When the space shuttle Endeavor rockets into orbit on December 2, 1993, NASA will begin the last phase of a three part program to recover from the disappointing beginnings of Hubble Space Telescope operations. The problem with the telescope was detected shortly after launch in April of 1990 and in June of that year NASA concluded that the primary mirror of the telescope had been manufactured with the wrong figure. NASA immediately began a detailed investigation to determine exactly what went wrong. Next, NASA and the HST community began a multi-effort program to maximize and improve the science produced by the telescope in its aberrated condition. Finally, plans were developed to repair the telescope's problems in orbit and to restore the observatory's full scientific potential. Much has been written about the failure to detect this flaw before launch. The impression of many outside the program is that Hubble is a complete failure. Far less has been written of the impressive efforts and successes as the recovery from this disappointing start evolved.

HST is unique among NASA missions in the level of preparation that has been performed for on-orbit servicing. Planning and training for HST servicing has been an important element HST development leading to an observatory that is uniquely designed with servicing in mind. An overview of the components of the first Servicing Mission are described along with a walkthrough of several of the planned extravehicular activities (EVAs) including replacing solar arrays, swapping out the WF/PC and HSP for WFPC2 and COSTAR, replacing the gyros and installing the GHRS repair kit. An integral part of preparing for HST's first servicing mission is determining how to recommission the observatory for science operations. The recommissioning period is referred to as the Servicing Mission Observatory Verification (SMOV) program and is a composite of the HST deployment Orbital Verification (OV) and Science Verification (SV) programs. We will examine how the lessons learned from the deployment commissioning have been addressed in development of the SMOV plans.

The Corrective Optics Space Telescope Axial Replacement (COSTAR) is designed to restore the high image quality, lost to a primary mirror manufacturing defect, that was originally expected from the Hubble Space Telescope (HST). Though its rapid development is both a technical and programmatic challenge, COSTAR is nearing completion at Ball Aerospace and is scheduled for deployment during the first HST servicing shuttle mission late this year. This paper describes the extensive optical testing that has been planned to assure the high quality imaging performance of the COSTAR-corrected HST. Independent, redundant tests at both component and system levels are employed to verify that design tolerances are sufficient and manufacturing tolerances are achieved. We report the results of the critical tests which have been completed to date and their implications for the on-orbit performance.

The Corrective Optics Space Telescope Axial Replacement (COSTAR) instrument is designed to provide optical correction for the current figure error in the Hubble Space Telescope (HST) primary mirror to three of the first generation instruments: Faint Object Camera (FOC), Faint Object Spectrometer (FOS), and the Goddard High Resolution Spectrometer. The FOC and FOS each have two optical channels and the GHRS one optical channel that are corrected. The optical correction is achieved by deploying a two mirror relay into the HST hub area in front of the Optical Telescope Assembly (OTA) focal surface for each optical channel. Structural motion of the mirror support system will affect alignment producing changes in wave front error (wfe) and line-of-sight (los). Changes of los during an exposure will blur the image and degrade image quality in addition to alignment degradation of wfe. The system analyses used to perform image quality trades and allocate budgets in the design phase and to help define requirements for the integration and test phase will be presented. These analyses were an integrated and iterative process among the optical, structural, and thermal analysis disciplines. Results of these analyses predict COSTAR performance will meet the image quality requirements.

The effect of internal light scattering caused by the microroughness of the Hubble Space Telescope primary mirror was measured and compared with theoretical models. It was found that the effect was much smaller than predicted and would not be a problem even in the UV.

The Hubble Space Telescope (HST) is an orbiting astronomical observatory, designed to operate as close as possible to ground based instrumentation, given the limitation of operating in a low earth orbit. The spacecraft design had to accommodate an absolute pointing accuracy of 0.01 arc seconds, a relative pointing stability of 0.007 arc seconds rms, the capability to maneuver 90 degrees in 18 minutes, and operate autonomously in a safemode control scheme for up to 72 hours. Furthermore, the design had to provide for a flexible, stored command methodology, and real-time command capability. This paper briefly reviews the spacecraft engineering hardware and software design. A detailed critique of the on-orbit performance of the spacecraft is provided. Enhancements and work-around, which have enabled HST to continue implementation of a successful science plan, are explained.

We report on the results of a dedicated on-orbit test which we have performed to measure the line-of-sight jitter of the Hubble Space Telescope. The test, which made use of one of the science instruments on board the spacecraft, had a very high sensitivity (a fraction of a milliarcsecond) and covered all frequencies up to 500 Hz. As was previously known from gyroscope data analysis, the bulk of the line-of-sight jitter occurs at frequencies lower than 5 Hz and is caused by motion of the spacecraft body reacting against the oscillation of its appendages. However, the test indicates that there are also minor contributions by various modes of the telescope structure in the 15 - 30 Hz range and the vibration of the primary mirror at 61 Hz when it is excited by thermally induced shocks or the running of the tape recorder. There are also traces of a component around 300 Hz induced by the tape recorder.

