The Hubble Space Telescope suffers from spherical aberration caused by a spacing error in the null lens used to fabricate and test the primary mirror. Corrective optics must be installed on-orbit in order to enable many key programs. An analysis of imaging data obtained on-orbit gives the same results as measurements on the faulty ground test apparatus, so such optics can be designed with confidence. The assumed conic constant on the primary mirror for all the corrective optics, -1.0139(5), is consistent with measurements by four major independent methods. Aligning the new optics will be very demanding, because of the large slope of the wavefront to be corrected. If the images are to be diffraction limited, the pupil at the corrective element must be aligned to better than one percent of its diameter. Some other residual effects of the spherical aberration will remain after installation of the corrective optics, primarily in the pointing and collimation of the telescope.

A method for correcting flaws in the primary mirror of the HST that is under intense study by NASA is discussed. The proposal is to install corrective optics in front of the existing science instruments using the Corrective Optics Space Telescope Axial Replacement (COSTAR). COSTAR is expected to recover the rated capability of the HST.

The Goddard High Resolution Spectrograph (GHRS) has completed Orbital Verification and is well into the Science Verification phase of its mission. The instrument performance has been flawless, and many significant early science observations have been completed. The GHRS digicon detectors are well calibrated including the determination of operating parameters, detector geometry, and noise sensitivity. Tests using calibration lamps and standard UV stars have confirmed the instrument sensitivity and spectral resolving powers of Lambda/Delta-Lambda = 2000, 20,000, and 90,000. The sensitivity has not changed since the 1984 baseline ground based calibration. The GHRS flight software has been thoroughly tested, and is controlling all instrument observing as expected. Basic target acquisition testing and GHRS alignment calibrations have been successfully completed, and targets are routinely being located within 2-3 arcsecs of the initial pointing. Observations have been successfully performed using both the 2.0 x 2.0 arcsec aperture, and the smaller 0.25 x 0.25 arcsec aperture. The extended point spread function caused by the spherical aberration of the HST primary mirror has been well measured, and observing methods to deal with it have been developed. The aberrated image allows approximately 70 percent of the total energy into the large science aperture, and 15 percent of the total energy into the smaller aperture. Numerous science assessment observations of interesting astronomical targets have been completed, and indicate the extreme usefulness of the GHRS to the scientific community.

An overview of the Faint Object Camera and its performance to date is presented. In particular, the detector's efficiency, the spatial uniformity of response, distortion characteristics, detector and sky background, detector linearity, spectrography, and operation are discussed. The effect of the severe spherical aberration of the telescope's primary mirror on the camera's point spread function is reviewed, as well as the impact it has on the camera's general performance. The scientific implications of the performance and the spherical aberration are outlined, with emphasis on possible remedies for spherical aberration, hardware remedies, and stellar population studies.

The Hubble Space Telescope High Speed Photometer has four image dissector tubes, two with UV sensitive photocathodes, two sensitive to the near UV and to visual light, and a single red sensitive photomultiplier tube. The HSP is capable of photometric measurements from 1200 to 7500 A with time resolution of 11 microseconds and has no moving parts. An initial analysis of the on-orbit engineering performance of the HSP is presented with changes in operating procedures resulting from the primary mirror spherical aberration and experience gained during the verification period.

The on-orbit performance of the HST + FOS instrument is described and illustrated with examples of initial scientific results. The effects of the spherical aberration from the misfiguring of the HST primary mirror upon isolated point sources and in complex fields such as the nuclei of galaxies are analyzed. Possible means for eliminating the effects of spherical aberration are studied. Concepts include using image enhancement software to extract maximum spatial and spectral information from the existing data as well as several options to repair or compensate for the HST's optical performance. In particular, it may be possible to install corrective optics into the HST which will eliminate the spherical aberration for the FOS and some of the other instruments. The more promising ideas and calculations of the expected improvements in performance are briefly described.

