When using artificial satellites to discover more about the outside world, optics have a very special, fundamental role to play. Whether observing the Earth in detail, including its vegetation and atmosphere or detecting pollution, analysing the chemical composition of stars or galaxies, measuring angular deviations or the velocity of celestial objects, the natural option is to use visible, ultraviolet or infrared instrumentation in order to extend our knowledge. Since its creation in the sixties, the French Space Agency CNES, just like other space agencies, has been heavily involved in observation projects and those aiming to physically characterize natural, terrestrial, planetary, stellar or galactic phenomena. At first fairly straightforward, experiments quickly benefitted from the huge advances made in technology, making it possible to construct and send into space veritable observatories stable in both attitude and temperature. Such observatories are also equipped with computing facilities and a transmission downlink capability. After the first scientific experiments, more ambitious space astronomy projects such as D2B were given the go-ahead by CNES. This satellite flew five experiments investigating solar and stellar radiation in the far ultraviolet: spectral analysis and photometry of stars located around the ecliptic, study of solar activity and determination of atmospheric absorption. Photometers and spectrometers working in the visible or far ultraviolet were orbited for this purpose (with a wavelength of up to 1 70 A when observing the iron line for example, and commonly in Lyman's range). The satellite flying these experiments weighed 105 kg and was developed within 5 years (it was launched in 1975). The total mass allocated for the instruments on board was around a third of the total mass. Obviously, the equipment flown was compact in the extreme. The seventies saw the start of large-scale space application projects. The harnessing of space for the study of natural resources and meteorological purposes began with the Meteosat and Spot projects. The latter used an optical instrument of a new design. Indeed, because the satellites on which the instruments were flown were heavier (a satellite launched by Ariane or Thor Delta being at least ten times heavier than one launched by a Diamant BP4 launch vehicle) the instruments themselves could aim for more ambitious missions and become more complex. The Meteosat radiometer thus provides a complete visible and near infrared image of the Earth every half an hour. The image has a spatial resolution of both 3 km and 5 km and the instrument uses two infrared channels: an atmospheric transmission window between 10.5 and 12.5 jim, and a water vapour absorption band between approximately 5 and 6.5 jim. The Meteosat satellites, designed in the early seventies by CNES then developed by the European Space Agency, have provided an unbroken service. They form part of a worldwide meteorological watch network and will be replaced later this decade by more efficient imaging instruments and sounders as part ofthe European 2nd Generation Meteosat project. As far as Earth observation with optical instruments is concerned, CNES has played a major role by setting up and developing the Spot programme, which consists in detecting, recording, processing and distributing high-resolution multispectral images ofEarth. The years leading up to the decision to proceed with this programme were spent developing the basic techniques used, such as the push-broom principle behind linear array CCDs. A multispectral linear array CCD camera was thus developed and tested during airborne experiments in 1 975. The Spot mission is highly reliant upon optical techniques: telescope nearly diffraction-limited and extremely stable in orbit, alignment of 6000 photodetectors over 78 mm with a precision of around one micron, high degree of integration for the optoelectronic components (dichroic filters, electronics) all built into a detection unit with extremely precise positioning constraints, calibration device using gradian-index fibres etc. Spot 1 , the first satellite in the Spot family, was launched in 1986 and still produces highquality images. Spot 2 and 3 have now been in orbit since 1993 . TheHRV (High Resolution Visible) instrument includes a mirror which can change the viewing direction by 13 .5° along a plane perpendicular to the ground track. Three of the instrument's spectral bands have a resolution of 20 m and another, wider band, has a resolution ofjust 10 m. There are in fact two HRV instruments on the Spot platform, both identical yet independent from each other. One of Spot's original features is its stereoscopic vision over two different orbits, retransmitting data on relief invaluable for cartography purposes. Other quite original optical instruments dedicated to specific missions have been used to explore the solar system. We could quote, for example, the mission to observe and analyse Halley's comet in 1 986. Several different instruments developed by European science laboratories, including visible and ultraviolet spectrometers and infrared imagers, made a significant contribution to extending our knowledge of the structure and chemical composition of the comet's core, and provided outstanding images of the comet itself (both core and tail).

The paper presents the Infrared Atmospheric Sounding Interferometer (IASI) being developed, in cooperation between France and Italy. IASI is a part of the operational meteorological payload managed by EUMETSTAT. The Baseline plateform for the first flight model of the Instrument is the METOP-1 polar platform to be launched during 2000/2001. IASI consists of a Fourier Transform Spectrometer (FTS) based on a Michelson Interferometer coupled to an integrated imaging system which allows characterization of cloudiness inside the FTS field of view. This instrument will provide spectra of high radiometric quality at 0.5 cm - 1 resolution from 15.5 micrometers to 3.4 micrometers with global coverage twice per day at 25 km horizontal resolution.

An evaluation model of a high precision mirror drive, based on the parallelogram principle, has been designed and manufactured at Officine Galileo so as to meet the instrumental requirements of the high resolution Fourier Transform Spectrometer (FTS for the Spectroscopy of the Atmosphere using Far InfraRed Emission (SAFIRE) project. Mechanical and optical tests, carried out on the scanning mechanism evaluation model, provided measurements of the stability and uniformity of the movement and showed that the selected mirror drive concept meets the required tolerances.

MIPAS, the Michelson Interferometer for Passive Atmospheric Sounding, is a high-resolution Fourier-Transform spectrometer designed to measure concentration profiles of atmospheric constituents on a global scale. It will observe the atmospheric emissions from the Earth horizon (limb) throughout the mid infrared region (685 - 2410 cm^-1, 14.6 - 4.15 micrometers ), which will allow the simultaneous measurement of more than 20 atmospheric trace gases, including the complete NO_y-family and several CFC's. MIPAS will provide global data coverage, including in particular the polar regions, where the stratospheric chemistry is currently exhibiting alarming changes.