The interferometric system used for the guidance of HST is sensitive to close binaries, and before the launch of HST it was believed that up to 25% of all guide stars pairs would lead to guidance failure due to duplicity. After nearly 3 years of operation the actual failure rate is below 1%. We have developed a computer simulator to understand the causes of the discrepancy. Our simulation results indicate a failure rate under 2% per guide-star pair in much closer agreement with the observed performance of the guiding system. We show that the failure criterion originally used was much too severe and that the proportion of guide stars in the critical separation range had also been overestimated.

We discuss the requirements and design of the calibration data base which supports the processing of Hubble Space Telescope observations. We describe procedures for ensuring that, despite the continuing changes to software and hardware, calibration files to match the instrument configuration for any observation are provided when needed.

Images and spectra obtained with the Hubble Space Telescope (HST) in its current configuration suffer from severe spherical aberration and are therefore routinely restored for visualization and scientific evaluation. Among the non-linear, iterative restoration methods investigated so far, the Richardson-Lucy (RL) algorithm stands out for the quality of the restorations it delivers at modest computational cost. In this contribution we summarize our experience so far with restoration methodology applied to HST imagery and make a first attempt at drawing some general conclusions about its usefulness.

The endeavour of Hubble Space Telescope (HST) proved once more that arguments such as high costs, extremely long preparation time, inherent total failure risks, limited life time and high over-subscription rates make each scientific space mission almost always a unique event. The above arguments immediately point to the need for storing all the data produced by spacecraft in a short time for the scientific community to re-use in the long term. This calls for the organization of science archives. Together with the Space Telescope Science Institute, the European Coordinating Facility developed an archive system for the HST data. This paper is about the experience gained in setting up and running the European HST Science Data Archive system. Organization, cost versus scientific return and acceptance by the scientists are among the aspects that will be covered. In particular, we will insist on the 'four-pillar' structure principle that all archive centers should have. Namely: a user interface, a catalogue accurately describing the content of the archive, the human scientific expertise and of course the data. Long term prospects and problems due to technology changes will be evaluated and solutions will be proposed. The adaptability of the system described to other scientific space missions our ground-based observatories will be discussed.

We describe the current design for Edison, the first large radiatively-cooled infrared space observatory, now under consideration by the European Space Agency. Without the large cryogen tanks, more of the spacecraft can be filled with light-collecting optics and, of course, the observatory has no built-in lifetime. Our proposal is for a telescope with a 1.7 m primary to be launched by an Atlas, Ariane 5, or Proton. The baseline orbit for the observatory is a 'halo' around L2, a location which allows additional radiating area to be placed anti-sunward. Models of the temperature behavior of the observatory indicate an equilibrium temperature via radiation alone of about 20 K. Use of near-future cryo-coolers may allow optical system temperatures as low as approximately 15 K. Consequently, Edison will be limited in sensitivity by the celestial thermal background at wavelengths shortward of about 60 micrometers and by celestial source confusion at longer wavelengths.

The Composite Infrared Spectrometer (CIRS) is an instrument currently under development at NASA Goddard Space Flight Center for the Cassini mission to Saturn. The CIRS optical design heritage extends back to the Infrared Interferometer Spectrometer (IRIS) which flew on Voyager. CIRS is the next logical step in the exploration of the atmosphere of Saturn and Titan. It will obtain more complete sets of data with broader spectral coverage, higher spectral and spatial resolution, and greater sensitivity. The CIRS optical design consists of four subassemblies: (1) a 50.8 cm diameter Cassegrain telescope, (2) a Mid-Infrared (MIR) Michelson interferometer, (3) a Far-Infrared (FIR) polarizing interferometer, and (4) a Reference interferometer (RI).

The International Gamma-Ray Astrophysics Laboratory (INTEGRAL) is a proposed joint ESA/NASA/Russia gamma-ray astronomy mission which will provide both imaging and spectroscopy. It is currently at the final stages of an ESA phase-A study which it is hoped will lead to it being adopted during 1993 as the second 'medium-class' mission within ESA's Horizon 2000 plan. Launched in less than 10 years time it will be the successor to the current generation of gamma-ray spacecraft, NASA's Compton Observatory (GRO) and the Soviet- French Granat/Sigma mission. The baseline is to have two main instruments covering the photon energy range 50 keV to 10 MeV, one concentrating on high-resolution spectroscopy, the other emphasizing imaging. In addition there will be two monitors--an X-ray monitor which will extend the photon energy range continuously covered down to a few keV, and an Optical Transient Camera which will search for optical emission from gamma-ray bursts.

The Integrated Modeling of Optical Systems (IMOS) Integration Workbench at JPL has been used to model the effects of structural perturbations on the optics in the proposed Orbiting Stellar Interferometer (OSI). OSI consists of 3 pairs of interferometers and delay lines attached to a 7.5 meter truss. They are interferometrically monitored from a separate boom by a laser metrology system. The spatially distributed nature of the science instrument calls for a high level of integration between the optics and support structure. Because OSI is designed to achieve micro-arcsecond astrometry, many of its alignment, stability, and knowledge tolerances are in the submicron regime. The spacecraft will be subject to vibrations caused by reaction wheels and on-board equipment, as well as thermal strain due to solar and terrestrial heating. These perturbations affect optical parameters such as optical path differences and beam co-parallelism which are critical to instrument performance. IMOS provides an environment that allows one to design and perturb the structure, attach optics to structural or non-structural nodes, trace rays, and analyze the impact of mechanical perturbations on optical performance. This tool makes it simple to change the structure and immediately see performance enhancement/degradation. We have employed IMOS to analyze the effect of reaction wheel disturbances on the optical path difference in both the science and metrology interferometers.