The Fine Guidance Sensors of the Hubble Space Telescope have two roles to play. They are the ultimate pointing and control instruments onboard the spacecraft and they are the primary astrometric instruments of the observatory. Because they are used for every scientific observation with the Hubble Space Telescope, independently of the ultimate scientific instrument being utilized, there is much more experience with them than with any of the other scientific devices. The Fine Guidance Sensors have already serendipitously discovered their first binary star from the Hubble Space Telescope Guide Star Catalog, have been exercised in most of their observing modes, and are able to fulfill their pointing and control functions up to their original specifications (in the absence of external influences). In addition, the imperfections in the primary mirror have minimally affected the performance of the Fine Guidance Sensors. An up-to-date summary of FGS engineering and science will be presented.

Both the amplitude and the phase of the incoming wave front viewed from the focal plane are reconstructed from the illumination recorded in defocused stellar images taken in flight by the HST. The reconstructed wavefront surface shows very little aberration. Compared to geometrical optics methods, the technique produces high spatial resolution wavefront maps with images taken closer to the telescope's focal plane. For the HST, the quality of the wavefront reconstruction is limited by the telescope jitter. The technique cannot be applied to ground-based telescopes at optical wavelength where the image is heavily blurred by the atmosphere. It is argued to be promising as a diagnostic tool for optical telescopes in space.

Attention is given to the science goals of the Next Generation Space Telescope (NGST), namely, detection and spectroscopy of gas giant planets to earthlike planets around nearby stars, study of nearby star-forming complexes with resolutions of 5-50 AU in the visible and IR, measurement of stellar populations in a variety of environments, spatial resolution and mapping of complex structures in the inner narrow line regions for AGN and QSOs, and investigation of the structure and evolution of galaxies at redshifts greater than 1. The NGST characteristics, milestones, long-term strategy, and launch capability are also discussed.

To assess the systems and technological requirements for constructing lunar telescopes in conjunction with the buildup of a lunar base for scientific exploration and as a waypoint for travel to Mars, the NASA Marshall Space Flight Center conducted concept studies of a 16-m-aperture large lunar telescope (LLT) and a 4-m-aperture precursor telescope, both operating in the UV/visible/IR spectral region. The feasibility of constructing a large telescope on the lunar surface is assessed, and its systems and subsystems are analyzed. Telescope site selection, environmental effects, and launch and assembly scenarios are also evaluated. It is argued that key technical drivers for the LLT must be tested in situ by precursor telescopes to evaluate such areas as the operations and long-term reliability of active optics, radiation protection of instruments, lunar dust mitigation, and thermal shielding of the telescope systems. For a manned lunar outpost or an LLT to become a reality, a low-cost dependable transportation system must be developed.

The paper discusses the optical requirements for future large space or lunar telescopes and their influence upon the fundamental system parameters. Two telescope concepts, a four-reflection three-mirror telescope, and a spherical-primary four-mirror telescope are introduced. Both have the potential for extreme high performance, and both use four reflections to accomplish this. Considering the present available coating capabilities, the reflectivity loss in the lower UV region due to the additional reflections, compared to a two-mirror system, seriously affects the utility of four-mirror telescopes for this part of the spectrum. Separate telescopes for the UV and for longer wavelengths may therefore be a practical solution, unless significant advances in coating technology are made in the foreseeable future.

NASA is considering a 16-m diameter optical telescope on the moon as a part of the Space Exploration Initiative. Fundamental concepts of engineering activities on the moon and how they can be applied to the establishment of a 16-m large lunar telescope (LLT) are discussed. These fundamental concepts include the engineering response of lunar soils and how they affect construction activities, namely, drilling, blasting, ripping, digging and compaction. A mirror support structure and foundation design concept is proposed. The foundation considered is a multiple contact points spud-can type footing. It does not appear that a deep foundation or the presence of bedrock is required to achieve the telescope foundation stiffness. The LLT system will include a regolith covered housing, the size of a small room, which will contain sensitive electronic equipment including charge coupled devices which need protection from cosmic radiation effects. A brief discussion is made on radiation, radiation transport and radiation effects on electronics and on humans. Radiation protection techniques and the different emplacement schemes for the LLT instrument housing for radiation protection are suggested. A structural concept of an early lunar based telescope is also presented.

