faint guide stars in the NIR camera. This controller provides a payload reference stable to the arcsecond level. Low-frequency pointing errors due to sensor noise and dynamic disturbances are suppressed by a 2-axis gimbaled FSM locate din the instrument module. The FSM servo bandwidth of 6 Hz is intended to keep the guide star position stable in the NIR focal plane to the required milli-arcsecond level. The mirror is kept centered in its range of travel by a low-bandwidth loop closed around the ACS. This paper presents the result of parametric trade studies designed to assess the performance of this control design in the presence of modeled reaction wheel disturbances, assumed to be the principle source of vibration for the NGST, and variations in structural dynamics. Additionally, requirements for reaction wheel disturbance levels and potential vibration isolation subsystems were developed.

Third in the series of NASA great observatories, the AXAF is scheduled for launch from the Space Shuttle in November of 1998. Following in the path of the Hubble Space Telescope and the Compton Gamma Ray Observatory, this observatory will image light at x-ray wavelengths, facilitating the detailed study of such phenomena as supernova and quasars. The AXAF project is sponsored by the NASA Marshall Space Flight Center in Huntsville, Alabama. Because of exacting requirements on the performance of the AXAF optical system, it was necessary to reduce the transmission of reaction wheel jitter disturbances to the observatory. This reduction was accomplished via use of a passive mechanical isolation system to interface the reaction wheels with the spacecraft central structure. In addition to presenting a description of the spacecraft, the isolation system, and the key image quality requirement flowdown, this paper details the analyses performed in support of system-level imaging performance requirement verification. These analyses include the identification of system-level requirement suballocations, formulation of unit-level isolation system transmissibility requirements, and quantification of imaging performance. Given in comparison to the non-isolated system imaging performance, the result of these analyses clearly illustrate the effectiveness of an innovative reaction wheel passive isolation system.

The SIRTF requires a visible light sensor at its focal plane to 1) calibrate the alignment between the externally mounted star trackers and the telescope boresight; 2) to establish the correspondence between the telescope coordinate system and the absolute J2000 reference frame; and 3) to provide starting attitudes for high accuracy absolute offset maneuvers. The Pointing Calibration and Reference Sensor (PCRS) functions as the primary absolute attitude reference for the SIRTF telescope. It measures the J2000 position of Tycho Catalogue stars to an accuracy of 0.14 arcsec 1-s per axis. To accurately measure Tycho objects, we have selected a silicon PIN photodiode operating in the Johnson V band, which we use with a cryogenic readout developed for the MIPS instrument on SIRTF. The PCRS employs a 4 by 4 Si:PIN detector array, using the outer rings for acquisition and the inner four pixels for precise measurements. Operation in the SIRTF focal plane presents us with several unique problems. Since the detector thermally links directly to the cryostat helium bath, it must operate at a temperature of 1.4K. Additionally, the power dissipation must be less than 0.1 mW to minimize the impact on helium lifetime. We describe low temperature characterization of Si'PIN detectors and readouts to verify their operability in the PCRS environment. Since the beryllium optics of the SIRTF telescope are diffraction limited only at 6.5 microns and longward, they yield a complicated point spread function at visible wavelengths. We present operational solutions to these and other challenges that allow the PCRS to meet its accuracy requirements with minimal impact on the rest of the SIRTF mission.

Preliminary studies of passively cooling the NGST utilizing a lightweight deployable subshield are described. The NGST mission concept of a passively-cooled large-aperture optical telescope is unique from any other mission flown to date. We show that achieving operational temperatures of less than 50 K appears feasible by passive cooling alone through a combination of (i) operating the observatory far from the Earth so that the Sun becomes the only significant source of environmental heating, (ii) selecting an observatory configuration that isolates all significant heat dissipation from the cold telescope, and (iii) employing a high performance sunshield to attenuate the incident solar radiation. The observatory configuration consists of the sunshield with cold telescope and instrument elements on the anti-sun side, and warm spacecraft avionics and propulsion elements on the sun-side of the sunshield. A sunshield thermal configuration trade study, preliminary telescope thermal analyses, and a mechanical concept for a lightweight deployable sunshield are presented. Also discussed are the remaining issues to be addressed.

ESA is studying the possibility to merge two of the next scientific missions of its HORIZON 2000 Programme, the fourth cornerstone mission, the far IR and sub-millimeter telescope (FIRST) and the third medium-sized mission M3, Planck. FIRST is a multi-user observatory, which targets the IR and sub-millimeter part of the electromagnetic spectrum, in the wavelength range from 85 micrometers to 600 micrometers . The Planck mission is a survey mission dedicated to mapping the temperature anisotropies of the cosmic background radiation. The merger of the mission is presently being studied in view of the programmatic constraints on both missions, the fact that they use a similar orbit, the partial parallel development of both missions and the potential cost savings. The cryogenic system of FIRST is based on a Superfluid Helium Dewar at 1.65K with a design lifetime of more than 4.5 years. The very low temperature, required in the bolometer instrument will be obtained from a dedicated ³He-sorption cooler. The cryogenic system of Planck uses a sequence of passive radiator, H₂ Joule-Thomson Sorption cooler, JT mechanical cooler, and dilution refrigerator.

Precision thermal control of large telescope system offers design challenges that require a balance among thermal, mechanical, optical, manufacturing, and systems engineering disciplines. These include optimizing the thermal design around optical sensitivities, choices of thermal control techniques, material selection trades to meet optical metering requirements consistent with other requirements, as well as general configuration trades. This paper presents an overview of the development of the AXAF telescope thermal design, including several major subsystem each with its own unique performance and resulting thermal control requirements. Thermal trades made for the telescope system, HRMS, optical bench, and fiducial transfer system are presented. Results are then applied in a general sense to large high precision optical telescope systems.

This paper presents an overview of the cryogenic refrigerator and cryogenic integration programs in development and characterization under the Cryogenic Technology Group, Space Vehicles Directorate of the Air Force Research Laboratory (AFRL). The vision statement for the group is to support the pace community as the center of excellence for developing and transitioning space cryogenic thermal management technologies. The primary customers for the AFRL cryogenic technology development programs are Ballistic Missile Defense Organization, the Air Force Space Based IR System Low program office, and other DoD space surveillance programs.

NASA is developing the NICMOS Cooling System (NCS) for deployment during Servicing Mission 3 of the Hubble Space Telescope (HST) in late 1999. The NCS is intended to provide mechanical cryocooling for the near IR camera and multi- object spectrometer (NICMOS) instrument that was installed during servicing mission 2 in February 1997. The NICMOS with NCS can potentially continue the near-IR capability of HST through the currently scheduled end-of-mission in 2010. The NCS hardware is currently in final integration and will soon start a series of rigorous ground and flight test that will prepare it for installation in the HST.