The control system requirements for the next generation space telescope are discussed, based on the authors experience with Hubble Space Telescope (HST), Advanced X-Ray Astrophysics Facility (AXAF) and Space Infrared Telescope Facility (SIRTF). Since the HST design phase, there have been significant strides in the guidance and control domain (i.e., fiber optic gyroscopes, solid state star trackers and non-linear control algorithms). The control system design will be determined by the predicted spacecraft configuration, mirror geometry (6 to 8 meters will be considered) and science requirements. Spacecraft dimensions have been estimated for the telescope aperture range of interest. Presently, the Energiya rocket can only accommodate a 6 m telescope, the proposed Heavy Lift Launch Vehicle apparently can accommodate a 7 m telescope. A low Earth orbit (600 Km) has been adopted for this study, the advantage of Shuttle servicing and an accompanying long spacecraft life, weighed heavily in this decision. However, the possibility of a long spacecraft life in a high altitude orbit, with the requisite attitude control redundancy and fault tolerance, may be feasible.

CdZnTe crystals useful for room temperature x-ray astronomy detectors have recently been developed. Uniform response and good energy resolution in the 3 to 300 keV range have been demonstrated for large area detectors and monolithic arrays. Spectral resolution of 2.9 keV for the 59.5 keV ²⁴¹Am line, and resolved 661 keV peak from ¹³⁷Cs were recently achieved at room temperature. Gain deviations of 1.2% were measured for the elements over a large area monolithic array. Imaging results from a 32 X 32 array with active area of 12.4 cm² are presented. The stability, uniformity, producibility and size of the crystals, and the properties of the resulting nuclear radiation detectors make CdZnTe an attractive candidate for applications in the space sciences.

The Optical/UV Monitor Telescope (XMM-OM) on the ESA X-ray Cornerstone mission XMM is designed to provide simultaneous optical and UV coverage of all sources viewed by the observatory in the X-ray band. The instrument consists of a 30 cm Ritchey-Chretien telescope. This feeds a compact photon counting detector that operates in the blue part of the optical spectrum and the UV (1600 - 5500 angstroms), and simultaneously a cooled CCD detector which registers the red light (5500 - 10000 angstroms). The XMM-OM will have a field of view of approximately 25 arcmin diameter, matching that of the X-ray cameras on XMM, and a spatial pixel size in normal operation of 1 arcsec in the blue, and about 1.8 arcsec in the red. Because of the low sky background in space, the sensitivity of the XMM- OM for detecting stars will be comparable to that of a 4-m telescope at the Earth's surface, and it should detect a B equals 24^th magnitude star with a photon counting detector in a 1000 s observation using unfiltered light.

A program is underway at the Naval Research Laboratory (NRL) to develop a high-resolution spectrometer for the study of astrophysical sources at EUV/soft X-ray wavelengths. The spectrometer design is simple in that the sole optic is a multilayer-coated spherical grating or mosaic of co-aligned gratings used at near-normal incidence, allowing large effective collecting area without the strict tolerance requirements of grazing incidence optics. Therefore, both high resolution and high throughput can be obtained over several selected narrow bandpasses. We present efficiency and resolving power measurements of spherical gratings which have parameters similar to that intended for our flight instrument. Two gratings were replicated from the same ruled master and then coated with a multilayer of molybdenum and silicon. A third sister grating was used as a control and overcoated with gold.

Future space telescopes seek to maximize collecting aperture for increased sensitivity and high spatial resolution yet are limited in mass due to launch weight restrictions. JPL has developed the Controlled Optics Modelling Package (COMP) to easily facilitate analyses of optical systems whose elements are perturbed. Development of the computer tool, IMOS (Integrated Modeling of Advanced Optical Systems) allows modeling of structurally and thermally induced deformations to interact with optical systems. Presented here are analyses on the Segmented Reflector Telescope, (SRT), to estimate and then minimize the effect of anticipated disturbances on the resultant optical performance. Such studies are a needed prerequisite for estimating the requirements for adaptive optics due to structural movements. A simulation study estimates the space-time power spectral density of the residual telescope phases from reaction wheel disturbances. Results show that significant disturbances are concentrated in the first few Zernike polynomials with 87% of all disturbances described by the first 11 terms leaving a 0.35 micrometers rms residual. The time bandwidth of the disturbances is between 20 - 25 Hz which placed the required corrections in the adaptive optics regime.