A 16-m telescope which NASA is proposing building on the moon surface for extraterrestrial observations is discussed. The telescope operates in the UV/visible/IR ranges of the light spectrum and is capable of detecting planets the size of the earth which are many light years away. Results are presented of a study conducted at the Marshall Space Flight Center to identify different design options for a pedestal to support the primary structure which supports the primary and secondary mirrors. Different foundation systems that can be used to anchor the telescope to the lunar surface are considered. Problems associated with constructing the telescope on the lunar surface are identified, solutions to these problems are suggested using presently available technologies, and new technologies are proposed.

The 16-meter diffraction limited lunar telescope incorporates a primary mirror with 312 one-meter segments; 3 nanometer active optics surface control with laser metrology and hexapod positioners; a space frame structure with one-millimeter stability; and a hexapod mount for pointing. The design data needed to limit risk in this development can be obtained by building a smaller engineering telescope on the moon with all of the features of the 16-meter design. This paper presents a 4.33-meter engineering telescope concept developed by the Summer 1990 Student Program of the NASA/JHU Space Grant Consortium Lunar Telescope Project. The primary mirror, made up of 18 one-meter hexagonal segments, is sized to provide interesting science as well as engineering data. The optics are configured as a Ritchey-Chretien with a coude relay to the focal plane beneath the surface. The optical path is continuously monitored with 3-nanometer precision interferometrically. An active optics processor and piezoelectric actuators operate to maintain the end-to-end optical configuration established by wave front sensing using a guide star. The mirror segments, consisting of a one-centimeter thick faceplate on 30-cm deep ribs, maintain the surface figure to a few nanometers under lunar gravity and thermal environment.

The designs and performances of lunar optical and submillimeter interferometer systems for the Space Exploration Initiative (SEI) are described. Attention is given to the conceptual design of these systems and to the concepts for the launch, transport, deployment, maintenance, and toleration of the lunar environment (both natural and induced by crew activity). Consideration is also given to the requirements for SEI resources (such as cargo space and crew time), the data return concepts, and the flight system technonogy develpment needs.

The moon offers particular advantages for interferometry, including a vacuum environment, a large stable base on which to assemble multi-kilometer baselines, and a cold nighttime temperature to allow for passive cooling of optics for high IR sensitivity. A baseline design for a Lunar Optical Interferometer (LOI) which exploits these features is presented. The instrument operates in the visible to mid-IL region, and is designed for both astrometry and synthesis imaging. The design uses a Y-shaped array of 12 siderostats, with maximum arm lengths of about 1 km. The inner siderostats are monitored in three dimensions from a central laser metrology structure to allow for high precision astrometry. The outer siderostats, used primarily for synthesis imaging, exploit the availability of bright reference stars in order to determine the instrument geometry. The path delay function is partitioned into coarse and fine components, the former accomplished with switched banks of range mirrors monitored with an absolute laser metrology system, and the latter with a short cat's eye delay line. The back end of the instrument is modular, allowing for beam combiners for astrometry, visible and IR synthesis imaging, and direct planet detection. With 1 m apertures, the instrument will have a point-source imaging sensitivity of about 29 mag; with the laser metrology system, astrometry at the microarcsecond level will be possible.

A conceptual design is described for a lunar submillimeter wavelength interferometer called SALSA, a Synthesis Array for Lunar Submillimeter Astronomy. Its design is greatly simplified over conventional submillimeter wavelength arrays because it takes advantage of a beam waveguide to relay signals to a centrally-located receiver system. The array is optimized to synthesize images at wavelengths between 60 and 300 microns, and it has an angular resolution of 10 milliarcsecs at 60 microns. Observations in this region provide unique insights into fundamental issues in astrophysics such as star formation, but are impossible from the earth's surface because of atmospheric absorption. The baseline design for SALSA consists of twelve, 3.5-meter diameter antennas arranged in a Y-shaped configuration consisting of three 0.5-km long arms, each with four antennas distributed according to a power-law function of distance. Such a beam-waveguide approach has significant advantages, since all the high-power-consumption items (active cryogenics, correlator) are at a central location and could share a single power source. In addition, the antennas can be quite simple and maintenance is greatly reduced. The primary technology challenges for developing such an array are the coherent detectors (mixers and local oscillator sources), and ensuring the dimensional stability of the optical elements under the extreme lunar thermal environment (85-385 K).