SVS has recently completed a phase II small business innovative research (SBIR) project called low cost space imager (LCSI). As part of the SBIR project, a sparse aperture telescope design concept was developed. This design includes an optical control system capable of correcting the primary segments to within 38 nm piston and 17 nrad tilt as required by the optical tolerance analysis. The optical system utilizes a common secondary and primaries arranged in a Golay-6 configuration. The primaries are spherical, which eliminates the need for translation and rotation control. A laboratory experiment to validate the controls concept has been completed. This experiment culminated in the demonstration of autonomous capture, alignment, and phasing of an optical system with a three segment primary to tolerances consistent with the space optical system. The implementation of the controls scheme in the laboratory experiment is done using Matlab/Simulink for controller design and code generation the code is implemented real-time on a VME based computer system. Closed loop piston control, which utilizes a four-bin sensing scheme, of an actuated mirror to 25 nm RMS mirror motion has been demonstrated. Additionally, autonomous capture and phasing of three segmented primaries has been demonstrated. The technique for the phasing capture involves real-time implementation of image processing techniques to measure the white light fringe visibility in the far field.

As part of the technology validation strategy of the next generation space telescope (NGST), a system testbed is being developed at GSFC, in partnership with JPL and Marshall Space Flight Center, which will include al of the component functions envisioned in an NGST active optical system. The system will include an actively controlled, segmented primary mirror, actively controlled secondary, deformable, and fast steering mirrors, wavefront sensing optics, wavefront control algorithms, a telescope simulator module, and an interferometric wavefront sensor for use in comparing final obtained wavefronts from different tests. The developmental cryogenic active telescope testbed will be implemented in three phase. Phase 1 will focus on operating the testbed at ambient temperature. During Phase 2, a cryocapable segmented telescope will be developed and cooled to cryogenic temperature to investigate the impact on the ability to correct the wavefront and stabilize the image. In Phase 3, it is planned to incorporate industry developed flight-like components, such as figure controlled mirror segments, cryogenic, low hold power actuators, or different wavefront sensing and control hardware or software. A very important element of the program is the development and subsequent validation of the integrated multidisciplinary models. The phase 1 testbed objectives, plans, configuration, and design will be discussed.

NASA's Search for Origins calls for ambitious astronomical mission to identify Earth-like planets orbiting neighboring stars and observe the formation of the earliest galaxies. These goals demand large, high performance optical systems integrated within precise and stable structures.One of the greatest challenges in developing origins mission is verifying astronomical performance prior to launch. The optical systems for these missions may require dimensions of meters or tens of meters making it extremely unlikely that they can undergo a full scale environmental simulation in their flight configuration. A similar challenge was encountered in the development of the AXAF mission, which combined difficult optical performance requirements with equally daunting cost and schedule constraints. This task was faced by NASA's Marshall Space Flight Center, the Smithsonian Astrophysical Observatory and an industry team led by TRW. Even before the discovery of spherical aberration on the primary mirror of the Hubble Space Telescope, the AXAF project planned for an extensive series of investments in optical testbeds, simulations, and selective testing of mission-critical subsystems. Planned as the first Origins mission, the Space Interferometry Mission will make microarcsecond astrometric measurements with optics deployed along a ten meter baseline precision structure. SIM lends itself to the same performance verification approach as that taken by the TRW Team on AXAF. Verification approaches for SIM will involve testing these subsystems specifically required for microarcsecond performance in a manageable configuration and relying on analyses and individual component testing to characterize the entire flight configuration system. This paper presented the experience gained from AXAF and outlines a strategy to be used in developing the testing approach for SIM.

We are developing a technique to measure segment misalignment of large telescopes based on wavefront estimation using phase-diverse images. We report the current results of an experiment to measure piston errors on the Keck II primary segmented mirror, through atmospheric turbulence, using phase-diverse phase retrieval. The segment piston errors are separated from the random turbulence by averaging phase estimates from many frames. Phase estimates from real data collected with segments intentionally moved in piston reproduce the observed speckle patterns well. However, average phase maps do not reveal the segment piston errors. Simulations show that the observed data were collected in a regime of turbulence where the current algorithm often fails, but would be expected to work very well when the adaptive optics system is operating. There is reason to believe that we can eventually make the algorithm work with these or similar data if apparent mismatches between the data and our current imaging model are removed.

Over the past two years, a team of researchers led by the Goddard Space Flight Center has developed a conceptual design for the NGST. The optical design of the optical telescope assembly (OTA) as well as the integrated science instrument module (ISIM) has presented many challenges. As currently envisioned, the NGST is an 8 m class telescope capable of diffraction-limited imaging at a wavelength of 2 microns operating at L2 at a temperature of around 40 K. The baseline design incorporates features such as a segmented primary mirror, deployable optical components, and active optics including a deformable mirror and fast steering mirror. In this paper, we describe the development of the conceptual design, discuss the trade-offs involving performance versus complexity, packaging, and cost, and then highlight some of the more important lessons that have been learned in the process. The interaction between the OTA and the ISIM is also discussed. It is hoped that this paper can provide insight and/or guidance to those who ar or will be working on the continuing refinement of the optical design of the NGST.

The new design of image slicer developed at Durham University has many advantages that make it particularly well suited for an integral field spectrograph on NGST. Integral field spectroscopy is a method to obtain the spectra of all the positions in a 2D image at the same time. This is usually done by cutting the field into many sub- fields that are re-imaged at the entrance focal plane of a spectrograph. Among these methods, a slicer is different in the way that it cut the field: in one direction only into 'slices' instead of in the 2 directions of space into 'pixels'. This implies that a slicer will give the highest product of the number of spectra by the number of spectral elements of resolution, maximizing the size of the field of view, the spatial resolution and the spectral length. Also, the huge focal ration degradation present when the spatial sampling is smaller than the diffraction PSF is significant in one direction only, so the optics of a slicer must be longer but no larger as must the other methods. Other advantages are the high transmission, the very large bandwidth since it is all reflective, and the ability to easily be cooled to cryogenic temperatures. For other methods to given the same performances, the spectrograph would have to be massively oversized. The new design is also much smaller than previous slicer designs. We will show that the multiplex advantage of a slicer on NGST is one to 2 order of magnitude larger when compared to the other methods. A few possible designs with up to 64000 spatial elements are described.

The optics and detectors for the NGST, will operate at IR wavelengths between 0.5 and 30 micrometers . To accomplish the requirements set for NGST, the telescope and science module will have to operate at temperatures below 60 K. To achieve cryogenic temperatures, several of the current designs for NGST use a large deployable sunshield to passively cool the telescope. The current concepts for the sunshield consist of 4 to 6 layers of thin film thermal control material supported by deployable struts. The sunshield will need to be about 30 by 15 meters, and will have to survive for 10 years in a deeps space environment. A program has been initiated to identify thin film materials that will meet the NGST sunshield requirements. The first step in this program is a literature research that has identified potential thin film materials and coatings for the sunshield. The second step will involve an initial screening of these materials, followed by more rigorous testing of selected candidate materials. This testing will characterizes the mechanical, thermal and optical properties before and after exposure to a simulated NGST sunshield environment. In addition, because the sunshield will be folded and stowed before launch, the candidate materials will be folded, stowed and unfolded before exposure to the simulated environment.