The CRISTA experiment (CRyogenic Infrared Spectrometers and Telescopes for the Atmosphere) is a limb sounding instrument designed for high spatial resolution measurements of infrared emissions from the Earth's middle atmosphere. The principal scientific aim is to study small and medium scale structures in the concentrations of minor constituents which serve as tracers for the dynamical processes acting on these species. In order to achieve a high spatial resolution CRISTA is equipped with three telescopes each followed by its own grating spectro-meter. The spectrometers cover a wavelength range of 4 micrometers to 17 micrometers (all three viewing directions) and 55 micrometers to 71 micrometers (one telescope only) with a resolving power of about 300. Up to 15 trace gases will thus be measured simultaneously on a 500 X 650 km² grid reaching from the lower strato-sphere to the upper mesosphere. For special soundings the measurements will occasionally be extended to an altitude of 150 km. The launch of CRISTA is planned for autumn 1994. CRISTA will be integrated into the free- flying platform CRISTA-SPAS and carried to a 57 degree(s) inclination orbit by the Space Shuttle. In the free-flying phase a measuring time of approximately on week is expected. The launch of CRISTA on CRISTA-SPAS will be together with the ATLAS-3 mission.

Global Ozone Monitoring Experiment (GOME) is a grating spectrometer designed to monitor from the ESA satellite ERS-2 absorption by ozone and trace gases in the earth atmosphere. The instrument works in the spectral range 240-790 nm, with a spectral resolution of 0.2 nm and a spatial resolution of 40 X 40 km². GOME makes use of high density holographic gratings as dispersing elements, and cooled photodiode arrays of 1024 pixels as detectors. Its characteristics, as well as the in flight calibration means, permit to use both differential optical absorption spectrometry and backscattering in the ultra-violet as retrieval methods. This paper describes the main features of the instrument, the results of the test campaign performed on the engineering model and the concepts adopted for ground and in- flight calibration.

The GOMOS instrument will provide world-wide daily mapping of the ozone concentration by the observation of spectra attenuation of stars setting behind the atmosphere. The instrument optics consist of two spectrometers and two fast photometers fed by a fixed Cassegrain telescope and mounted behind a steerable flat mirror.

SCIAMACHY (SCanning Imaging Absorbtion spectroMeter for Atmospheric CHartographY) is a spectrometer in the UV, VIS and near IR region, with which the concentration of atmospheric trace gases both in the troposphere and in the stratosphere can be determined. It has been selected for ENVISAT of which the launch is scheduled in 1999. SCIAMACHY is a common German-Dutch project. The optical configuration consists of 2 scan mirrors (a nadir/elevation mirror and an azimuth mirror), a telescope and a complex spectrometer, with which a spectral range from 0.24 - 2.4 micrometers is covered in 8 channels with array detectors. Axillary optics consist of two on-board calibration light sources, a sun/moon follower and a polarization measurement device (PMD). The sun/moon follower enables SCIAMACHY to track the sun or moon during occultation and sun/moon calibration measurements. The PMD is a simple seven channel spectrometer with basically sensitivity for one polarization direction only. The PMD is used to correct for the difference in sensitivity for different polarization states of the regular spectrometer channels. The optical system is described in detail elsewhere. The operational success of an instrument like SCIAMACHY depends strongly on the accuracy of the calibration. Proper calibration enables comparison of measurements with those of other instruments, that measure atmospheric trace gases. Besides it gives information about degradation of the spectrometer during its lifetime, so correction of the measurements can be performed to make the real trends in detection of ozone (and other gases) visible. For on-ground calibration of the diffuser and of the spectral channels at room temperature a special facility has been developed at our institute. In the following chapters some essential aspects of the calibration of SCIAMACHY and of the calibration facility are described.

The `Spinning Enhanced Visible and InfraRed Imager' will be flown on the Meteosat Second Generation satellites. The mission objectives and major performance requirements of the instrument are presented. The features of the 12 channels covering a wide spectral range from 0,5 micrometers in the visible region to 14 micrometers in the infrared domain are described, with their dedicated applications: Basic Multispectral Imagery, Cloud Motion Vectors, High Resolution Imagery, Air Mass Analysis and Surface Parameters. The instrument design and operating principle are described. Telescope and Scan Assembly, Focal Plane and Cooler Assembly, and Instrument Control & Data Handling regroup the most challenging optical instrumentation technologies. Expected instrument radiometric performances are summarized; improved spatial, temporal, spectral and radiometric resolutions will allow the image data to be used in future Numerical Weather Prediction processes. The overall program development plan foresees the manufacturing and testing of 3 satellites, the first launch being planned for the dawn of the next millennium.

The instrument design and performance are described in this paper. C-ATLID is a backscatter LIDAR using a solid-state Nd-YAG (1.06 micrometers wavelength) and a 0.6 m diameter telescope. A linear scanning (+/- 23) is used in order to achieve the required swathwidth (700 Km). The selected concept consists in a lightweight scanning telescope associated to a contra-rotative flywheel. The detector is a silicon Avalanche Photodiode. The instrument fits on a half nadir sub-panel of the ENVISAT platform, and the thermal control is designed to be independent of the neighbor instruments.

In meteorological and climatological fields, the scientific community will increasingly need global measurements of key atmospheric parameters with high spatial resolution (horizontal as well as vertical): the spaceborne lidars are the most suitable instruments for those missions. While backscatter lidar (ATLID, currently studied as ESA) is presently first candidate for space deployment, the next generation of lidars will be DIAL and Doppler wind lidars, presenting a higher level of complexity, mainly due to the large power and complex signal processing required. The present considered wind lidars are based on CO₂ lasers, whose space compliance still needs confirmation, while alexandrite lasers are considered for water vapor and temperature measurements, but they need flashlamp pumping which poses a lot of several thermal constraints and lifetime problems: on the other side, the recent developments achieved in solid-state technology allow to envisage diode pumping as most promising possibility for both previous applications.

MERIS is a passive optical instrument, that will fly on the First Polar Orbiting Earth observation mission ENVISAT 1, scheduled for launch mid 1998. The study and development of this instrument is currently carried by an international team led by AEROSPATIALE. The instrument primary mission goal is to monitor bio-optical ocean parameters on a large scale. Secondary goals of MERIS include atmospheric investigation on cloud and aerosols parameters and on land surface processes. The instrument will perform 15 spectral images, programmable in width and position with a spectral sampling interval of 1.25 nm within the visible spectral range of 400 nm to 1050 nm. MERIS images will have a swath width of 1100 km and spatial resolution of 300 m.