The properties, origins, and previous operational experiences with lunar dust are discussed, with emphasis on the implications for world-class astronomy on the moon. The mechanisms that may govern the behavior of the fine particles are suggested, and working hypotheses for mitigating the dust hazard are advanced. Future experiments, both on the moon and in terrestrial simulations, that will assist in establishing effective and suitable means of limiting deleterious effects of dust on observatory operations are outlined. Dust studies of components returned by Surveyor 3 are presented. The performance of laser retroreflectors under conditions of moon dust is discussed.

The effects of the hostile lunar environment are assessed, and potential techniques for adverse-effect mitigation are developed. The environmental concerns addressed include Galactic cosmic ray (GCR) effects on telescope electronics, lunar dust obscuration and damage to optical surfaces, and micrometeor cratering of the optics and support structure. The feasibility of shielding the electronics from the GCR flux and associated secondaries is investigated as one option for noise reduction. An alternative approach to noise reduction uses shorter integration ties and multiple images for background subtraction. Dust abatement techniques such as stabilizing the lunar soil at the launch and telescope sites and covering the optics during high contamination-risk times are evaluated. The micrometeorite flux and associated surface cratering are assessed for their impact on the lifetime and integrity of the telescope.

The advantages of using a liquid mirror in a lunar telescope are summarized. Interferometric measurements of a 1.5-m diameter liquid mirror are presented which show that liquid mirrors can have the very high optical quality required of a lunar telescope. Some of the liquid metals that could be used are discussed.

To assess the design feasibility of the Space Infrared Telescope Facility (SIRTF) and to identify parameters that might impose constraints on performance such as frequencies of vibration, structural concepts for both the telescope and spacecraft are developed and evaluated. Trade studies of key design features are carried out using FEM and analysis. In most cases, the margin of safety was greater than 0.50. An example of stress in the octagonal equipment bus panels is shown. The strap-supported mass is predicted to deflect 6 mm relative to the outer shell when subjected to 8.0 G in the X direction, and to deflect about 5 mm in the Y and Z directions when subjected to 10.0 G. Deflections for the top of the solar panel are predicted to be about 35 mm when subjected to the 10.0 G quasi-static load in the Z direction. The liquid helium tank, thermal isolation, and primary mirror and mount are discussed.

The paper discusses the Infrared Space Observatory (ISO) optical subsystem, a cryogenically cooled telescope featuring main baffle and sunshade. The telescope, a 60-cm Ritchey-Chretien type, focuses the beam to the four scientific instruments located in its focal plane. The extremely low temperature, 1.8 K, is provided by the payload module cryostat, filled with superfluid He. The design of these elements is supported by different mathematical models and by dedicated elementary tests linked to operation in the cryogenic environment. Tests performed at ambient and cryogenic temperatures demonstrate the good behavior of the optical subsystems for thermal, mechanical, and optical performance.

The three mirrors comprising the Infrared Space Observatory telescope are discussed. The mirrors are made of fused silica substrate using a Ritchey-Chretien configuration. The primary mirror is aspherical, has an aperture diameter of 640 mm, and a fast F number of F/1.6. The mirror is mechanically lightweighted to a ratio of 70 per cent and is figured to lambda/90 RMS at the specified wavelength of 5 microns. The aspherical secondary mirror has a diameter of 90 mm. The pyramidal mirror's four sides reflect the incoming light in four perpendicular directions toward the scientific instruments. Consideration is given to the manufacturing steps of these mirrors, including the lightweighting process, the thermal and mechanical interface machining, the acid etching, the aspherical grinding and polishing, the coating, and the assembly on their support structure. Wavefront error measurements show diffraction-limited performance for each mirror as well as for the whole telescope.