A lunar-based solar observatory (planned for expansion) will support the Space Exploration Initiative (SEI) by providing solar flare alerts and high quality scientific data. One candidate instrument for the proposed observatory, a vector magnetograph, will provide scientists with vector magnetic field data at a level of resolution of approximately one-half an arc second, the scale of important physical processes. Scientists will use the vector data from this instrument to develop flare prediction capabilities. The instrument, consisting of a 30 cm Cassegrain telescope system, will be placed on the lunar nearside, making long duration studies of solar activity (of order 14 days) possible. The vector magnetograph will have a temporal resolution on the order of three minutes, an operational lifespan of five years, and will be serviceable by crew of the First Lunar Outpost (FLO). This instrument, based on a currently operating vector magnetograph at NASA's Marshall Space Flight Center, is under study for placement on the Moon using either Johnson Space Center's Common Lunar Lander or crew from the First Lunar Outpost. This paper will review the scientific need to place a vector magnetograph on the Moon, outline the design for such an instrument, describe the preliminary requirements for the launch vehicle and lander, and will recommend a more penetrating study to determine the optimum optical design for the instrument, the best materials, and the environmental effects.

This paper gives an update on the performance of the Faint Object Camera--launched with the Hubble Space Telescope--since the last report two years ago. The primary camera, the f/96 relay, continues to work well, but the f/48 relay has recently developed serious problems. The stability of the f/96 relay has been very good with the only change being a small apparent decrease in UV sensitivity. Preliminary results for the f/48 DQE are presented. In-orbit UV flat fields have been obtained and the f/96 objective prisms and polarizers have been calibrated.

During the Science Verification phase of the Hubble Space Telescope mission, it was determined that the Faint Object Spectrograph's (FOS) Red detector displayed significant image motions which correlated with orbital changes in the geomagnetic field. The Blue detector exhibited similar but less pronounced motions. The cause of this motion was determined to be inadequate magnetic shielding of the instrument's Digicon detectors. The results of these motions were decreases in onboard target acquisition accuracy, spectral resolution, and photometric accuracy. The Space Telescope Science Institute and the FOS Investigation Definition Team, set about correcting this Geomagnetically-induced Image Motion Problem (GIMP) through a real-time on-board correction scheme. This correction required modifications to almost all aspects of the HST ground system as well as additional NSSC1 flight software and the use of an existing software 'hook' in the FOS microprocessor firmware. This paper presents a detailed description of the problem, the proposed solution, and results of on-orbit testing of the correction mechanism.

We present the results of an investigation of the in-orbit performance of the Digicon detectors in the Faint Object Spectrograph (FOS), conducted as part of the commissioning phase of the Hubble Space Telescope. This paper includes orbital results on detector background noise, sensor image stability, and photometric stability along with several typical FOS observations. This information should be of general interest to designers of future spacecraft detectors and to astronomers observing with the FOS instrument.

The Hubble Space Telescope High Speed Photometer (HSP) has successfully completed orbital and science verification testing and is currently executing scientific proposals. The performance of the HSP has been satisfactory except for sensitivity of one of the detectors and low transmission of the prism apertures used for two color photometry. The impact of the telescope performance and ground system limitations on instrument performance is discussed. The HSP's use of the telescope is unique in several respects. Target acquisition depends critically on knowledge of aperture locations and the ability to execute precise small angle maneuvers. The HSP requires precise spacecraft scans for occultation observations. The photometric performance of the HSP is especially dependent on spacecraft pointing stability. The HSP's time series data do not naturally fit into the image oriented data format of HST.

The scientific goals of the Goddard High Resolution Spectrograph (GHRS) require data of very high integrity, with high S/N, spectral resolution, wavelength calibration and photometric accuracy. The instrument was designed to have optical, structural, thermal and detector performance capable of delivering nearly photon limited ultraviolet spectra. Many important characteristics such as geometrical formats, sensitivity functions, resolving power and linearity were predicted from design specifications, and were tested and verified. Instrumental properties which may limit the ultimate performance, such as stray light, distortions and geometrical instabilities were anticipated but not precisely modeled. A careful test program, comprising several distinct pre- and post-launch phases identified, characterized and in several cases eliminated potentially troublesome aspects. In the spirit of 'lessons learned' this paper describes a number of optical and detector related artifacts, how they were identified and measured, and either eliminated by reworking the hardware, or calibrated and compensated for during data reduction. Builders of future instruments should be aware of the types of anomalies we encountered so they may be avoided in the design phases, or well calibrated in the testing phase. We will discuss aspects of the testing programs, both technical and programmatic, which contributed to the successful commissioning of the GHRS and its current return of high quality spectroscopic data.

Observations of a standard star (BD+75D325) have been used to measure the Hubble Space Telescope (HST)/Faint Object Spectrograph (FOS) scattering characteristics in the wavelength range 115 to 250 nm. Spectra of the standard star were obtained as the star was progressively offset from the optical line of sight axis of the telescope, both in the in- dispersion and cross-dispersion directions. These data have been reduced and analyzed to determine the scattering function of the telescope-spectrograph combination. Two primary results have been obtained. (1) The resulting scattering function exhibits three characteristics: (a) the inner core ((theta) < 4') is dominated by the large Point Spread Function (PSF) of the HST; (b) the outer wings of the scattering function (4' < (theta) < 32') show a (theta) ^-3 dependence consistent with predictions for the HST Airy disc; and (c) the wavelength dependence of this scattering function follows (lambda) ^-1, suggesting that the ultraviolet (UV) micro roughness contribution to the scatter is quite small, and hence the HST primary mirror is very smooth at ultraviolet wavelengths. (2) The FOS scattering contribution is limited only by grating scatter, and is consistent with pre-launch grating calibration measurements.