The vegetation payload's mission is in simple terms to take, record and transmit to the ground medium resolution (1 km) optical images of the globe in four registered spectral channels ranging from the visible to the medium infrared (MIR). The payload, which is built by AEROSPATIALE, is planned for launch in 1997 on the SPOT4 satellite, and will take advantage of a close commonality with the HRVIR high-resolution instruments, sharing spectral bands and geometrical references with them. The complete independent payload includes a wide-angle medium resolution push-broom optical instrument, electronic capability for processing and recording the optical information, and an L- and X-bands image telemetry subsystem which sends the data to the ground in real time and dump modes. It also features independent service subsystems and an original instrument calibration device. The main new features are the wide field optics telemetry, an improved electronic mass memory subsystem, and identical B2, B3 and MIR spectral bands with HRVIR. The Vegetation system, which include the payload as space segment, is financed by EEC, France, Sweden, Belgium and Italy, and is developed by CNES (French Space Agency).

A comparison has been made of spectral radiance calibrations of SBUV-2 instruments using two techniques. In one, a source of spectral radiance is obtained by illuminating a spectralon panel diffuser whose BRDF has been measured by NIST with a 1000 W FEL quartz-tungsten- halogen standard of spectral irradiance from NIST. In the other, the spectral irradiance of the aperture of an internally illuminated spherical integrator is measured relative the same NIST standard of spectral irradiance used with the panel diffuser. The spectral radiance of the sphere is calculated for a source of irradiance which is uniform and radiates as a Lambertian source over the area of the aperture. The agreement between the average of four spectral radiance calibrations of SBUV-2 instruments obtained with the spectralon panel diffuser technique with those obtained from five spectral radiance calibrations with the internally illuminated spherical integrator technique is better than one percent in the wavelength range of 250 - 340 nm. Repeated use of two spectralon panel diffusers over a two year period indicates a stability of diffuser BRDF of better than one percent.

A new instrument for Earth polarized observation is investigated. The POLDER system includes a telocentric wide field angle lens, a CCD matrix detector and, in between, a rotating wheel holding different spectral filters and polarizers assuming a registration function. Characteristics of the camera are presented.

HRTIR for High Resolution Thermal Infrared Radiometer is an earth observation instrument candidate to the European Space Agency polar platform beyond Envisat 1. A preliminary design of the instrument has been performed in order to identify the most critical points and breadboard them. The instrument is a push broom concept providing an on ground spatial resolution of 50 m for a swath width of 50 km and a temperature sensitivity of 0.1 K in 3 spectral bands in the 8 to 12.5 micrometers range. A compact dioptric system has been selected for the optics and the focal plane consists of three linear arrays of 1000 HgCeTe photovoltaic elements hybridized on a CCD multiplexer and cooled down to 50 K inside a cryostat by mechanical cryocoolers. The HgCdTe IRCCD with a cut-off wavelength longer than 12.5 micrometers has been identified as the most critical technology and breadboarded. A complete detection chain with a long wavelength linear array of 222 pixels obtained by butting of three sub-arrays, a CCD multiplexer, a driving and processing electronics up to digital signal has been manufactured. The linear array is housed in a cryostat similar to the foreseen flight model but coupled to a laboratory cryogenic system. The IRCCD has been tested at unit level and the complete detection chain have been characterized in laboratory in conditions close to the flight. The test results have demonstrated the feasibility of the IRCCD at long wavelength with excellent performances. The instrument radiometric performances have been validated from the breadboard test results.

Since year 86, the spot satellites provide images of the earth for commercial applications. Whereas the Spot IV satellite, to be launched in 97, will include a medium IR imaging capability, a more significant improvement of the system performances will be achieved with the future Spot IV observation modes a 5 m panchromatic resolution, as well as an along track stereoscopic capability. Trade off activities on concurrent concepts of instrument have been performed during feasibility phase, they have confirmed that the reuse of the Spot IV instrument concept provides the best compromise in terms of performance with respect to programmatic. Even so the demanding resolution implies significant improvement of every subsystem of the instrument. A new focal plane including 5 observation channels has to be designed. To cope with the 5 m resolution and the 60 km image width a monolithic 12000 pixels of 6.5 microns squared CCD array is to be developed. Data rate up to 4 Megapixels per second will also need to develop new components for the video electronics. The optics constituting the Schmidt telescope will require outstanding performances in terms of wave front error. A figure ranging (lambda) /18 for the complete telescope is needed, this will be achieved thanks to computer assisted polishing technique. For the same reason the minimisation of the in orbit mirror surface distortions will need to develop specific isostatic mounts. High sensitivity of the optical combination with regards to defocus will also lead to design improvement in the field of thermo- and hygroelastic behaviors of the structure and in the fixation concept of the telescope mirrors. Lastly new techniques for the optical alignment and the performance measurements will have to be developed to assess ultimate in orbit performances.

The imaging pushbroom scanner MOS measures the spectral radiance of the backscattered radiation of the earth surface in the VIS/NIR spectral region from orbit altitudes. The two main problems, the investigation of ocean/land and atmospheric properties, require a modular optomechanical and -electronical spectrometric device: MOS-A especially for data acquisition in the absorption band of the atmospheric oxygen in 4 spectral channels with 1,4 nm bandwidth and MOS-B with 13 channels of 10 nm bandwidth between 0,4 micrometers and 1,0 micrometers . The spectral and radiometric properties of MOS are chosen with respect to the spectral characteristic of the upwelling radiation, which is influenced by the object spectral reflection function. They are a compromise between the dominant measuring quantities like radiometric, geometric and time resolution requirements. These problems in connection with the design and calibration concepts will be discussed.

A Fabry-Perot spectrometer concept suited for satellite observations of OH and other trace gases which emit in the far infrared region is presented. Long term observations can be accomplished by using high T_c detectors in conjunction with mechanical Stirling coolers which have been developed for space applications. In comparison with Fourier transform spectrometers and heterodyne devices the instrument is small, sensitive and robust. Its spectral resolution (in excess of 10⁴) is shown to be sufficient to completely avoid spectral interference in OH observations.