An integrated Large Deployable Reflector (LDR) analysis model was developed to enable studies of system responses to the mechanical and thermal disturbances anticipated during on-orbit operations. Functional requirements of the major subsystems of the LDR are investigated, design trades are conducted, and design options are proposed. System mass and inertia properties are computed in order to estimate environmental disturbances, and in the sizing of control system hardware. Scaled system characteristics are derived for use in evaluating launch capabilities and achievable orbits. It is concluded that a completely passive 20-m primary appears feasible for the LDR from the standpoint of both mechanical vibration and thermal distortions.

The overall AXAF program is summarized, with particular emphasis given to its science instruments. The science objectives established for AXAF are to determine the nature of celestial objects, from normal stars to quasars, to elucidate the nature of the physical processes which take place in and between astronomical objects, and to shed light on the history and evolution of the universe. Attention is given to the AXAF CCD imaging spectrometer, which is to provide spectrally and temporally resolved imaging, or, in conjunction with transmission grating, high-resolution dispersed spectral images of celestial sources. A high-resolution camera, an X-ray spectrometer, and the Bragg Crystal Spectrometer are also discussed.

The development of multilayer reflective coatings now permits soft X-ray, EUV and FUV radiation to be efficiently imaged by conventional normal incidence optical configurations. Telescopes with quite modest apertures can, in principle, achieve images with resolutions which would require apertures of 1.25 meters or more at visible wavelengths. The progress is reviewed which has been made in developing compact telescopes for ultra-high resolution imaging of the sun at soft X-ray, EUV and FUV wavelengths, including laboratory test results and astronomical images obtained with rocket-borne multilayer telescopes. The factors are discussed which limit the resolution which has been achieved so far, and the problems which must be addressed to attain, and surpass the 0.1 arc-second level. The application of these technologies to the development of solar telescopes for future space missions is also described.

Attention is given to the design of a Mills Cross imaging interferometer in which the arms are fully filled with mirror segments of a Ritchey-Chretien primary and which has sensitivity to 27th magnitude per pixel and resolution a factor of 10 greater than Hubble. The optical design, structural configuration, thermal disturbances, and vibration, material, control, and metrology issues, as well as scientific capabilities are discussed, and technology needs are identified. The technologies under consideration are similar to those required for the development of the other imaging interferometers that have been proposed over the past decade. A comparison of the imaging capabilities of a 30-m diameter FFT, an 8-m telescope with a collecting area equal to that of the FFT, and the HST is presented.

This paper describes a proposed third-generation Hubble instrument for extra-solar planet detection, the Hubble Extra-Solar Planet Interferometer (HESPI). This instrument would be able to achieve starlight cancellation at the 10 exp 6 to 10 exp 8 level, given a stellar wavefront with phase errors comparable to the present Hubble telescope wavefront. At 10 exp 6 starlight cancellation, HESPI would be able to detect a Jupiter-like planet next to a star at a distance of about 10 parsec, for which there are about 400 candidate stars. This paper describes a novel approach for starlight suppression, using a combination of active control and single-mode spatial filters, to achieve starlight suppression far below the classical limit set by scattering due to microsurface imperfections. In preliminary lab experiments, suppression by a factor of 40 below the classical scatter limit due to optical wavefront errors has been demonstrated.

The basic design of the different units constituting the full CFRP telescope for the PRONAOS submillimetric mission is described. The alignment technique is discussed, and results of optical performance measurements are presented. The instrumentation comprising the telescope consists of a reference CFRP box made of two floor sandwich panels distanced by a frame of flat sandwich panels. It provides all the mechanical interfaces internal to the telescope as well as all those needed with the gondola. The secondary structure is also made from CFRP beams organized in a framework which provides the fixations for the thermal protection panels and which ends in an electroactuated aperture door. The PRONAOS telescope's deployed configuration is illustrated. The adequacy of the semiactive mirror concept to meet very low areal mass while obtaining ultimate surface accuracy in the submillimeter wavelength domain is demonstrated.