In the summer of 1996, three study teams developed conceptual designs and mission architectures for the NGST. All three conceptual designs provided scientific capabilities that met or surpassed those envisioned by the Hubble Space Telescope and Beyond Committee. Each group highlighted areas of technology study included: deployable structures, lightweight optics, cryogenic optics and mechanisms, passive cooling, a non-orbit closed loop wavefront sensing and control. NASA and industry are currently planning to develop a series of ground testbeds and validation flights to demonstrate many of these technologies. The developmental cryogenic active telescope testbed (DCATT) is a system level testbed to be developed at Goddard Space Flight Center in three phases over an extended period of time. This testbed will combine an actively controlled telescope with the hardware and software elements of a closed loop wavefront sensing and control system to achieve diffraction limited imaging at 2 microns. We will present an overview of the system level requirements, a discussion of the optical design, and results of performance analyses for the Phase 1 ambient concept for DCATT.

Current concepts for the NGST call for an 'open' telescope design in order to passively cool the telescope and instrument to cryogenic temperatures. We show that, in spite of the lack of an external baffle, straylight due to off- axis sources is negligible compared to the zodiac light. We also show that the instrumental thermal emission is in general dominated by scatter from the sunshield and not by emission from the optics. The back surface of the sunshield needs to be at about 180 K or less to reduce the self emission of the observatory to a negligible level in the near-IR, and at about 90 K or less in the case of the mid- IR.

Various efforts are underway to demonstrate hardware for the NGST. One such effort is the development of the DCATT testbed. This testbed is an NGST effort geared to demonstrating in hardware the end-to-end system functionality. The system includes a segmented telescope, active optics subsystem with a deformable mirror (DM), and a wavefront sensor. The degree to which the DCATT can demonstrate this functionality depends crucially on its performance. A system performance analysis of this testbed is presented. The analysis is based on the design of the DCATT developed by Goddard and JPL. In the analysis, the performance of the system as a function of key system factors is calculated. These factors include the following: control, environmental, fabrication, alignment, and design. The performance after correction by the DM is required to be diffraction-limited at a wavelength of 2.0 microns. The flowdown of this performance is called the corrected error budget. To fully characterize the testbed performance as a system, one must develop a budget for the performance of the system before action of the DM. The flowdown of this performance is called the uncorrected error budget. The top line of this budget is related to the correction capability of the DM.

This paper provides a high level summary of STIS, the space telescope imaging spectrograph, and describes the principle hurdles overcome during the servicing mission orbital verification in order to bring STIS to operational status as HST's optical and UV spectrograph. Also described are the approach taken to develop STIS for effective GO use and the approach taken to the on-orbit calibration of this complex and capable instrument. Lastly, we provide a description of the current operational and calibration status of STIS, including many recent results. STIS is performing extremely well as a science instrument. Monitoring projections suggest it should continue to operate effectively throughout its projected 13 year mission, barring unforeseen effects or complications.

The advanced camera for surveys, an instrument containing two CCD cameras and a MAMA detector, is being build by Ball Aerospace and Technologies Corporation for NASA. The instrument will be placed in the Hubble Space Telescope during a space shuttle mission in December 1999. The CCD detectors need to operate at a temperature below -80 degrees C in order to avoid unacceptable dark current. This cooling is achieved through detailed thermal design which minimizes the parasitic load to a 4K by 4K array with 15 micron pixels and cools this wide field channel detector with a combination of thermo-electric coolers (TECs). This paper will describe the innovative thermal design necessary to maintain the WFC CCD at its cold operating temperature while providing the means to reject the heat generated by the TECs. It will focus on optimization techniques developed to manage parasitic loads including material selection, surface finishes and thermal isolation. The paper will also address analytical techniques developed to characterize TEC performance. Finally, a comparison with the STIS CCD design currently operating in the Hubble Space Telescope will be made.

The Advanced Camera for Surveys (ACS) is an instrument containing two charged-coupled device (CCD) cameras and a multi-anode multi-channel array (MAMA) detector being built by Ball Aerospace and Technologies Corporation for NASA's Goddard Space Flight Center. The instrument is scheduled to be installed in the Hubble Space Telescope during a space shuttle mission in December of 1999. The CCD detectors need to operate at a temperature below -80 degrees C in order to avoid unacceptable dark current. This cooling is achieved with thermo-electric coolers (TEC) mounted in evacuated assemblies that contain the detectors. Heat that is generated by the TECs must be dissipated to space. Since the CCd assemblies are centrally located within the instrument enclosure, a method must be provided for transferring this heat to a heat rejection surfaces. Heat pipes have been selected for this purpose since they are frequently used in space applications for passively transferring heat from sources to remotely located radiating panels. The alignment of the CCDs is critical, however, so the loads induced into the detectors and the optical bench containing the sensor assemblies through heat pipes must be minimized. Consequently, the CCD heat pipes have been designed with a flexible section to minimize either thermally generated or launch induced structural loads. Structural and thermal testing has shown that these heat pipes will allow the ACS detectors to attain their operating temperature while meeting alignment stability requirements. This paper presents the design of and test results from the ACS flexible heat pipes.

The advanced camera for surveys (ACS) is a third generation science instrument scheduled for installation into the Hubble Space Telescope (HST) during the third servicing mission scheduled for 1999. ACS, along with the previously installed space telescope imaging spectrograph and near IR camera/multi-object spectrograph, consume significantly more power than the first generation of instruments. Additionally, the larger apertures of these instruments make parallel operations scientifically exciting. These parallel operations demand that all of the instruments operate in their highest power states simultaneously for extended periods of time. These and other factors have resulted in much higher temperatures inside the aft shroud where the ACS will be installed. As a result, new approaches are required to transfer heat inside the instrument and reject it away from the telescope. This paper describes the unique thermal systems required by the ACS. These include capillary pump loops and flexible and rigid heat pipes.

Advanced optical bandpass filters for the Hubble Space Telescope (HST) advanced camera for surveys (ACS) have been developed on a filer-by-filter basis through detailed studies which take into account the instrument's science goals, available optical filter fabrication technology, and developments in ACS's charge-coupled-device detector technology. These filters include a subset of filters for the Sloan Digital Sky Survey which are optimized for astronomical photometry using today's charge-coupled- devices. In order for ACS to be truly advanced, these filters must push the state-of-the-at in performance in a number of key areas at the same time. Important requirements for these filters include outstanding transmitted wavefront, high transmittance, uniform transmittance across each filter, spectrally structure-free bandpasses, exceptionally high out of band rejection, a high degree of parfocality, and immunity to environmental degradation. These constitute a very stringent set of requirements indeed, especially for filters which are up to 90 mm in diameter. The highly successful paradigm in which final specifications for flight filters were derived through interaction amongst the ACS Science Team, the instrument designer, the lead optical engineer, and the filter designer and vendor is described. Examples of iterative design trade studies carried out in the context of science needs and budgetary and schedule constraints are presented. An overview of the final design specifications for the ACS bandpass and ramp filters is also presented.