IASI is an Infrared Atmospheric Sounding Interferometer devoted to the operational meteorology and to atmospheric studies and is to be installed on-board the second ESA Polar Platform called METOP-1; It is planned to be launched in the year 2000. The required operating lifetime is 4 years with a target of 7 years plus 4 years of ground storage. The cold Optical Unit is located at the focal plane of the Michelson Interferometer. This sub-assembly is designed to divide the IR input flux into 3 spectral bands and to focus it on the 3 arrays of detection. It includes a spectral separation using 2 beam splitters dividing the incoming flux into 3 spectral bands which are 3.4 to 5.0 micrometers , 5.0 to 8.26 micrometers and 8.26 to 15.5 micrometers . Each array stands behind an objective and a set of 4 microlenses. This unit defined the aperture and the field of view of the instrument and operates at 100 K with passive cooling. An overall design of the cold box has been performed for the mechanical structure and for the optics taking into account the space requirements. This structure is interfaced with a passive radiator and is illustrated by several figures. A thermal analysis of this unit has been computed to check the thermal gradient by means of a finite-element software involving several thousands of nodes. A specific optical ray tracer delivered the optimization and the tolerancing of the optical design. The results show that, with severe tolerances, the optical losses and cross-talk meet the requirements. The optical diagram has been drawn, as well as the CNES instrument design. Several critical items have been manufactured to check that they can meet the required performance (photoconductive detectors, microlenses, AR coatings, packaging).

There has been long-standing interest in satellite-borne imaging IR sensors for a variety of applications in the commercial, environmental, and defense sectors. Recent availability of high quality, low-cost IR focal plane arrays and high-speed processing hardware has lead to the development of several optically-filtered radiometric imaging sensor systems. However, a more versatile satellite system results from a spectrally agile sensor. In this paper we present a design and performance analysis of an imaging grating spectrometer system which simultaneously collects high-spectral-resolution and high-spatial-resolution data in the NIR or SWIR region. The data can be presented as radiometrically calibrated spatial images for any wavelength and bandwidth in the sensor bandpass. Additionally, spectra can be produced for each spatial region of the image. Spectral and spatial data gathered with a prototype sensor of similar design are presented to illustrate these data products and the potential utilization and merits of the design concept. The overall design and cooling techniques are compatible with light-satellite applications requiring a minimum operating period of one year.

GERB is an instrument which has been designed by a team led by British Aerospace Space Systems Ltd. to measure the Earth Radiation Budget from a Geostationary Satellite. The results presented in this paper are the outcome of a Phase A design study of an AO instrument to meet a set of science requirements defined by ESTEC and a Science Advisory Group (SAG). The instrument has been designed for accommodation on the Meteosat Second Generation Satellite ( MSG ) due for launch in mid 2000. The design could however be adapted to suit the 3 - axis stabilised platforms of other agencies. The Phase A design is a proof of concept study and the overall conclusion is that an instrument satisfying both the scientific requirements and spacecraft constraints is feasible. The instrument design allows unique temporal measurements to be made of the balance between short and long wavelength radiation leaving the Earth's atmosphere. The data will be highly complementary with the data sets gathered by instruments in low Earth orbit.

This paper concerns the development of instrumentation to detect the presence of small clouds < 1 km square. This cloud imager will be part of the Pakistani BADR-B micro-satellite, currently planned for launch with ERS-2. The project will take advantage of the opportunity of a low-cost, short time scale launch on BADR-B, to produce early data demonstrating the technology available for `small cloud' detection. This technology will be of particular interest to future Earth Observing satellite instruments involved in the measurement of Sea Surface Temperature. The cloud imager will consist of a CCD camera operating at visible wavelengths, aiming to achieve 250 X 250 m resolution on the ground, and with a wide field of view of +/- 8.5 (equivalent to approximately 200 X 150 km on the ground). The challenge in the design and build of this instrument was the need to maintain the performance with the restrictions on available mass, power, space and schedule (12 months to delivery). This paper will describe the design and testing of the instrument, with particular emphasis on the lens system and CCD detector; it will also include some discussion of the scientific objectives.

The MERIS instrument is a medium resolution spectro-imager developed by AEROSPATIALE for the first ESA polar platform, ENVISAT 1. This instrument is mainly devoted to oceans observation between 400 and 1050 nm with a large field of view corresponding on ground to 1100 km with a central 260 meters resolution. Up to 15 bands can be selected from ground, with a spectral bandwidth from 1 25 to 30 nm, and the overall field of view is achieved by five identical adjacent modules. CERCO is in charge of these Optical Sub-Assemblies. The paper presents the concept of an elementary module and the works done to guarantee the camera optics compatibility with environmental constraints. It also gives the status of breadboarding activities concerning critical items such as concave grating, second order blocking filter and optical parts coatings.

Previous designs for infrared space observatories assumed that the best method of cooling these systems was a single large tank of liquid helium. Although effective, as demonstrated by the IRAS and, soon, the ISO missions, this technique produces limited lifetime, small telescope aperture, and inflexibility in optical design. More modern spacecraft designs, including the Japanese IRIS and European Edison concepts, instead adopt multiple cooling strategies: radiative, cryogenic, and/or mechanical. This alternative philosophy permits each technique to be applied to the most appropriate task, taking advantage of strengths of each technology and minimizing weaknesses. In this paper we give a brief history of the development of some cooling technologies, emphasizing the advantages of the designs adopted for a pair of possible future infrared space missions, IRIS and Edison. We also briefly discuss negative aspects of the IRAS legacy in infrared spacecraft design and we speculate on what types of missions might follow IRIS and Edison.

The Stellar Interferometer Tracking Experiment (SITE) is a Space Shuttle flight experiment proposed by the MIT Space Engineering Research Center and NASA's Jet Propulsion Laboratory. The SITE instrument is a two-aperture stellar interferometer with a baseline of four meters and a detection bandwidth of 200 - 780 nanometers. The objective of SITE is to validate in the space environment the detectors and fringe-tracking control systems necessary for future space-based astrometric and imaging interferometers. The cophasing and coalignmnet requirements of the stellar beams for such instruments demand nanometer pathlength and milli-arcsecond jitter control in order to acquire precise fringe amplitude and phase measurements. SITE will evaluate and quantify the effects of vibration isolation, structural quieting, and active pathlength and beam tilt control technologies on the ability to capture and track the central interference fringe from a star. This paper describes the conceptual optical design of the SITE instrument.