The alignment concept of ORFEUS, a short-term scientific space payload scheduled for launching by the STS in January 1993, is discussed. ORFEUS comprises two alternatively operating spectrometers (Echelle and Rowland) implemented in a CFC telescope with a 4-m tube length and an aperture of 1000 mm. The lightweight primary mirror has a focal length of 2426 mm. In order to achieve the required spectrometric high telescope resolution in the UV range (40-125 nm), a sophisticated alignment concept was developed. The centering of the alignment diaphragm (diameter: 15 microns) in the focus of the primary mirror has to be provided in the vertical tube position by means of an autocollimation telescope. The spectrometers have to be integrated into the horizontal telescope aligned within a special antigravity device to reduce optical surface deformations and to ensure the optical performance of the primary. The alignment of all optical components is to be performed in the visible spectral range.

A family of imaging, pulse-counting, photoelectric detector systems, the Multi-Anode Microchannel Arrays (MAMAs), are now under active development for use on a number of space ultraviolet astrophysics missions at far-ultraviolet (FUV) and extreme-ultraviolet wavelengths between about 300 and 28 nm. Specifically, MAMA detectors are being fabricated and tested for use in two instruments on the ESA/NASA Solar and Heliospheric Observatory mission, for the NASA Goddard Space Flight Center's Hubble Space Telescope Imaging Spectrograph, and for the prime FUV spectrograph of the Far Ultraviolet Spectroscopic Explorer FUSE/Lyman mission. The construction and performance characteristics of the different MAMA detector systems are described, and techniques for improving the spatial resolution of each of the detector systems by the use of custom application specific integrated circuits in the electronics are discussed.

This paper provides a review of a custom multiplexer circuit designed for use with a 256 element InSb linear array in the CRAF/Cassini VIMS instruments. The requirements, operation noise model and test results from a prototype 1 x 64 array are discussed. The infrared focal plane array (FPA) preliminary design and the impact of the new multiplexer on the instruments' predicted performance will be discussed. Emphasis will be placed on the multiplexer that was designed for the CRAF/Cassini missions. The FPA assembly combines electronic and optical components into a single hermetically sealed hybrid package. The detector configuration is that of a linear dual-multiplexed indium antimonide array with 256 elements, each 103 x 200 on 123-micron centers.

The study presents recent performance results for a 2D InSb hybrid focal plane array. The short wavelength (1 to 3 microns) response at liquid helium temperatures was improved, making the device more useful for astronomy and other scientific applications. The detectors exhibit high short wavelength quantum efficiency and good uniformity.

A brief description of IR sensor devices for astronomical observation in the 4-17 micron wavelength band using Si:Ga detectors is given. These devices are to equip ISOCAM, a camera which will operate from the Infrared Space Observatory, the European satellite expected to be launched in May 1993, and C10-mu, a French astronomical camera based at the Canadian French Hawaii Telescope. These sensor devices are polylithic dies: the photoconductor array is hybridized by indium bumps to the readout circuit. Reliability tests show that neither thermal cycles nor strong acceleration or vibrations degrade the mechanical behavior of such a structure. A comparison between ISOCAM and the C10-mu detector is presented in tabular form.

During the outgassing of orbiting astronomical observatories, the condensation of molecular species on optical surfaces can create difficulties for astronomers. The problem is particularly severe in ultraviolet astronomy where the adsorption of only a few atomic layers of some substances can be very damaging. In this paper the removal of adsorbed atomic layers using carbon dioxide snow is discussed. The rate of removal of adsorbed layers of isopropyl alcohol, Freon TF, and deionized distilled water on Teflon substrates was experimentally determined. The removal of fingerprints (containing fatty acids such as stearic acid) from optical surfaces is also demonstrated. The presence and rate of removal of the multilayers was monitored by detecting the molecular dipole field of adsorbed molecular species. For isopropyl alcohol, Freon TF (trichlorotrifluoroethane), and water adsorbed multilayers were removed in under 1.5 seconds. Fingerprint removal was much more difficult and required 20 seconds of spraying with a mixture of carbon dioxide snow flakes and atomized microdroplets of isopropyl alcohol.