The Goddard High Resolution Spectrography, (GHRS), has been in operation since the launch of the Hubble Space Telescope in April 1989. While this instrument has proven it's value to the scientific community, a number of operational and hardware problems had to be addressed to enable it's continued use. This paper will detail the symptoms, analysis and resolution of each of these problems. Issues to be covered include the effect of spherical aberration on the target acquisition software routines, performance of the optical element carrousel motor and electronics, and the impact of a failure in the side one low voltage power supply.

The Wide Field Planetary Camera (WF/PC) onboard the Hubble Space Telescope contains contaminants which condense on the windows in front of each CCD detector. These contaminants are UV opaque and increase with time to the extent that after several months they block 50% of the flux at 300 nm. Also, when the contaminants are warmed above -40 degree(s)C and then returned to the normal CCD operating temperature of -87 degree(s)C, particles form and severely degrade the image quality. The windows may be temporarily cleaned by raising their temperature to 0 degree(s)C. However, this results in a change in the structure of the flat field due to the partial removal of the UV flood which was applied after launch to suppress Quantum Efficiency Hysteresis in the CCDs. Repeated decontaminations will reintroduce the QEH and necessitate another time consuming UV flood and recalibration of the instrument. After 22 months of on-orbit operation, the contaminants could no longer be fully removed by the decontamination procedure. This paper describes the current state of the contaminants, what has been deduced concerning their properties and sources, the results of our efforts to remove them, and some lessons for future space-based instruments using cryogenic UV sensitive detectors.

The Faint Object Camera is a unique Scientific Instrument of the Hubble Space Telescope; operating it at top performance is critical for the quality of the science data and a most efficient usage. This paper describes the steps taken to optimize detector performance, minimize the effects of thermal variations, and maximize the allocated observing time.

The nature of the Hubble Space Telescope's (HST) low Earth orbit imposes scheduling restrictions and interruptions in the data collection periods for it's compliment of scientific instruments. During many of these times the Faint Object Spectrograph (FOS) is in a full operational configuration and is taking detector background measurements which are continually reported in HST's engineering telemetry stream. These data are primarily used to monitor the instrument for changes in behavior resulting, principally, from intermittently noisy diodes in its digicon arrays. These same data may be used to monitor temporal changes in the charged particle environment of HST's near-earth orbit. We present here the results of a study of two years of on-orbit FOS background data obtained serendipitously during periods while the FOS in an operational state, but not exposing on external, or calibration targets. These in situ data, which represent more than 100,000 discrete samples (equivalent to more than 1100 orbits) have allowed us to accurately measure variations in the background proton flux seen by the FOS. An analysis of these variations have permitted us to model the geomagnetic environment of the South Atlantic Anomaly (SAA) as a function of time as well as the change in detector background as a function of geomagnetic latitude.

The Hubble Space Telescope High Speed Photometer (HSP) thermal control system uses a software control system instead of mechanical thermostats to control heaters. The most unreliable part of a conventional thermal control system, the thermostat, is eliminated in this design. In addition, the software control design provides great operational flexibility impossible to obtain with thermostats. The heaters can be controlled by a 'software thermostat' with its in-flight adjustable set points. The control system can also be operated in a variety of other modes, namely, a constant power mode, a power profile mode, and a direct commanding mode. The system can provide a given amount of energy into the heaters over a wide range of input bus voltages because bus voltage is sensed by the control software. Heater power is switched by the same relays that are needed in a thermostat system for power control. The system has proven to be adaptable to the changing needs of the Hubble Space Telescope mission. A similar system was designed and built into the Diffuse X-Ray Spectrometer instrument that was launched in January 1993. Experience from that mission is also described.

Observations of the sky background obtained with the Faint Object Spectrograph during 1991 - 1992 are discussed. Sky light can be an important contributor to the observed count rate in several of the instrument configurations especially when large apertures are used. In general, the sky background is consistent with the pre-launch expectations and showed the expected effects of zodiacal light and diffuse galactic light. In addition to these sources, there is, particularly during the daytime, a highly variable airglow component which includes a number of emission lines. The sky background will have an impact on the reduction and possibly the interpretation of some spectra.

The installation of COSTAR will improve the imaging performance of the Hubble Space Telescope such that the Faint Object Camera performance will approach that predicted before the discovery of the spherical aberration. However, this is not achieved without some undesirable side-effects. Despite these, it will be possible to achieve scientific goals with the COSTAR-corrected Faint Object Camera that are not currently feasible.

We present a discussion of the various sources of FOC Point Spread Functions (PSFs), and the problems associated with their use. In particular, we highlight the time variability of the PSF halo structure and indicate some of the causes. We examine the usefulness of the PSF modelling software, TinyTim, and show that although this software creates visually similar images, these similarities are to a large extent superficial. We conclude by showing that the theoretical PSFs produced by TinyTim are inadequate for the restoration of high S/N FOC point source images. We also conclude that, because of 'breathing', there is a strong likelihood that empirical PSFs, whether pre-existing or specifically obtained, may not be sufficient either.