The advanced camera for surveys (ACS), scheduled for installation on the HST observatory in December 1999, is nearing completion at Ball Aerospace. This versatile camera, comprising 3 detector systems covering the wavelength range from the far UV through 1.1 micron, a large complement of filters, polarizers, prism and grism dispersers and a coronagraph, must be fully characterized before launch. We present plans for the instrument-level optical performance verification and calibration which will be performed later this year. Our intent is to perform a comprehensive characterization of the ACS to facilitate plans for its use aborad HST and to optimize the scientific usefulness of the immense data volume that ACS will provide. In order to comply with the aggressive delivery schedule and relatively restrictive budget, the calibration program will make use of much of the tools and apparatus developed at Ball for previous HST instruments and the data acquisition process will be improved, applying the lessons learned from those earlier programs.

The advanced camera for surveys (ACS) will be installed in the Hubble Space Telescope during the third servicing mission in May 2000. The ACS has three cameras, each of which is optimized for a specific set of science goals. The wide field camera, is a high throughput, wide field, optical and I-band camera that is half critically sampled at 500 nm. The high resolution camera (HRC) is optimized for the near- UV, has a 26 inch by 29 inch field of view and is critically sampled at 500 nm. The solar-blind camera, is a far-UV, photon counting camera that has a relatively high throughput over a 26 inch by 29 inch field of view. The WFC employs a mosaic of two SITe 2048 by 4096 CCDs with 15 micrometers pixels and a SITe backside treatment, while the HRC channel is designed around a 1024 by 1024 CCD with 21 micrometers pixels, and a near-UV backside treatment developed at the Steward Observatory. In this paper we review the performance of the devices currently selected for flight, and discuss the design of their flight packages.

The Flight Software (FSW) for the Hubble Space Telescope (HST) advanced camera for surveys (ACS) is complex, real- time, embedded software system. It is responsible for non- board command and telemetry processing, instrument maintenance, detector and mechanism control, execution of scientific observations and science data collection - in other words, control of over 40,000,000 dollars of hardware which is beyond the physical grasp of the ground system. To ensure reliable and expected performance, it is subject to an extensive test and integration process. The test program has matured from its first use during second generation near-IR camera multi-object spectrometer and space telescope imaging spectrograph FSW development, to a proven methodology currently used on ACS, a third generation instrument. The test philosophy has its foundation in solid software development practices, and most importantly, focuses on lengthy testing with high-fidelity flight and ground operations simulations and early integration with flight hardware. This paper will describe the test and integration process for the ACS FSW, with particular emphasis on the expectations at each level and the program resources required to meet those goals. We will provide quantitative measures of the benefits of the approach, using actual examples from ACS FSW test and integration. Finally, the paper will highlight the lessons learned and provide ideas which may be applied to future flight software development efforts.

The Hubble Space Telescope (HST) advanced camera for surveys (ACS) employs a wide variety of spectral filtration components including narrow band, medium band, wide band, and far UV (FUV) long pass filters, spatially-variable filters, VIS/IR polarizers, NUV polarizers, FUV prisms, and a grism. These components are spread across ACS's wide field, high resolution, and solar blind channels which provide diffraction-limited imaging of astronomical targets using aberration-correcting optics which remove most aberrations form HST's optical telescope assembly. In order for ACS to be truly advanced, these filters must push the state-of-the-art in performance in a number of key areas at the same time. Important requirements which these filters must meet include outstanding transmitted wavefront, high transmittance, uniform transmittance across each filter, spectrally structure-free bandpasses, exceptionally high out of band rejection, and a high degree of parfocality. These constitute a very stringent set of requirements indeed, especially for filters which are up to 90 mm in diameter. The development of unique optical metrology stations used to demonstrate that each ACS filter will meet its design specifications is discussed. Of particular note are specially-designed spectral transmissometers and interferometers.

We present an overview of the expected performance and science goals of the cosmic origins spectrograph (COS), a fourth generation instrument to be installed aborad the Hubble Space Telescope (HST) during the fourth HST servicing mission scheduled for late 2002. COS is a UV spectrograph optimized for observing faint point sources with moderate spectral resolution. The instrument has two channels: a far- UV channel that is sensitive in the 1150-1775 angstrom wavelength range and a near-UV channel that operates between 1750-3200 angstrom. The COS science team program concentrates on QSO absorption line systems and the IGM, dynamics of the ISM in galaxies and galaxy halos, UV extinction in the Milky Way, horizontal-branch stars in globular clusters, and volatile gases in the atmospheres of solar system bodies.

The Space Telescope Imaging Spectrograph (STIS), which was successfully installed into the Hubble Space Telescope (HST) in February 1997, has the capability to perform a powerful set of autonomous target acquisition functions. The STIS target acquisition capabilities have been tested during the initial commissioning of the instrument and are currently being used to acquire astronomical targets to support the HST Guest Observer program and the ongoing instrument calibration activities. STIS can perform several types of autonomous target acquisition sequences, all of which make use of the STIS instrument's imaging capabilities and are controlled by flight software executing out of the STIS control section microprocessor, an Intel 80386. The types of acquisitions include the location of point sources, the location of the brightest area in a diffuse or extended target, the location of point sources in crowded fields, and the centering of targets within any of the STIS spectroscopic slits. In order to move located targets into slits and center them there, the STIS control section flight software makes requests for HST pointing corrections, via special engineering telemetry sent to the HST main flight computer. The accuracy of the target acquisition tested in orbit has exceeded pre-flight specifications.

The transmitted wavefronts of optical filters for he Hubble Space Telescope (HST) advanced camera for surveys (ACS) are characterized using the Wildly and Openly Modified Broadband Achromatic Twyman Green (WOMBAT) Interferometer developed in the NASA/GSFC Optics Branch's Diffraction Grating Evaluation Facility. Because only four of thirty-three of ACS's optical bandpass filters transmit the 633 nm light of most commercial interferometers, a broadband interferometer is required to verify specified transmitted wavefront of ACS filters. WOMBAT's design is a hybrid of the BAT² interferometer developed for JPL used for HST wide field and planetary camera II filters and a WYKO 400 phase shifting interferometer. It includes a broadband light source, monochromator, off-axis, parabolic collimating and camera mirrors, an aluminum-coated fused silica beamsplitter, flat retroreflecting mirrors for the test and reference arms, and a UV-sensitive CCD camera. An outboarded, piezo-electric phase shifter holds the flat mirror in the interferometer's reference arm. The interferometer is calibrated through interaction between the WYKO system's software and WOMBAT hardware for the test wavelength of light entering the beamsplitter. Phase-shifted interferograms of the filter mounted in the tests arm are analyzed using WYKO's vision for optical testing software. Filters as large as 90 mm in diameter have been measured over a wavelength range from 200 to 1100 nm with a ACS fixed bandpass and spatially-variable bandpass filters for a variety of wavelengths.