The large lunar telescope is a proposed moon-based telescope which incorporates a sixteen-meter segmented primary mirror. An error budget is developed for the active control system of the primary mirror. A control methodology for the primary mirror is then described which utilizes piston sensors for measuring the relative piston error between adjacent segments as well as a separate sensor which measures the tilt of each segment with respect to the pointing direction of the telescope. A trade study is conducted in which the following types of tilt sensors are examined to determine their applicability to this program: stellar wavefront sensors, such as a Hartmann-Shack or a shearing interferometer; holographic optical elements; interferometers; scanning systems; and some nonoptical systems which electronically measure the relative tilt between adjacent segments. In addition, two independent methods of quantitatively verifying the performance of the telescope using either a phase retrieval algorithm or an image sharpening technique, both of which are based on the quality of a stellar image, are presented.

A technique has been developed which permits toroidal, and coma-corrected toroidal, diffraction gratings to be replicated from spherical master gratings by the use of elastically-deformable substrates. Toroidal gratings correct for astigmatism and, thus, make it possible to construct stigmatic spectrometers that employ a single reflective diffraction grating. These spectrometers are particularly useful for the extreme-ultraviolet (EUV) wavelength range, where reflection coefficients are low, since the single optical surface provides for dispersion, focusing, and astigmatism correction. The fabrication procedures for the pure toroidal, and coma-corrected toroidal, gratings are described, and initial test results are presented. The use of the toroidal gratings in a high-resolution sounding-rocket EUV spectroheliometer, and in both the coronal diagnostics spectrometer and the ultraviolet coronagraph spectrometer on the ESA/NASA solar and heliospheric observatory mission, is described briefly, and the use of this technique for the fabrication of a coma-corrected toroidal grating for the prime Rowland spectrograph of the FUSE/Lyman mission is briefly discussed.

Present-day glass mirrors for telescopes, including the most recent results obtained with aluminum mirrors developed within the European EUREKA procedure (LAMA program) are reviewed. The major advantages of the aluminum-alloy solution, which can be extrapolated today for large size, are discussed. It is shown that aluminum-alloy meniscus blanks, polished on a thin nickel coating, are appropriate to manufacture mirrors of astronomical quality. With the technique of electron-beam welding, large sizes can be envisaged. The development of active optics makes it possible to easily compensate for real-time deformations. The good thermal diffusivity of aluminum alloys leads to a better and faster thermal equilibrium than all other glass structures.

In this paper the fabrication of ultralight mirror segments is described. The mirrors are made from HTP (high thermal performance material), better known as a third generation derivative of the Space Shuttle heat shield tile. The HTP material has a density of only about 0.14 gms/cc and is made from tangled fibers of aluminum oxide and silicon dioxide sintered together as a non-isotropic material. HTP material is also stiff, and undergoes very small shrinkage and expansion due to thermal effects. When one side of an HTP surface is coated with a suitable glassy substance, such as fused silica, fine mirrors can be made. HTP material also outgasses quickly, thereby preventing annoying virtual leaks which can result in condensation of gaseous species onto optical surfaces. Experiments have shown that a one-centimeter-thick specimen of HTP material will outgas from one atmosphere pressure down to the ambient pressure at low earth orbit in eight minutes when the pumping rate is 200 liters/sec.

Located on a 2076 m summit in Arizona, the present all-reflective McMath optical system consists of a 2.0-m CERVIT flat mounted as a heliostat to follow the sun, a 1.6-m 86.4 m focal-length quartz concave positioned within an inclined underground tunnel, and a 1.5-m CERVIT flat which directs the image to different fixed instrument stations. The building is adequate to accommodate a 6.0-m tracking feed and a 4.0-m concave, resulting in an f/22 beam. A 4.0 m aperture is desirable for adequate flux and resolution at 12 microns where a number of Zeeman sensitive atomic lines are found, lines which are a diagnostic for solar magnetism. At 12 microns, the diffraction limit is 0.75 arcsec, and this resolution might be realized a significant fraction of time because of improved seeing at these IR wavelengths. Direct vector measurements of solar magnetic fields would become possible because effective Zeeman splitting is proportional to wavelength, both the linear and circular Stokes amplitudes are proportional to their vector field components, and instrumental polarization becomes negligible at 12 microns. The telescope would also be used at night by the solar/stellar community.