The Coronal Diagnostic Spectrometer (CDS) to be carried on the joint ESA/NASA solar observatory satellite, SOHO, in late 1995 operates in the extreme ultra-violet (15 - 80 nm) and is thus particularly vulnerable to molecular contamination of the optical surfaces. Such contamination arises from materials present in the assembly and test environment and also from outgasses products from the various elastomers and other non-metals used for instrument construction. The need for secure surfaces free from contamination and the requirement for very low optical scatter drives the need for a comprehensive system of contamination control. We present our current work from a study in which we are attempting to identify typical contaminants found in space instrumentation, to understand and characterize their properties and to measure effects on the performance of an EUV optical system.

A new solution to the holographic grating equations designed for use in high resolution FUV applications is presented. The solution is shown through ray tracing to provide a sizable resolution enhancement over conventional Rowland circle grating solutions through the minimization of various aberration terms, especially astigmatism. This new design promises to also provide diffraction limited performance across a very large wavelength range.

The requirements for a second generation Hubble Space Telescope (HST) Science Instrument (SI) are similar to those of other space-borne instruments: the shuttle launch loads must be survived; the instrument must have specific dynamic characteristics; mass, size envelope, and electrical power constraints are imposed; thermal interfaces are defined; and a minimum on- orbit lifetime is required. The Near Infrared Camera and Multi-Object Spectrometer (NICMOS) differs from first generation and other second generation HST SIs in that it is an infrared instrument. The NICMOS detectors must be cooled to 58 K. This leads to a demanding instrument design that includes as an integral part of the optical bench design, a solid nitrogen (SN₂) cryogen dewar with a five year minimum lifetime goal. The dewar requires over 50% of the total instrument weight budget and occupies a significant portion of the available size envelope. Designing for five year cryogen lifetime while achieving a structural design that will meet launch loads and optical stability led to many design conflicts. The system level trades along with the structural and cryogenic lifetime analyses used to resolve these conflicts and arrive at the innovative NICMOS design will be discussed.

This paper discusses the mechanical design and assembly of the Near Infrared Camera and Multi-Object Spectrometer (NICMOS) focal plane assembly (FPA). The FPA consists of a mercury-cadmium-telluride (MCT) detector array hybridized to a silicon multiplexer (MUX), a sapphire carrier, an alumina ceramic multi-layer board (CMLB) including electrical components, a base plate, and flex cables. The FPA is designed for the following conditions; (1) shock and vibration during launch, (2) Coefficient of Thermal Expansion (CTE) of dissimilar materials at cryogenic temperature, (3) outgassing limitations to meet NASA's specifications, and (4) optical assembly tolerances. Also, the FPA is designed to be easily integrated into its dewar with provisions for mechanical as well as optical alignment. The FPA is assembled by building up two subassemblies in a parallel path, and then integrating the two subassemblies with the flex cables for the final assembly. These procedures are described in this paper, including alignment tolerances required and measured.

NICMOS is the near infrared second generation instrument for the Hubble Space Telescope. The NICMOS instrument consist of three infrared cameras with spectral response between 1 and 2.5 micrometers whose fields of view are 11, 19.3 and 51.1 arc seconds. Each camera has an independent filter wheel with 20 positions. In the wide field camera, two grisms will be substituted for two of the filters. Some unique challenges arise because (1) NICMOS must interface with HST, (2) it is a low background infrared device with warm optics and (3) it has a large cryogenic dewar enveloping the detectors. This paper summarizes the optical configuration of the instrument as well as some of the requirements that drive the design. Expected performance will be presented.

For detectors using microchannel plates (MCPs), the nonuniform response introduced by the finite size of the MCP pores has a significant effect when the size of a resolution element is comparable to the spacing between the pores (approximately 10 - 15 micrometers ). For the Far Ultraviolet Spectroscopic Explorer (FUSE) spectrograph, which will employ a delay line detector, the instrument plate scale, nominal slit width (1 arcsecond), and well-corrected optical design combine to produce slit-limited images 25 micrometers in width with resolution elements 32 micrometers wide, and a nominal resolution of (lambda) /(Delta) (lambda) equals 30,000. At these scales, the MCPs will sparsely sample spectral line images, resulting in significant pixellation effects. We have constructed a computer model of a microchannel plate detector to simulate these effects and evaluate the performance that can be expected from the FUSE detector. These simulations have been compared to actual images obtained with a laboratory version of a delay line detector. Slit patterns imaged onto the detector were chosen to simulate the resolution expected from the FUSE spectrograph in order to provide an estimate of the expected resolving power and test the effects of several detector parameters on resolution. Details of the model are described, and a comparison of the results with laboratory data is made. The implications for FUSE are also discussed.