The Near Infrared Camera and Multi-Object Spectrometer (NICMOS), is a `second generation' instrument for the Hubble Space Telescope (HST). As such it is an integral part of the HST mission concept of periodic instrument replacement. NICMOS provides a new capability of near infrared astrophysical investigations in the 0.8 - 2.5 micrometers spectral region utilizing three cameras at different spatial resolutions for imaging, grisms for multi-object spectroscopy, polarizers for polarimetry,and a coronagraphic spot and mask for coronagraphic imaging. NICMOS has a five year mission lifetime with a solid nitrogen dewar that maintains the three 256 X 256 pixel HgCdTe detectors at the operating temperature of 58 Kelvin. Insertion of NICMOS is scheduled for STS Mission 91 in March 1997. The NICOS design is a result of the opportunities and constraints of the HST program which has a unique concept; periodic enhancement of the instrumental capabilities on orbit. This concept, closer to traditional ground based operations, provides a very cost effective capability for new space science opportunities and poses unique challenges in the instrument concepts produced to take advantage of the opportunities.

The NU-VIMS is a new instrument concept for high resolution spectral imaging within the spectral band of 0.25 to 5.0 micrometers . The concept utilizes state-of-the-art detector and grating technologies to extend the capabilities of the Cassini/VIMS-V prototype while maintaining its compact and lightweight format. The NU-VIMS optical design achieves remarkable performance while relying only on spherical surfaces in a single optical channel with only one grating and either one or two detector arrays. Though the concept was developed in anticipation of the MORO, Rosetta, and Pluto-Charon deep space missions, the instrument could also be mounted on a lightweight satellite for low cost remote sensing of the earth.

The Rear Slit Camera (RSC) will be used in the SUMER (Solar Ultraviolet Measurements of Emitted Radiation) instrument on SOHO to investigate the exact position and quality of the solar limb imaged on the entrance slit of the SUMER spectrometer. The diffracted visible sun light will be focused on an image sensor array. The analogous signals are transferred to the internal RSC electronics and transferred to the SUMER Data Processor Unit (DPU). According to the major design constraints redundancy of the optomechanics and electronics of the RSC was not required, because the reliability of the camera was not estimated to be system critical. The scientific objects, the technical development and verification of this camera shall be detailed. The achievements of the tight cooperation between Kayer-Threde and the Max Planck Institute for Aeronomy, in terms of technical performance and costs shall specially be emphasized.

The optical performances of the Ultraviolet Coronagraph Spectrometer (UVCS) of the Solar and Heliospheric Observatory (SOHO) mission have been tested. A laboratory evaluation unit (LEU) of the spectrometer assembly (SPA) consisting of the structure equipped with breadboard models of the entrance slit assembly (ESA), a grating drive mechanism (GDM) mounting a toroidal grating for the Ly(alpha) channel and a multi-anode microchannel array (MAMA) detector has been integrated and aligned. Both tests with visible and UV radiation have been performed. Aberration and stray light measurements have shown satisfactory performances of the instrument almost in compliance with the scientific requirements. A LEU and a flight unit of the White Light Channel (WLC) have been integrated and aligned in a proper light-tight housing and in the flight spectrometer assembly respectively. Measurements of the polarimeter modulation curve and the relative error have shown performances within the specificated requirements.

The XMM Optical Monitor (XMM/OM) is a co-aligned telescope devoted to make observations of the X-ray sources both in the UV, visible and near-IR spectral bands, simultaneously with the X-ray instrument on the X-ray Multi-Mirror (XMM) satellite. The OM telescope is a Ritchey-Chretien with 300 mm clear aperture, for real time identification of sources up to magnitude mv equals 24. In the design of the telescope, particular care was paid in the selection of the optomechanical architecture and in the thermal and structural analysis, since the adopted optical scheme requires high stability of the structure. The paper highlights the major critical aspects and the criteria followed in the trade-off and design phases.

Imaging time-tagging photon detectors are active development for a wide range of high-time resolution applications including real-time image stabilization and analysis, and speckle interferometry and speckle image reconstruction. This paper reviews the different classes of imaging time-tagging detector systems and their performance characteristics. Probable areas for future developments of time-tagging detector systems and their components are outlined.

The detection offaint light sources, such as distant planets in astronomy; the fluorescence of biological trace elements in biosensors; and the spectral emissions from spectroscopy experiments; ideally requires instrumentation that can log the arrival of single photons in a temporally and spatially resolved fashion. Until recently, the most widely used sensor for such low light level optical measurements was the photomultiplier tube (PMT) but solid state solutions are now replacing them as the preferred detector. There are still a number of applications which include astronomical low light level imaging and photon counting spectroscopy which use conventional PMT and multichannel plate detectors because, until now, the technology has not been sufficiently flexible to replace them. Three solutions are currently available for low light level observation in the application areas mentioned above. Firstly, there are vacuum tube (PMT or multichannel plate) based systems. An example ofwhich is the PAPA Faint Object Camera [1] which indeed enables very low light level imaging to be performed but is extremely expensive and fragile: Secondly, Charge Coupled Device (CCD) based systems. These are very successful in many imaging applications, but have some limitations when low light level signals are to be addressed, and finally Avalanche Photodiode (APD) detectors. These detectors are not currently used for imaging but provide a robust replacement for the PMT in many applications. This work has involved the development of a novel, Avalanche Photodiode based optical integrated circuit, which enables the imaging of low light level signals and may potentially be a realistic alternative to PMTs in all of the above application areas. This paper describes the development of this novel and completely self contained integrated circuit for low light level imaging applications [2]. The technology developed to implement his integrated circuit has been called SPARCE.CMOS. (Single Photon ARray Control Electronics).