Two multi-anode microchannel array (MAMA) detectors were fabricated at Ball Aerospace Technology Corporation for the Space Telescope Imaging Spectrograph (STIS) which was installed into the Hubble Space Telescope in February 1997. The photometric stability of the opaque CsI and semi- transparent Cs₂Te sealed MAMA tubes has been characterized as a function of operating voltage and illumination conditions. A total exposure of 5 by 10⁷ counts per pixel results in < 1 percent change in the detection quantum efficiency (DQE), which is attributed to conditioning of the microchannel plate (MCP) during tube processing. Employing good engineering practices to power supply design mitigates the effects of these components in long term detector stability. Other factors contributing to the photometric stability include the use of a curved channel MCP, photocathode processing, hydrocarbon free ultra high vacuum processing and sealed tube processing techniques. Contributions to the low resolution mode DQE stability are discussed along with empirical results on high resolution mode stability.

The Space Telescope Imaging Spectrograph (STIS) operates from the UV to near IR providing a general purpose, imaging spectroscopic capability. An internal, two mirror relay system corrects the spherical aberration and astigmatism present at the STIS field position. Low and medium resolution imaging spectroscopy is possible throughout the spectral range and over the 25 arcsecond UV and 52 arcsecond visible fields. High resolution echelle spectroscopy capability is also provided in the UV. Target acquisition is accomplished using the STIS cameras, either UV or visible; these cameras may also be used to provide broad band imaging over the complete spectral range or with the small selection of available bandpass filters. A wide selection of slits and apertures permit various combinations of spectral resolution and field size in all modes. On board calibration lamps provide wavelength calibration and flat fielding capability. We report here on the optical performance of STIS as determined during orbital verification.

The space telescope imaging spectrograph (STIS) was designed as a versatile spectrograph capable of maintaining or exceeding the spectroscopic capabilities of both the Goddard High Resolution Spectrograph and the Faint Object Spectrograph (FOS) over the broad bandpass extending from the UV through the visible. STIS achieves performance gains over the aforementioned first generation Hubble Space Telescope instruments primarily through the use of large a real detectors in both the UV and visible regions of the spectrum. Simultaneous spatial and spectral coverage is provided through long slit or slitless spectroscopy. This paper will review the detector design and in-flight performance. Attention will be focussed on the key issue of S/N performance. Spectra obtained during the first few months of operation, illustrate that high signal-to-noise spectra can be obtained while exploiting STIS's multiplexing advantage. From analysis of a single spectrum of GD153, with counting statistics of approximately 165, a S/N of approximately 130 is achieved per spectral resolution element in the FUV. In the NUV a single spectrum of GRW + 70D5824, with counting statistics of approximately 200, yields a S/N of approximately 150 per spectral resolution element. An even higher S/N capability is illustrated through the use of the fixed pattern split slits in the medium resolution echelle modes where observations of BD28D42 yield a signal-to-noise of approximately 250 and approximately 350 per spectral resolution element in the FUV and NUV respectively.

This paper continues the series of reports on the performance of the faint object camera as its on-orbit lifetime draws to a close. We review the performance of the COSTAR-corrected FOC during routine science operations, concentrating on f/48 longslit spectroscopy, imaging polarimetry, absolute sensitivity, focus effects and geometric distortion monitoring. Plans for the closeout of the instruments before its withdrawal in the third HST servicing mission in 1999 are discussed.

The advanced camera for surveys (ACS), which will be installed on-orbit in HST during the 1999 servicing mission, is being developed jointly by the Johns Hopkins University (JHU), Ball Aerospace and Technologies Corporation, and NASA Goddard Space Flight Center (GSFC). On ACS, GSFC has contracted separately to JHU for the Principle Investigator (PI) and his science team and to Ball for development and fabrication of the instrument. In addition, GSFC is providing significant amounts of flight hardware. Led jointly by the PI, the Ball Program Manager, and the GSFC Technical Officer, an integrated product team approach has been implemented that includes members from all three organizations, ensuring an integrated approach to meeting the instrument objectives. An innovative, performance-based contracting structure has been implemented that reflects the dual objectives of meeting the science requirements within the cost and schedule constraints.

The Extreme UV Explorer satellite (EUVE) was launched on June 7, 1992 with seven microchannel plate detectors behind four telescopes. All seven detectors have been operating continuously since then, cycling the high voltage bias to half voltage during the daylight portions of the orbit as well as during passage through the South Atlantic Anomaly. This paper will present the time history of the detector performance characteristics, including spatial and spectral response, gain, and flat fields. We will also discuss our experiences with the thin-film filters used to define the detector EUV bandpasses including spatial and spectral response, gain, and flat fields. We will also discuss our experiences with the thin-film filters used to define the detector EUV bandpasses including the development of 'micro' pinholes in the Al/Ti/C filters. We then illustrate specific examples of detector problems and their solutions, such as 'dithering' the spacecraft pointing to average out the small scale image distortions and off-axis pointing to avoid an on-axis 'deadspot'.

The FUSE satellite will make high spectral resolving power measurements of astrophysical objects in the 905-1187 angstrom wavelength region from low-earth orbit. Its high effective area and low background will permit observations of solar system, galactic, and extragalactic targets that have been too faint for previous instruments at this high resolution. The spectra produced by the FUSE instrument are complex, 2D images which must be transformed into simple 1D spectra for analysis. In anticipation of the complications involved in this conversion, and because calibration spectra taken on the ground imperfectly represent the results expected on orbit, a detailed computer model of the instrument and satellite has been developed. This model includes the expected performance and alignment of the optical elements and detectors, in addition to other factors such as the spacecraft pointing stability. Using this model, realistic simulated spectra can be generated in order to exercise the processing pipeline and develop data extraction techniques. A description of the computer model is presented, and sample spectra are shown.

The Center for Astrophysics and Space Astronomy (CASA) recently delivered to the Johns Hopkins University the Far Ultraviolet Spectrograph Instrument for integration into the far ultraviolet spectroscopic explorer (FUSE) satellite. In addition to the optical design of the FUSE instrument, the CASA/FUSE team was responsible for development of major optical components of the spectrograph and the final assembly and alignment of the instrument.In this paper we present the optical design, alignment methodologies employed, and performance characteristics of the instrument as delivered to the Johns Hopkins University. In addition, we discuss how we determined the resolution of the instrument capable of resolving powers in excess of 30,000. We also discuss the contamination control and monitoring and stability testing of the instrument, i.e. vibration, thermal distortion, and longterm stability testing.

