The concept of an adaptive receive telescope array (RTA) for coherent optical space communications is presented. The RTA consists of N equals 2^K, e.g. 16, subtelescopes, N polarization-maintaining single-mode fibers, N optical phase actuators, a binary tree of N - 1 symmetrical polarization-maintaining directional couplers, N - 1 optical power sensors, and a digital control unit. The output interface, a polarization-maintaining single-mode fiber, can be efficiently coupled to a subsequent coherent receiver. Within a subtelescope's field-of-view, the control unit adapts the subtelescope phases (pistons) to the direction of the incident wavefront, thus maximizing the strength of the optical output field. The RTA is transparent, i.e. it operates independently of the modulation format employed. The feasibility of the RTA concept was demonstrated in a laboratory experiment. The implemented four-aperture antenna operates at a wavelength of 1064 nm. At an optical power level of 1 nW per subaperture, the experimental system combines the optical input signals with an efficiency greater than 99%. A step-shaped change of input wavefront direction is automatically compensated within 1 ms.

The National Space Development Agency of Japan (NASDA) plans to conduct an optical inter-orbit ling experiment in cooperation with the European Space Agency (ESA). ESA will launch the ARTEMIS geostationary satellite equipped with the SILEX optical terminal. NASDA will launch the Optical Inter-orbit Communications Engineering Test Satellite (OICETS) equipped with the LUCE optical inter-orbit communications equipment into low earth orbit. The link experiment will be conducted between these satellites with associated ground equipment in Europe and Japan.

This paper gives an overview on the current development status of the SOLACOS program and presents the highlights of the program. SOLACOS (Solid State Laser Communications in Space) is the national German program to develop a high performance laser communication system for high data rate transmission between LEO and GEO satellites (Inter Orbit Link, IOL). Two experimental demonstrator terminals are designed and developed in the SOLACOS program. The main development objectives are the Pointing Acquisition and Tracking subsystem (PAT) and the high data rate communication system. All key subsystems and components are straightway developed to be upgraded in follow- on projects to full space qualification. The main design objective for the system is a high degree of modularity which allows to easily upgrade the system with new upcoming technologies. Therefore, all main subsystems are interconnected via fibers to ease replacement of subsystems. The system implements an asymmetric data link with a 650 MBit/s return channel and a 10 MBit/s forward channel. The 650 MBit/s channel is based on a diode pumped Nd:YAG, Integrated Optics Modulator and uses the syncbit transmission scheme. In the syncbit system synchronization information which is necessary to maintain phase lock of the local oscillator of the coherent receiver is transmitted time multiplexed into the data stream. The PAT system comprises two beam detection sensors and three beam steering elements. For initial acquisition and tracking of the remote satellite a high speed CCD camera with an integrated image processing unit, the Acquisition and Tracking Sensor (ATS) is used. In the tacking mode the beam position is sensed via the Fibernutator sensor which is also used to couple the incoming signal into the receiver fiber. Incoming and outgoing beams are routed through the telescopes which are positioned with a 2 axis gimbal mechanism and a high speed beam steering mirror. The PAT system is controlled by a digital signal processor. For beam control advanced PAT algorithms are under development.

The European Space Agency is developing a small optical intersatellite link terminal under the Small Optical User Terminal (SOUT) program led by British Aerospace. This has mass of around 25 kg and is based on GaAlAs diode laser technology. Special features include a periscopic coarse pointing assembly, small refractive telescope, combined acquisition and tracking sensor, fiber coupled laser transmitter, and double-sided optical bench. The optical configuration which allows these features to be combined is described and the main optical design drivers are highlighted. In particular, details of the optical components and optical bench layout are described. The status of the SOUT breadboard currently under development is also reported.

The beacon is a powerful non-coherent CW laser source which is developed in the frame of the Semi-conductor Inter-satellite Link Experiment (SILEX). Its functionality is to provide a high divergence beam during the first tracking sequence. It is made up of 19 laser diodes coupled into one multi-mode fiber, the exit face of that fiber being integrated at the focal plane of a collimator. A breadboarding phase has been conducted at the beginning of the phase C/D in order to demonstrate that the requirement of 8 KW/Sr within a divergence of 11.75 mRd will be achieved. That phase is now completed and the overall breadboard performance is discussed in this paper with respect to the SILEX requirements.

Within the framework of a MATRA MARCONI SPACE FRANC contract for the European Space Agency, MATRA DEFENSE - DOD/UAO have developed, produced and tested 9 laser diode collimators, 52 optical components (anamorphoser, mirrors, dichroic splitters, redundancy module) and 9 interferential filters. All these space equipments must be integrated into the optical head of the SILEX (Semi-conductor Laser Intersatellite Link Experiment) bench. The SILEX experiment consists in transferring data from a low altitude satellite (SPOT 4) to a satellite in geostationary orbit (ARTEMIS) via beam generated by a laser diode (60 mW Cw). Very low emitted flux and long distance between the two satellites gives rise to the following technical difficulties: high angular (1 (mu) rad) and transverse stability requirements, requirement for high transmission and high rejection narrow band filters, in order to differentiate the transmit and receive channels, necessity of a very good optical wavefront, wavelength range 815-825 nm, 843-853 nm.