Delay line detectors have been chosen for the Far Ultraviolet Spectroscopic Explorer1 mission to be launched in 2000. The demands of the FUSE detectors include large format (220mm x 10mm format), high spatial resolution (15im x 35im FWHM) and linearity, high image stability, low power consumption and weight, and counting rates in excess of 3 x i0 events sec1. The FUSE program builds on the previous work, which includes two delay line detectors (95mm x 27mm double delay line format) that have already been successfully employed in the ORFEUSASTROSPAS2 ultraviolet spectrometer launched by shuttle in September 1993. We present the plans for the FUSE detector program, and results from double delay line (DDL) detectors that are under investigation to meet the requirements of the FUSE program. Our current detector achieves 15im x 25p,m FWHM (<4000 x 500 resolution elements) over the 65 x 15mm format used for the FUSE demonstration detector (90% of the flight detector segment format length), with good linearity (±1 resolution element) and high stability. State of the art analog to digital converter (ADC), gated integrator, and digital signal processor (DSP) technology have been employed to develop novel event position encoding electronics with high count rate capability (<5 x104 events sec). Microchannel plates with lOj.tm pores and an 80: 1 pore length to diameter ratio, with a 70mm x 20mm format have been used in a Z stack configuration to provide the photon amplification (gain 2 x 10). These show good pulse height distributions (<35% FWHM) even with uniform flood illumination, and background levels typical for this configuration (<1 event cm 2 sec 1). Flat field images are dominated by the microchannel plate fixed pattern noise due to the multifiber boundaries, and are stable. High efficiency photocathodes, such as KBr have been extensively studied, and provide quantum detection efficiencies of 40-50% in the 900 - 1200A range for FUSE.

The analysis of the imaging properties of an interferometric device essentially depends on the parameters that govern the Fourier synthesis operation: extension and density of the experimental frequency coverage (resolution, field, robustness), size and nature of the input errors (input data for the study of the error propagation). Among these parameters, the Field- to-Resolution Ratio (FRR) proves to play a key role, in particular, for array configuration optimization: the number of contiguous elements that can be resolved in the synthesized field is bounded by (FRR). The robustness of the corresponding reconstruction process is of course involved in the precise definition of this concept. The corresponding analysis is based on a new method of Fourier synthesis (WIPE), in which all these aspects are clearly specified (WIPE reminds of CLEAN, a well-known deconvolution technique in astronomy).

A cophasing system architecture has been derived for an imaging multi-aperture optical space interferometer. This system is based on cophasing on a guide star. Breadboarding and testing of the main part of the system, with emphasis on the critical items (delay line, cophasing interferometer, Optical Path Difference (OPD retrieval methods), has provided an evaluation of the expected performance. This paper describes the reference interferometry mission used for the study, the overall OPD stabilization concept, the cophasing system and associated OPD restitution algorithms, and the first test results.

A key technique for optical aperture synthesis instruments is the cophasing of the beams from the individual apertures. A cophasing system includes a number of delay lines for the adjustment of the optical path lengths. The design of a high-resolution delay line is based on the mission requirements of a space-borne interferometer. A cat's-eye type of retroreflector is the selected option for the optics. Three delay lines, including actuators and control electronics, have been built and have been tested in a dedicated setup, which includes a star simulator and a cophasing interferometer. The test results support the feasibility of the presented delay line concept.

In a synthesized aperture implemented for stellar interferometry application, the star light usually travels by mirrors train to reach the interferometric mixing station. Using fibers the number of freedom degrees to be controlled is reduced. Parameters to be servo are the fiber length, the dispersion compensation and the polarization control. We report experimental results demonstrating the fiber length stabilization with a 5 nm accuracy, the dispersion compensation over a 300 nm spectral bandwidth (600-900 nm) and the control of polarization effects.

The High Throughput X-Ray Spectroscopy Mission (XMM) is a `Cornerstone' Project in the ESA long-term Programme for Space Science. The satellite observatory uses three grazing incidence mirror modules coupled to reflection grating spectrometers and X-ray CCD cameras. In order to achieve a large effective area, each XMM mirror module shall consist of 58 Wolter I mirrors which are nested in a coaxial and cofocal configuration. This high packing density requires the production and integration of very thin mirror shells with diameters included between 300 and 700 mm. In 1991-93, a development program was run which aims to demonstrate the feasibility of such mirrors. Demonstration models which integrate mirrors having different sizes were manufactured using CFRP replication and Nickel electroforming technologies. These were X-ray tested. The proposed paper summarizes the activities and the test results obtained during this program.

The Reflection Grating Spectrometer (RGS) onboard the ESA satellite XMM (X-ray Multi Mirror mission) combines a high resolving power (approximately 400 at 0.5 keV) with a large effective area (approximately 200 cm²). The spectral range selected for RGS (5 - 35 angstroms) contains the K shell transitions of N, O, Ne, Mg, Al, Si and S as well as the important L shell transitions of FE. The resolving power allows the study of a wide variety of challenging scientific questions. Detailed temperature diagnostics are feasible as the ionization balance is a unique function of the distribution of the electron temperature. Density diagnostics are provided by studying He-like triplets where the ratio of the forbidden to intercombination lines varies with density. Other fields of interest include the determination of elemental abundances, the study of optical depth effects, velocity diagnostics by measuring Doppler shifts and the estimate of magnetic fields through the observation of Zeeman splitting. The resolving power is obtained by an array of 240 gratings placed behind the mirrors of the telescope, dispersing about half of the X-rays in two spectroscopic orders. The X-rays are recorded by an array of 9 large format CCDs. These CCDs are operated in the frame transfer mode. They are back illuminated as the quantum efficiency of front illuminated devices is poor at low energies because of their poly-silicon gate structure. To suppress dark current the CCDs are passively cooled. In order to obtain the effective area of about 200 cm², grating arrays and CCD cameras are placed behind two of the three XMM telescopes. A model of RGS was tested last autumn ('93) at the Panter long beam X-ray facility in Munich. The model consisted of a subset of four mirrors, eight representative gratings covering a small section of the inner mirror shells and a CCD camera containing three CCDs. The purpose of these tests was to verify the resolution and sensitivity of the instrument as a function of X-ray energy. Extensive simulations, using a Monte Carlo raytracing code, are used to interpret these tests. Preliminary results of these tests will be discussed and compared to the calculated response.

At the heart of NASA's Advanced X-ray Astrophysics Facility-Imaging (AXAF-I) lies the High Resolution Mirror Assembly (HRMA). The HRMA consists of four nested, confocal Wolter Type-I grazing incidence mirror pairs ranging in diameter from 0.7 to 1.2 meters. Because of its unusual optical design and stringent image quality requirements, novel approaches to imaging performance tolerancing and analysis have been developed in support of the HRMA thermomechanical design. Here, the methods and techniques employed to demonstrate compliance with alignment, strain, obstruction, and nonvignetting ghost image control are discussed; including tolerance flow-down methodology, finite element analysis (FEA) to ray trace coupling, and probabilistic `monte carlo' techniques.