The Space IR Telescope Facility (SIRTF), the last of NASA's 'Great Observatories' is entering its development phase. Ongoing advances in IR detector technology, coupled with innovative choices in orbit and system architecture, have maintained the vitality of SIRTF's scientific capability at a small fraction of the original development cost. The great sensitivity of SIRTF and its high observing efficiency promise to yield a rich legacy of science results. SIRTF is on a fast-track development schedule, with launch in December 2001. While the current baseline calls for a minimum 2.5-year cryogenic lifetime, recent programmatic and engineering development suggest that a 5-year lifetime is within reach. More than 75 percent of the SIRTF observing time will be available to the general community. We summarize the scientific capabilities and the technical specifications for the mission, including descriptions of the three-instrument payload. We will focus on the SIRTF science observations concepts, and describe SIRTF's seven observing modes - the modes by which the community will interface with the Observatory. The pre- and post-launch user services available at the SIRTF Science Center will also be presented. We include a listing of events likely to be of interest to potential SIRTF users between now and launch.

Telescopes are critical elements of NASA's space program. Very high resolution telescopes are needed to study planets of neighboring stellar systems and life beyond earth. Telescope resolution is limited by aperture diameter but current technology limits telescope apertures to about 10- meters in diameter. The Earth's atmosphere refracts sunlight such that the sun's image appears about a half degree above its real position during sunset. If we could build a space telescope using the Earth's atmosphere as an objective lens the aperture of such space telescope would be the diameter of the earth. Telescope resolution could be enhanced by up to seven orders of magnitude and would enable detailed images of planets in far away stellar systems.

Optical interferometry and interferometric synthesis imaging are being recognized as very important to the future of high resolution space-based astronomy. In this paper, an overview of optical interferometric imaging is presented and the effect of starlight beam misalignment in optical synthesis imaging is studied using a computer model. These studies will be augmented by laboratory synthesis imaging experiments designed to investigate these issues further.

This paper analyses the various ways of carrying out near IR multi-object spectroscopic studies in space. We show that ground-based observations would have limited results except in the 1-1.5 micrometers wavelength where large telescope of the 8m class would be approximately equivalent to a 1m in space. Beyond 2m, even an instrument such as the adaptive-slit near IR (ANIS) would be much more efficient. Due to their position in space, the traditional masks used in ground- based telescopes cannon be used. New technologies must be developed. Here, we present a multi-object spectrograph called ANIS based on micro-mirror arrays and designed for NGST PathFinder3. It would be able to perform a near IR spectroscopic/photometric mini-survey of the sky over a few square degrees. Thanks to its large field of view, ANIS would be complementary to NGST. Its goal would be to probe the Universe in the 0 < z < 5 range and we can consider ANIS as a scientific precursor for the NGST.

We present an optical design for a spaceborne instrument, of about half m aperture, to perform a combined astrometic and photometric survey via a scan similar to that of Hipparcos. A CCD detector array with time delayed integration will permit an astrometic mission accuracy better than 50 microarcseconds for stars brighter than 10th magnitude. 1 1/2 orders better than Hipparcos. The passband is nominally 0.4 to 0.9 microns. For the instrument to have both high measurements rate and high accuracy, the optical system just satisfy several requirements. It should have aberration well under diffraction, for high precision in centroiding and as a means of keeping unmolded shifts of the image centroids small. The system should have a wide field of view so that there is a large overlap of successive scans, have a large field of view for scientific throughput, and have low image distortion so that the stellar images moved at constant rate along columns of detector pixels. The design presented meets these requirements using aspheric surfaces that are manufacturable. We have demonstrated that the instrument will determine the temperature of an observed star without requiring a dispersive element or color filters. The design is thus free of transmissive elements, and protected from the systematic errors that they might have induced, e.g., due to thermal variation variation and to chromatic effects. This study was inspired by our previous consideration of scientific throughput. Our study of data reduction from a scanning astrometic survey mission demonstrated that there is a substantial gain in mission accuracy if the spacecraft precesses without discontinuities such as those that result from gas jet firings. Our study of methods of processing the spacecraft showed that smooth rotation would be possible using solar radiation pressure, but only if the spacecraft rotation rate were increased. Maintaining the integration time for each object would require an optical design of shorter focal length. Meanwhile, our study of mission accuracy as a function of focal length showed that another increase of accuracy would result from shorter focal length, via the greater number of lower-accuracy measurements. Therefore we performed this optical study to find a design with shorter focal length, having a proportionate increase in infield of view. We conceived and investigated a family of short focal length, wide-field designs, and developed a methodology to facilitate selection from among them. The new baseline design achieves diffraction-limited images over a 2.2 degree FOV with a 1.1 degree square central blockage, and has a 7.5 m focal length.

Studies were initiated in 1995 at the Lockheed Martin Advanced Technology Center in Palo Alto to explore both the application of new technologies and the economical utilization of commercial products to the design of a new generation of scientific research satellites. A 2.4-meter Solar System Observatory (SSO) has been designed to carry out as its primary mission imaging and spectroscopy of comets and of the outer planets form geosynchronous orbit. Such a Hubble-class telescope with a science payload consisting of four UV/EUV spectrographs and a high- resolution imager having 0.06 arcsec spatial resolution can now be built and launched within the budget of a NASA Discovery Mission. Following a one-year science program under the direction of the principle investigator, the SSO would transition to a guest observer facility. Although optimized for cometary and planetary measurements, SSO would have outstanding capability for a variety of astrophysical measurements. SSO would also serve as a prototype for other similar low-cost space observatories that could be optimized for stellar, extragalactic and other applications.

Closed loop wave front correction of low order Zernike polynomials has been demonstrated using a phase diversity wavefront sensor. The Lockheed-Martin Advanced Technology Center phase diversity brassboard was used to demonstrate low bandwidth correction of aberrations consisting of the Zernike polynomials describing focus, coma and spherical. The method of Lofdahl-Scharmer is used to estimate and correct fixed aberrations in an optical system. The General Regression Neural Network method is used to estimate slowly varying aberrations in the same optical system. Closed loop experimental results from these tests are presented.

UVSTAR, UV Spectrograph Telescope for Astronomical Research operates in the 500-1200 angstrom waveband; it has capability for long slit spectral imaging of extended cosmic sources. UVSTAR has recently flown as a Hitchhiker-M payload on the STS 85 mission of the SHuttle Discovery. UVSTAR is a joint collaboration between the University of Arizona and the University of Trieste. The instrument consists of a movable platform and an optical system The platform provides fine pointing within +/- 3 degrees from the nominal view direction, which is near the shuttle +Y axis, i.e. perpendicular to the long axis of the Shuttle and in the plane of the wings. The optical system has two channels, each formed of a telescope and Rowland concave-grating spectrograph with intensified CCD detector. The first channel, FUV, operates in the 850-1250 angstrom spectral range, the second, EUV, has covered the 500-900 angstrom region. UVSTAR includes capabilities for independent target acquisition and tracking. Here we report FUV observations, obtained in August 1997, of the sdO star BD +28 degrees 4211 which is a secondary flux standard and of the central star of the planetary nebula NGC 246, a hot degenerate star which shows strong OVI lines in the optical region. The UVSTAR spectrum of NGC 246 displays remarkable P Cygni profiles indicating a very fast stellar wind.