The theoretical encircled energy and wide angle scatter of the SUMER Demonstration Telescope (SDT) mirror was analyzed at the wavelength of 123.6 nm. The modeling was accomplished with two software packages: (1) Metrology Data Processor (METDAT) and (2) Optical Surface Analysis Code (OSAC). The modeling is based on measured mirror surface figure error data and roughness characteristics covering all important spatial frequencies affecting the imaging in the Far Ultraviolet (FUV) wavelength region. The performance of the SDT mirror including the encircled energy and wide angle scatter was also directly measured at 123.6 nm. We found an excellent agreement between the measured and theoretical encircled energy up to about 3 arcseconds from the peak and wide angle scatter up to about 20 arcminutes from the peak at 123.6 nm. The 80 percent encircled energy diameter of the SDT mirror is about 2.6 arcseconds and the amount of scattered light drops to about 5.0 X 10^-10 of peak irradiance 20 arcminutes from the peak. The performance of the mirror in the FUV is degraded by the mid-frequency errors.

Lightweight, high performance optical systems have historically relied upon ultralightweight optical components to achieve high stiffness, low weight, high quality optical surfaces exhibiting high thermal stability. Composite Optics, Incorporated (COI) has independently pursued state-of-the-art graphite fiber reinforce composite (GFRC) substrates for microwave and infrared (IR) applications. Eastman Kodak Company (Kodak) and COI have participated in a joint evaluation of hybrid optical mirrors fabricated from low coefficient of thermal expansion (CTE) graphite composite materials and ULE^TM low CTE glass. While glass can be polished to achieve an optical quality surface, relative to other mirror substrates, GFRC attractively offers high specific stiffness and low steady state and transient distortion characteristics as shown. This joint effort between Kodak and COI has resulted in the demonstration of processed optical surfaces within 0.05 waves rms (at 0.63 micrometers ). Optical surfaces have remained stable to within 2 waves rms over a wide range of temperatures (-13 to 65 degree(s)C). The optical performance demonstrated meets the requirements for long wavelength systems, with promise of satisfying visible wavelength performance with further development.

A novel micromachined electrostatically controlled deformable mirror has been fabricated and characterized. This device combines the fields of microinstruments, adaptive optics and controls to form a silicon-based mirror assembly that is relatively simple to process, inexpensive, lightweight, and integrable with drive and sensing electronics. Electrostatic control of a thin membrane mirror is demonstrated with low voltage actuation and without the need for complex construction of PZT or other translator-type arrays. In addition, the low- stress Si-rich Si_xN_y film used as the deformable membrane mirror is thermally matched to the silicon supporting frame. Custom design of the mirror shape can be implemented by redesigning the electrode pattern on an insulating substrate separate from the thin film mirror. Test results from a pull-only circular mirror with a single actuator are presented as a proof of concept for low voltage actuation of a low-stress Si_xN_y flexible membrane. The Si_xN_y which forms the membrane is under tensile stress. This tensile stress increase the voltage required for deflection of the membrane, but insures a linear relationship between the center deflection of the mirror and the applied pressure. This should significantly simplify the controls algorithm required for closed-loop operation of this device.

Segmentation of the primary mirror offers an inexpensive method to produce large, active telescopes or beam directors. We discuss the goal of segment control and our analytic solution configuration functional. We show that the nature of the segmentation, control, and solution generates an additional source of diffractive errors that must be accounted for in the design of these systems. These diffractive errors and not r₀ ultimately set the segment size.

A study for the optical design of an UV-imaging camera is briefly reported. We emphasize the guidelines that drove the design choices adopted, as trade-off between optical quality and efficiency. Optical solutions for an additional long focal length channel are also given. This study is performed in the framework of the SUV project, a 170 cm Ritchey-Chretian space telescope to be made with the collaboration of Russia, Ukraine, Italy and Germany.

Many large optical telescopes have a wide field camera as one of their primary instruments. Camera designs with all reflective surfaces are useful because they give stable image quality over a broad spectral band. Systems with two conic mirrors can correct the residual aberrations, field curvature and astigmatism, of Ritchey-Chretien telescopes. We present first- and third-order optical design relations which can facilitate the formation of design concepts during preliminary trade studies. We apply the relations to the Hubble Space Telescope in the design of a new wide field camera which compensates the existing spherical aberration, as well as the astigmatism and field curvature, of the telescope.

The Advanced Satellite for Cosmology and Astrophysics, ASCA, is a joint Japanese-United State X-ray astrophysics observatory which was launched on February 20th 1993. The scientific payload comprises four identical grazing incidence X-ray mirrors complemented by two charge-coupled devices and two gas scintillation proportional counters as the focal plane detectors. This paper presents the latest work carried out to improve the quality of ASCA images using the Lucy-Richardson deconvolution method. The ability to resolve two point sources is studied under various conditions of separation, relative intensity, and signal-to- noise. The method is also tested with an extended source. The minimum separation which can be resolved is 30 arcseconds, corresponding approximately to the radius of the core of the PSF. There is also some advantage to be gained in the relative orientation of the sources. Sources of unequal intensity must be separated further in order to be resolved, for example, when one source is half the intensity of the other source the minimum separation is 45 arcseconds. A signal-to-noise ratio of 5(sigma) is the lower limit for resolving two sources of equal strength 30 arcseconds apart. Deconvolution of the simulated image of the supernova remnant CAS-A is successfully carried out and the resulting image has about one arcminute resolution, limited by the PSF core width.