This paper briefly reviews the more traditional materials that have been used for Soft X-Ray (SXR) and Extreme Ultraviolet (EUV) filters and then discusses several applications where new multilayer combinations of materials are being employed. The new applications include projection lithography and the detection of Energetic Neutral Atom (ENA) populations from spacecraft. Also briefly discussed is the use of polyimides for spacecraft filters.

The MARS 94 spacecraft is devoted to a mission to the planet Mars. Planned to be launched in October 1994, it shall reach the planet in September 1995. The lifetime is one Mars year. A second spacecraft will follow two years later. This paper presents the characteristics of the IR detection modules for the IR OMEGA spectrometer which consists of InSb multiplexed detectors. The purpose of this Meudon Observatory instrument is to observe mineralogy, water, ice and atmosphere in the IR spectrum. SAT has developed the Identification Model, Development Model and Fight Models of detectors, each fitted with 2 arrays and IR filters in a dedicated space housing equipped with a cold flex line and a connector. An array is composed of 128 multiplexed InSb photovoltaic detectors surrounded by a specific IR filter. Each filter is divided into 2 zones. For the IR Short Wavelength filter, one zone operates from 1 to 1.8 micrometers and a second one from 1.8 to 2.7 micrometers ; for the IR Long Wavelength filter, the 2 zones are 2.7 to 4.7 and 4.7 to 5.3 micrometers . The housing is designed in collaboration with the Observation de Paris-Meudon. The InSb photodiodes of sensitive areas 90 X 120 micrometers , and pitch of 120 micrometers , operate at 77 K, with the following characteristics: by means of a scanning operation, the sensitive areas are measured at half maximum of height and the diameters are: 120 +/- 1 et 92 +/- 0,7 mum. The signal is illustrated by the I(V) curve: the main parameters are: I_sc equals 10 - 100 pA and R₀ equals 10 - 25 G(Omega) . The quantum efficiency is (eta) equals 60% - 65% and the peak responsivity 2,5 - 2.6 A/W. The detectivity of non multiplexed pixels is: D* at (lambda) pic (FOV, 1800 Hz, 1 Hz) equals 2 X 10¹¹ cm Hz^1/2 W^-1. Measurements are made under low flux corresponding to the conditions of the OMEGA experience, the planet Mars being a cold body of about 270 K. The 1/f noise is very low: fc equals 1 - 2 Hz at reverse bias voltage of Vp equals -200 mV. The cross-talk under photon flux between adjacent diodes is < -40 dB. The multiplexed readout is obtained with a Si CCD from Thomson developed for the SPOT IV Mid IR channel. This CCD normally operates at room temperature and is space - qualified for this Earth observation project. It has not been modified for our 77 K conditions, only the drive electronics, bias conditions and phase clocking have been adapted to the 128 detectors. This 2 phase - CCD operates in a specific mode: discharge of a capacitance with a subtraction of electrons corresponding to the optical signal. The integration time is T_i equals 5 ms for a charge of Q_max equals 0.5 pC with I equals 100 pA. The performances of multiplexed InSb diodes at 77 K are given by the mean total noise of 2800 electrons including a 2300 noise contribution due to the CCD. Environmental tests have been performed and results are reported including mechanical tests (vibrations, shocks), thermal cycling, electrical tests (electrostatic discharge), and irradiations under 3 and 6 krad.

We report the experimental generation of a linear optical path modulation of 20 (lambda) range with an accuracy of (lambda) /200 ((lambda) equals 633 nm) using a servo control system driving an optical fiber stretcher. The error signal is provided by mixing interferometric signals with an electronic oscillator corresponding to the pure sinusoidal variation of the detected interferometric signal under a pure linear optical path modulation.

Some solutions for the case of two-mirror three-reflection telescope assuming to use really two mirrors (first reflection on a surface strictly equal to the one related to the third reflection) are briefly described. This work is a by-product of the efforts of a larger team for the conceptual design of wide field UV space mission.

The active real-time cophasing of an interferometer, that is, the fulfillment of the condition (Delta) (OPD) >> (lambda) (OPD is the Optical Path Difference between every pair of light beams, each coming from every telescope of the array is a crucial requirement for high resolution and long exposure image formation. So the optical concept is a possible design of a Cophasing System (CS hereafter) for a space interferometer, e.g. the Multimirror Ultraviolet Solar Telescope (MUST), will be presented. A collimator and a pair of achromatic wedges are two of its components; the former has the target to collimate light beams which enter in the CS preserving the instrumental errors due to aberrations much less than (lambda) in the Optical Path Length (OPL) of the light beams; the latter allows to choose any region of the Field of View (FoV) of the beams by their simple rotation (two degree of freedom correspond to a FoV point) in order to have high contrast features used in telescopes pre- alignment subsystem included in the CS. Ray tracing results on these optical components will be show. Their tolerance analysis will also be discussed. A mechanical approach for each wedge rotation will be shown together with a preliminary CAD arrangement of the subsystem in a cylindrical package of diameter approximately equals 300 mm and approximately equals 100 mm height.

The precise location of telescopes in a stellar interferometer makes position servo necessary. We propose a metrology configuration for synthesized aperture using a fiber linkage with the mixing station. Interference layers parallel to star wave fronts are generated in order to materialize a reference plane. Fringe position sensors generate error signals and drive the telescope position assembly to place the fiber input in a reference star wave plane.

SCIAMACHY (SCanning Imaging Absorption spectroMeter for Atmospheric CartograpHY) is a spectrometer developed for atmospheric space research, which covers the UV to near infrared spectral range, and is presently scheduled for flight on ENVISTAT-1 (1998). The principal scientific objective is the measurement of the global distribution of atmospheric trace gases, aerosol and cloud. Special emphasis is placed on the measurements in the stratosphere and the free troposphere. The current status of the instrument development is reported with details on the near infrared detectors. The predicted performance of the instrument in terms of signal to noise has reached an acceptable level in the infrared. Critical polarization issues have been identified and their solution is discussed.