The Far UV Spectroscopic Explorer (FUSE), currently undergoing integration and scheduled for a 1998 launch, is an astrophysics satellite designed to provide high spectral resolving power over the interval 905-1187 angstrom. It consists of four normal incidence primary mirrors which illuminate separate Rowland circle spectrograph channels equipped with holographic gratings and delay line microchannel plate detectors. The mirrors are fabricated from Zerodur blanks, which were 70 percent lightweight and then figured to off-axis parabolas with (lambda) /40 RMS surface figure errors. Each mirror is mounted to its own composite sandwich plate, which serves as a bed for heaters and also isolates the mirror from forces and moments induced by the tip/tilt/focus actuators. A flight-like qualification unit is built up in order to verify that the mirror maintains an acceptable optical figure after assembly and environmental testing. Unexpected optical distortions during assembly and environmental testing of the qualification unit resulted in substantial modifications to the assembly procedure, as well as alteration of component and satellite thermal test limits. Known or suspected sources of distortion which warranted investigation included: assembly- induced thermal test limits. Known or suspected sources of distortion which warranted investigation induced: assembly- induced strain, thermal relaxation of the mirror flexure adhesive, changes in the moisture content of the composite plate facesheets, and warpage of the composite plate with initial thermal cycling. This paper describes how these problems were diagnosed and addressed in order to provide mirrors meting the optical performance requirements of the FUSE program.

An x-ray all-sky monitor based on a lobster-eye focusing optics concept is a very promising satellite for the future astronomical mission. The angular resolution of the optics based on Angel's geometry, implemented as an array of square pore channel pates is, in theory, limited by the physical size of individual channels. In practice, the metric uniformity, in particular channel misalignment, and surface roughness of the channel plates are the prime factors limiting the efficiency and resolution of the focusing optics. Most of the methods to test the metric uniformity suggested earlier allowed to study the quality of the pates only in local areas. We suggest another method of estimation of the global uniformity, and in particular the multifiber misalignments, over the entire plate area based on moire interferometry. It is shown that for conventional MCP with 60-pore multifiber diameter this technique in principle can detect the multifiber angular misalignments and twists with an accuracy of about 1.2 mrad. The channel long axis misalignments may also be measured with accuracy of 35/L/p mrad, where L/p is the channel length to interchannel distance ratio. We believe this technique is a powerful tool for the preliminary selection of channel plates to be used in focusing optics.

Several designs of filters for use in vacuum UV imaging systems are discussed. These designs incorporate all reflective optics,and are characterized by comparatively high in-band throughout, very low out-of-band transmission and sub-arcsecond spatial resolution. In addition, they an be tuned over ranges useful for vacuum UV astronomical observations. Results from a simplified laboratory version of the filters intended to prove the concept are presented.

Advanced spaceborne optical system currently under consideration, such as the Next Generation Space Telescope, will require the use of large UV segmented mirrors that can perform at cryogenic temperatures. The mirrors must be rugged, low risk, reliable, and capable of surviving launch. A design approach that utilizes passive segments in order to meet the optical system design requirements at cryogenic temperatures offers significant advantages in weight, performance, and reliability. A comparison of passive mirror designs using alternate mirror materials and the results of prototype testing at cryogenic temperature will be presented.

The Gravity Probe-B cryogenic star-tracking telescope provides the inertial pointing reference, as established by a distant 'fixed' guide star, with milli-arc-second resolution per year for the NASA/Stanford general-relativity gyroscope experiment. The design of the f/27 modified- Cassegrain telescope is briefly reviewed. Then discussed are two of its unique aspects: (1) focal-plane roof-edge diffraction and (2) fused-quartz bonding for 2.5 Kelvin applications. The star image in the telescope is split by roof prisms to generate quadrant pointing information within few arc-seconds about the guide-star direction. The corresponding roof-edge diffraction effects due to the roof- prisms were compared with theoretical calculations, which successfully interpreted the test result and indicate the need for enlarging post-focal-plane detection apertures, as compared with ray-trace situation. On the other hand, a novel fused-quartz bonding technique was developed and found superior in many aspects to index-matching epoxies, optical contacting, and Corning's proprietary frit bonding for both room-temperature and cryogenic applications. The bonding was simple, economical, yet extremely reliable and optically precise. It resulted in strengths near that of fused quartz, with less than 10 nano-meter interface thickness demonstrated.

DIVA (deutsches interferometer fuer vielkanalphotometrie und astrometrie) is a project of a small satellite, aiming to measure positions, proper motions, parallaxes and spectra of several million stars. DIVA will carry two Fizea interferometers with a baseline of 100 mm using a novel telescope design. It consists of a Gregory configuration with high secondary magnification and a four-component field lens system at the intermediate focus. We present the optical layout which allows diffraction-limited imaging over a 0.5 degree of view in the wavelength range 400-1000 nm. Critical aspects of the design are discussed. The present status of the project is briefly outlined.

The AXAF Telescope, built by Eastman Kodak Company's Image Acquisition Systems division, contains the world's largest and most advanced dimensionally stable composite metering structures. Tight dimensional stability requirements were satisfied requirements were satisfied for the 26-foot long optical bench by designing a laminate with low axial and hoop CTE and use of state-of-the-art cyanate ester resins to limit hygroscopic effects. Strict control of material and processes minimized CTE/CME variability. The design also had to consider launch loads in excess of 30,000 pounds developed from the 3,500 pound high-resolution mirror assembly and cantilevered 1,050 pound science instruments module. A complete verification program was implemented consisting of materials, parts, and static load qualification and acceptance testing of this 'one of a kind' structure. Composite materials were also used in the design of thermal-structural enclosures for the HRMA. These structures act as integrated thermal and optical baffles and structural support for the optical transmission gratings. The material and laminates were carefully selected to balance thermal and structural requirements. These structures were designed, fabricated, and tested under stringent thermal, structural, and cleanliness requirements.

This paper describes the design and prelaunch performance of the tip-tilt mirror (TTM) system developed for the XUV Cassegrain telescope aboard the ISAS sounding rocket experiment. The spatial resolution of the telescope is about 5 arcsec, whereas the rocket pointing is only controlled to be within +/- 0.5 degree around the target without stability control. The TTM is utilized to stabilize the XUV image on the focal planes by tilting the secondary mirror with two-axes fixed-coil type actuators. The two position- sensitive detectors in the telescope optics and in the TTM mechanical structure from the normal and local closed-loop modes. The TTM has four grain modes with automatic transition among the modes. The low gain mode is used in the initial acquisition, and in case the TTM loses the tracking. The high gain mode is used in the normal tracking mode. This arrangement provides us with the wide initial acquisition angle with single TTM system as well as the high pointing accuracy once the tracking is established. The TTM has a launch-lock mechanism against the launch vibration of 16G. The closed-loop control with command and telemetry interface is done by the flight software against the launch vibration of 16G. The closed-loop control with command and telemetry interface is done by the flight software on the DSP processor. The use of the fast processor brings in the significant reduction in the weight and size of the control- electronics, more flexible control system, and shorter design and testing period.