Coded mask X-ray and gamma-ray telescopes are the only way of obtaining true images in the photon energy range from approximately 10 keV to a few MeV. The detectors used must be position sensitive, and the types employed in gamma-ray coded mask telescopes up to now have had limited energy resolution. With a view to developing position sensitive detectors which have the energy resolution attainable with Germanium we have procured and characterized in the laboratory a detector comprising a small array of high purity Germanium elements each 15 X 15 X 50 mm. Although having only nine elements, its construction is such that is should later be possible to build larger modules in the same way and finally to assemble modules into a large detector plane array. The nine element array is being incorporated into a coded mask telescope which will be tested in a balloon flight. Laboratory tests on the array detector and comparisons with simulations are reported and the anticipated performance of the small array telescope considered. The feasibility of a large instrument based on this approach, which is under study for a space mission, and its expected capabilities are discussed.

The importance of calibration subsystems as part of overall system design has grown with the increasing sophistication and complexity of remote sensing and imaging instruments. In general they provide spectral and radiometric reference data in-situ under remote and sometimes unpredictable instrument conditions which are used to correct electronic, optical and detector nonlinearities. The recent difficulties with GOES satellites underline the importance of reliable calibration sources for space applications. Although the calibration component is a small fraction of the budget of any space qualified instrument, its failure can result in a catastrophic loss of data. A group of sources will be described which have been developed for in-flight and pre-flight calibration of a variety of space astronomy and space physics experiments with different requirements in terms of wavelength coverage, power budget, size requirements and radiation hardness.

Some innovative approaches to the design of a 16 meter filled-aperture, UV to IR, high spatial resolution, wide field-of-view space telescope are presented. The purpose of this paper is to stimulate the discussion of innovative concepts for a second generation space telescope. The ideas in this paper are not tested or analyzed. They are simply concepts that might prove to be applicable and which will have to be tested and developed, and possibly rejected. Comments on the concepts presented in this paper will be welcomed by the author.

This paper first considers the design of robotic telescopes to monitor faint objects in crowded fields. It shows that the mechanical design problems have been solved by the use of precision control and modelling software developed for the latest large telescopes. Modern design methods mean that these telescopes can be produced relatively cheaply. The largest part of the cost of a robotic telescope is the software to enable it to work as an autonomous robot. Conventional software techniques are inadequate and inefficient for many purposes associated with robotic operation. These include: to optimize and monitor their operation and efficiency, to schedule their observing, to evaluate their environment, to generate confidence in the target acquisition pattern recognition parameters, to evaluate the quality of the CCD images and the photometry of the objects within the images, and to return reduced data to the astronomer with the required indices to gives the astronomer confidence in the data. The paper evaluates AI, neural nets and fuzzy logic techniques applied to these different problems.

There are several problems in astrophysics that cannot be addressed until very high resolution spectroscopy ((lambda) /(Delta) (lambda) > 100,000) in the far ultraviolet ((lambda) < 2000 angstroms) can be achieved simultaneously with high efficiency. Issues such as atomic isotopic abundances in the interstellar medium, velocity structure in interstellar clouds, and fine and hyperfine line structure in atomic transitions require 100,000 - 200,000 class resolution and high sensitivity; a capability that currently does not exist. Historically, resolutions this high have been obtained with echelle spectrographs, which require two gratings, and must suffer the losses due to reflective efficiency and diffraction efficiency on both gratings. These losses are much more significant in the far ultraviolet than in the visible. We present a means of obtaining very high resolution spectroscopy in the far ultraviolet with a single, holographic grating, which should be significantly more efficient than classic echelle designs.

The performance of space telescopes, space instruments, and space radiator systems depends critically upon the selection of appropriate spectrally selective surfaces. Many space programs have suffered severe performance limitations, schedule setbacks, and spent hundreds of thousands of dollars in damage control because of a lack of readily-accessible, accurate data on the properties of spectrally selective surfaces, particularly black surfaces. A Canadian effort is underway to develop a resource base (database and support service) to help alleviate this problem. The assistance of the community is required to make the resource base comprehensive and useful to the end users. The paper aims to describe the objectives of this project. In addition, a request for information and support is made for various aspects of the project. The resource base will be useful for both ground and space-based instrumentation.

The Astrometric Imaging Telescope, an orbiting 1.5 m low-distortion Ritchey-Chretien, will use a large format CCD to record star trails as the CCD is dragged across the image plane. Star-trail separations, when averaged over thousands of pixels, yield photon-noise limited centroids with 10 micro-arcsecond accuracy. In this paper, we will discuss the important CCD and optical design parameters that affect astrometric accuracy. For the CCD, these include charge transfer efficiency, pixel-to-pixel relative quantum efficiency, sub-pixel QE gradients, and systematic pixel dislocations. For optical design, they are tolerancing to parameters such as secondary mirror decenter and tilt, and conic constants. We present a point design for a system that can achieve 10 micro-arcsecond accuracy over a long-term mission. End-to-end modeling, including high precision diffraction calculations, is used to validate the design.