The data on spacecraft glow are discussed. The results of the observations of this phenomenon from the orbital stations confirming the contribution of solar and geomagnetic activity to the glow intensity are presented. The possible hypotheses about the mechanisms of this contribution are analyzed and the space weather parameters which affect the glow brightness are examined. The conclusion is made that there are two reasons that amplify the glow brightness: the increase of the upper atmosphere density on the low-Earth orbit (LEO) after a series of solar flares and global magnetic storms, and the increase of the intensity of solar extreme ultraviolet (EUV) radiation scattered in geocorona to the night side after isolate global magnetic storms. In the latter case the contribution of O⁺⁺ ions to scattering should be taken into account, especially if subsequent large storm have the same type of commencement.

ISLA--the International Stratospheric Laboratory for Astrophysics--is proposed as the consequential evolution from present ground- and space-based astronomical observatories. ISLA operates remotely controlled at approximately 15 km altitude. Its diffraction limited 4 m telescopes permit imaging with 30 milli-arcsec resolution, while the interferometric combination of its telescopes reaches milli-arcsec resolution. The extremely low residual water vapor allows nearly unhindered observations from the infrared to the sub-mm spectral range. The efficiency of imaging and spectroscopy increases by about 10³, due to the improved light concentration, and interferometric observations will even be facilitated by factors of 10⁴ to 10⁵ compared to ground-based observations. ISLA combines the advantages of modern pressurized balloons with those of rigid airships. Several diffraction limited, extremely light telescopes of 2 to 4 m diameter are stationed on ISLA, together with the optics necessary for interferometry. The size and numbers of telescopes on ISLA will surpass any present and realistic future space telescope projects. The advantages of easy maintenance and exchange of instrumentation assure that the ISLA observatory will permanently be at the state of art. Scientific challenges to be tackled with ISLA are the search for extra-solar planets, details of star forming regions, surfaces of giant stars, nuclei of galaxies, gravitational lenses and the evolution of novae and supernovae etc. Many more presently unasked important scientific questions will arise than can be answered, but most likely there will be fascinating totally unexpected discoveries from ISLA.

A six-feature, all-sky star field identification algorithm has been developed, integrated with a CCD-based imaging camera and tested at the Table Mountain Observatory. The calibration of the CCD-based camera to minimize the error in the measured star parameters due to instrument error and experimental conditions is described. A set of observatory tests on star fields with this intelligent camera are presented.

Newly developed optical systems especially for astronomical purposes. Among these are: 1) Two mirror system free from spherical aberration, coma, astigmatism and distortion. Up to primary F-numbers of 1 and 8 meter diameter it's diffraction- limited for fields of 0,5 degrees. 2) Three mirror system free from spherical aberration, come, astigmatism and curvature of image field. For F-number 4 and a field of 2 degree's diameter aberrations are below 0,15 arc-seconds. 3) Three mirror system free from all third order aberrations. The system is very short and delivers a diffraction-limited, distortion-free image on a flat, 1 degree field. 4) Two mirror system with three lens corrector. For an object field of 2 degree's and F-number 4 image aberrations are below 0,3 arc-seconds in the wavelength range from 0,4 to 0,7 micron. 5) Two mirror correctors for all classical mirror systems with focal reducer and multi-focal length capability. For F-number 6 and a field of 1 degree aberrations are below 0,1 arc-seconds. 6) Mirror-lens system consisting out of three thin spherical elements. Image quality is about two times better than in a Schmidt-type camera with field flattening lens at same field diameter and F-number. 7) Optical system including a lens system of high symmetry. The lens system is capable of giving diffraction-limited and distortion-free image of 180 degree's object field.

Performances for nowadays optronic systems require focal plane arrays (FPA) with an increasing number of detectors. The `push- broom' technic is well adapted to earth observation in the visible range with the availability of long linear CCD'S offering thousands of pixels. In the infrared, line scan systems are preferred at the present time because technological difficulties have to be overcome in order to get long linear arrays. Among the most important, are: (1) Difficulties to have a large cold focal plane with a temperature uniformity of a few degrees. (2) Difficulties to get good detection material over large surface. Mechanical or optical butting technology can be used there but with dead pixels and/or side effects. (3) Very low cold shield efficiency due to the geometry of the long linear array. (4) Very high development costs. MATRA DEFENSE UAO has made the design of a new infrared FPA concept which has the advantage to overcome all drawbacks listed previously (patented design). The idea consists to transform the pixel arrangement geometry of a 2D array which is available off the shelf into a long linear FPA using a coherent infrared fiber optic reformatter. In order to demonstrate the feasibility of this new FPA concept, a camera breadboard has been built. This task has been supported by the French MOD (STTE). This paper describes this breadboard and gives main technical performances.

The image linearity of a charge division position readout anode, for instance, a Wedge and Strip Anode (WSA), is controlled by several factors. In addition to the drift field uniformity and pattern geometry requirements, linearity can be affected by the surface characteristics of the anode itself. We have undertaken experiments to establish the mechanisms which lead to non-linear response and suggest ways to avoid it. We present measured image non linearity of image intensifier tubes using two different types of charge division position readout anode; the Wedge and Strip anode (WSA) and the Spiral Anode (SPAN) devices. Both intensifier tubes were manufactured by DEP bv., Holland and utilize S20 or Caesium Telluride semi- transparent photocathodes on Magnesium Fluoride, Fused Silica or Calcium fluoride windows, proximity focused to a microchannel plate (MCP) stack. All detector variants built thus far use a stack of three MCPs in a Z configuration. We have undertaken experiments using a de- mountable detector of similar design but without window or photocathode. The role of secondary electrons produced on the anode in charge redistribution was investigated by using an intermediate conductive grid of high open area between the MCP and anode. The grid was used to define the electric field in a region just above the anode in order to either suppress the range of the anode secondary electrons, or prevent them from returning to the anode by collecting them on the grid. We present some preliminary results from these experiments and suggest modifications to the intensifier design to maximize the detector performance.

The Spectrum-UV mission, a general purpose ultraviolet observatory endowed with a 170-cm aperture telescope for imaging and spectroscopy in the 912 to 3600 angstroms range, will be launched in a seven days, high elliptical orbit in the late '90s by a Proton booster. The advanced feasibility study, carried on by an international study team from the participating Countries (Canada, Germany, Italy, Russia and Ukraine) is close to completion. The focal plane instrument complement for spectroscopy are described, and in particular the high resolution Echelle spectrograph and the Rowland one.