Simple Ronchi testing of Ritchey-Chretien space telescope optics is discussed. Bright star is used in the infinity and Ronchi grating is situated in front of focus of the telescope under test. Ronchi patterns are to be recorded by means CCD camera. The same camera is to be used for the sky observations. Space telescope optics aberrations can be calculated from the recordings of Ronchi patterns. Aberrations can be used for the main mirror shape corrections in the space. Foucault knife test can be used instead of the Ronchi grating for independent testing of the telescope optics too.

We report progress in the development of very lightweight uv-vis-ir mirrors for space- and ground-based applications. The mirrors are made by replication using spaceflight qualified graphite cyanate ester composite materials. We have developed a process that successfully overcomes the problems of fiber print-through, vacuum instability, and appearance of bond lines on the surface. These problems have thwarted previous attempts in the development of composite optics. We describe our process and present some recent results. These include the fabrication of mirrors with highly smooth surfaces, low mid-frequency ripple, and areal density 2 kg/m² at 60 cm aperture. We also present data on bond lines and active optical figure control.

We have achieved reliable, repeatable coatings of solgel- deposited glass on Carbon Fiber Reinforced Plastic sandwich panel mirrors of sizes 10 to 90 cm. Very lightweight CFRP panels up to 2 meter size have been produced with high thermal and temporal stability, but replicated surface accuracy limited to millimeter applications. The solgel coatings of 20 to 200 micrometers thickness provide a surface which can be optically ground, polished, and figured to provide IR and optical quality mirrors. We have demonstrate this with both flat and concave spherical mirrors.

At the Project Design Center at the JPL, a concurrent design environment is under development for supporting development and analyses of space instruments and telescopes in the early, conceptual design phases. This environment is being utilized by a Team I, a multidisciplinary group of experts. Team I is providing study and proposal support. To provide the required support, the Team I concurrent design environment features effectively interconnected high-end optics, CAD, and thermal design and analysis tools. Innovative approaches for linking tools, and for transferring files between applications have been implement. These approach together with effective sharing of geometry between the optics, CAD, and thermal tools are already showing significant timesavings.

Using our concept it has become possible to explain many phenomena which have not been adequately understood earlier including some results of 'Voyager' US missions. The superdiamagnetic model is based on the assumption that the planets' rings consist of the superconducting particles. The huge number of superconducting particles in the planet's magnetic field demonstrated behavior comparable to superdiamagnetic liq uid film. The superdiamagnetic concept of the planets' rings makes it possible to add classical theories, and to resolve as yet unsolved problems as will shown below.

The Division of Astronomical Sciences of the National Science Foundation (NSF) support the design and construction of telescopes and instruments for ground-based astronomical observations in the visible, IR, and radio regions of the spectrum. Priorities are given to instrumentation deemed most likely to provide responses to the scientific questions central to current astronomy and astrophysics.

We are developing a novel solar blind, high resolution, photon counting detector for applications in space UV astronomy. Our concept is to utilize a charge injection device (CID) as the readout stage behind a microchannel plate (MCP) intensifier. This detector will take advantage of the flexible readout options afforded by the addressable CID architecture to provide high local frame rates around bright features in an image. In this concept, the detector bandwidth can be used most efficiently, reading pixels around a bright star more frequently than those in a nearby dim cloud of gas, for example. The demonstration apparatus described in this paper incorporates a 25 mm diameter intensifier tube fiber optically coupled to a commercially available 30 frame-s^-1, 512 X 512 pixel, progressive-scan CID2250 camera. The 10 MHz analog video from this camera is digitized and processed by a centroider module that calculates the position of each event in real time with subpixel precision, thus providing high spatial resolution limited by the MCPs. The pore-resolved images presented in this paper validate the intensified CID concept. We plan to incorporates custom driving electronics and an experimental CID with on-chip address decoders for high speed random access of detector subarrays of arbitrary size and location. Our goal is to demonstrate a solar-blind UV photon counter with 200-300 counts-s^-1 point source and 3 X 10⁵ counts-s^-1 global rate capability with up to 4000 by 4000 elements.

As part of the extensive ground testing of the AXAF, x-ray detectors of standard design were used as secondary absolute calibrators. In order to realize the overall facility calibration goal, these detectors need to be calibrated in turn on a primary x-ray standard. This exercise consists of accurately determining the detectors' response to monochromatic x-rays of different energies. Our data were obtained on various beamlines at the BESSY synchrotron storage ring, which is a very accurately calibrated standard x-ray source. Here we report progress in understanding the response function of the solid-state devices (SSDs). The response is modeled using JMKmod in the XSPEC package, and includes effects of pileup, incomplete charge collection, fluorescent escape, and other secondary processes. Knowledge of the SSD response function will permit precise removal of SSD detector effects form AXAF calibration data, and therefore allow accurate calibration of the AXAF system's effective area and other performance parameters.

A nulling interferometer for direct detection and spectral studies of the light from extra-solar planets would face daunting technical challenges. We outline a candidate optical architecture, discussing the major challenges in handling the starlight and controlling the optics to produce a deep on-axis null with high transmission a fraction of an arcsecond away.

There is a growing need for large aperture, ultralightweight, deployable optics for various science, military and commercial compact satellites. This paper will examine the engineering and manufacturing considerations that must be addressed in order to satisfy the requirements for these sought after optics. In order to limit the scope of this paper, only Graphite Fiber Reinforced/Polymer Matrix Composites (GFR/PMC) will be under consideration because of the potential to satisfy ultralightweight mirror requirements. The requirements associated with specular mirror concepts that Composite Optics, Incorporated (COI) has proposed to Air Force Research Laboratory and NASA Langley Research Center for visible range optics and LIDAR optics, respectively, will also be our interest. Moreover, it is the intent of this paper to illustrate how COI's proposed design/manufacturing concepts for visible and LIDAR optics have evolved based on overcoming, or working around, material constraints and/or undesirable characteristics associated with GFR/PMC.

To illustrate the efficiency of using of a multi-mode deformable plane mirror to record holographic gratings we have computed the three gratings for the Cosmic Origin Spectrograph. Their working conditions are severe, since they have to correct the residual spherical aberration of the HST. Nevertheless, all the images obtained are largely diffraction-limited with regard to the resolution.

The Wide-Field IR Explorer (WIRE) is a small spaceborne cryogenic IR telescope being readied for launch in September 1998. Part of NASA's Small Explorer program, WIRE will carry out a deep pointed survey in broad 24 and 12 micron passbands designed primarily to study the evolution of starburst galaxies and to search for protogalaxies. The strategy for the WIRE survey and its stare-and-dither technique for building up long exposure times are described. An overview of the WIRE instrument is presented, with emphasis on the results of ground characterization and expected on-orbit performance of the WIRE optics and the Si:As focal plane arrays. The result of the ground characterization demonstrate that WIRE will meet or exceed the requirements for its science objectives. A brief overview is given of the primary and additional science that will be enabled by WIRE.