This paper compares and discusses the attempts to overcome the "seeing" resolution limit imposed by spherical aberration for the Hubble Space Telescope and by Atmospheric turbulences f the case of ground -based telescopes - From recently reposed experimental data it is shown that wavefront reconstruction algorithms are capable of restoring nearly fully the Hubble’s spherical aberration of the primary mirror. Also from previously reported results of ground-based telescopes which utilize the powerful spectral imaging technique at high and low light levels, we compare for different object magnitudes the two types of telescope resolutions "seeing" resolution, SNR and point source resolution.

The conclusions, recommendations, and target design parameters for a 100-m class optical space interferometer that has been recommended by a study group of the ESA for launching around the year 2005 are summarized. The desirability of an interferometer on the moon, covering the entire wavelength range from UV to sub-mm, with a space interferometer as a possible intermediate step, is considered.

The advantages of photon-counting detectors such as the Ranicon over other detectors in space-based astronomy in the area of time-resolved imaging and spectroscopy are discussed. Details of a system to record the positions and absolute arrival times of individual photons are described, with emphasis on the time-tag module and detector electronics and data collection system. The use of a GPS system for accurate absolute timing when synchronizing observations from different observatories is suggested.

The feasibility of multiple spacecraft stationkeeping for submillimeter and optical interferometry is examined. A condition for interferometry is that two or more spacecraft must control their relative positions with better than 1 mn accuracy indefinitely in both radial and transverse directions although separated by as much as 1 Km in LEO and 100 Km in GEO. They must also maneuver through a useful area of the U-V plane of an arbitrary astronomical source. The problem is first outlined and a solution which utilizes gravity gradient forces to do most of the work and ion thrusters for additional maneuvering is proposed. All the perturbing forces are shown to be small compared to the ion thruster requirements. An inertial position and attitude control strategy is suggested which utilizes existing or soon to be available sensors and actuators. Finally, the fuel and power system mass requirements are estimated and found to be within reason for a 10 year mission.

A novel concept for making a lunar telescope either in the form of an optical interferometer or as a large aperture segmented array is described. Individual mirror elements can be made in situ by the replication process. An arrangement of magnets or electromagnets and high temperature superconductors are used to move and point the mirror elements. The telescope can be fabricated and operated by preprogramming supplemented by remote control from earth. The advantage of this approach is that it employs proven technology, requires very little transportation of material, little or no excavation, and is within the payload capability of current launch vehicles. A prototype is being made to demonstrate feasibility.

Low power (2mW) lasers mounted on a small satellite in a highly eccentric orbit can provide a bright and spectrally well-defined reference source for calibration of ground-based adaptive optic systems. Because the reference is spectrally well-defined it can be efficiently filtered in broad-band imaging applications and yet can provide a very bright reference source for wavefront detectors when imaging faint sources. Dependent on the size of the atmospheric isoplanatic patch, the satellite reference may be useful for calibrating observations of selected objects for periods in excess of 1 hr, leading to limiting magnitudes for detection of up to +30. The area of sky for which the reference is valid is restricted (order 1 sq deg of sky per telescope per year). The reference is valid for phasing aperture synthesis telescope arrays of kilometric scale. Orbital maneuvers for target selection and to increase the sky coverage will be considered.

The paper examines an off-axis three-mirror telescope with a flat local surface. Designed to image aurora in UV light from a satellite, this 20-cm telescope has an f/2 primary and an f/8 tertiary focus. The focal surface has a diameter of 4 cm which subtends 1.3 deg. The telescope is engineered to have very low distortion so that resolution would not be degraded during integration of images moving on the CCD detector at the same rate as that of the CCD readout.