The Receiver Front End (RFE) is the optical receiver of ESA's Semiconductor Laser Intersatellite Link Experiment (SILEX). Optical free space communication between satellites is characterized by narrow beams and therefore by demanding requirements for pointing accuracy. This applies for the steering of the laser beam in transmission, for the pointing of the receiver's field of view (FOV), and for the alignment between transmitted and received beams. The RFE housing, the optical system, the lens and detector's mounting have to be designed to meet the stringent requirements for angular stability. This paper concentrates on the mechanical and thermal aspects which strongly influence the performance. Thermal expansion effects are of major concern when keeping the optical axis stable. All materials have been matched to the thermal expansion characteristics of the hybrid circuit which contains the detector. Assuming only homogeneous temperature changes during life, no stress or angular deviations have to be expected. The relative changes of dimensions in any direction stays equal at different temperatures. The verification of opto-mechanical performance requires sophisticated measurement tools. Measurements have to be performed in order to determine the lateral stability of lens and detector. A dedicated autocollimator was developed which measures the angular stability of the optical axis after vibration, thermo-vacuum test and under temperature changes. It also serves as a test transmitter. Measurement accuracies of 5 (mu) rad have been achieved. For the measurements the RFE is mounted onto a test fixture. A reference mirror on the fixture is the stable reference which has to be more stable than the equipment itself.

The potential of diffractive optical elements for advanced laser communication terminals has been investigated. Applications include beam shaping of high- power laser diode arrays, optical filter elements for position detection and hybrid (refractive/diffractive) elements. In addition, we present a design example of a miniaturized terminal including diffractive optics.

In the area of free space optical communications a significant amount of attention has been given to the overall problems of the acquisition and tracking of satellites and the accurate alignment and positioning of optical components on board a spacecraft. Accomplishing these functions with minimal investment in hardware size, weight, and power is essential to the successful evolution of space laser communications (Lasercom). This paper presents two types of nonmechanical laser beam control devices. A nematic liquid crystal (LC) lens and wedge have been fabricated and tested. These electro-optic devices can provide the capability of acquisition, fine tracking, point ahead and alignment of a laser beam in Lasercom terminals. Characteristics such as optical efficiency, switching speed, beam steer range and beam divergence have been measured. Device design and performance parameters are described.

The rapid growth and implementation of fiber optic sensor devices have evolved to the stage of demonstration and in-situ applications, whereas normally pressure and temperature parameters are measured. New photonic space opto- electronic systems could sense in a distributed way impact, damage assessment and pressure variations in structural integrity testing of spacecraft and platforms. The system consists of several loops (up to 8 per control unit) of fiber optic sensors or cables buried or implemented in smart structures or complex materials in wings or structures. Signal processing has been performed by FDM/TDM techniques to interrogate and to demultiplex unambiguously the fiber optic sensors. The control units could be placed locally or centrally in a network configuration to interrogate up to 256 loops or single sensors.

The measurement of electrostatic charge is fundamental in physics. A new method of performing the measurement, developed in this case for application to spacecraft and employing optical techniques is described. The principles of the instrument, together with the relative advantages compared to existing instrument designs are detailed. Reference is also made to the design and development of an instrument to be flown on a forth coming satellite mission (STRV1a).

HDOS's Model HD-1003 Miniature Star Tracker provides spacecraft designers with exceptional attitude determination performance in an all-solid state design, which weights only 7 lb. Key to achieving the requisite performances is the radiation tolerant, moderate field of view (FOV) optical system which provides, in conjunction with the charge-coupled device (CCD) detector, sufficient sensitivity and area coverage to ensure an average of 6 stars in each FOV during each sample interval. Multiple star data, processed by the tracker's advanced microprocessor, facilitates star identification and improves accuracy (especially in the all-stellar mode) by up to three times when compared to current 1-star trackers. HDOS's on-going HD-1003 development program has now entered the hardware stage. This paper describes the unique optical and electronic capabilities of the HD-1003, which will be of significant interest to future spacecraft designers, especially those involved with 'Small-Sats'.

For the CASSINI mission, Officine Galileo are presently developing, under Jet Propulsion Laboratory (JPL) contract, a CCD based star tracker, named Stellar Reference Unit (SRU). The SRU will be used in a new attitude determination approach developed by JPL, in which 'gyroless' operation can be implemented. The unit has a wide angle (15 degree(s) full cone) Field of View (FOV) and a maximum sensitivity up to +6 Visual Magnitude for an integration time of 400 msec. Exposure time can be commanded via Attitude and Articulation Control Subsystem Flight Computer (AFC) from 0 to 65535 msec. Optics characteristics and centroiding algorithm for star position determination, are defined by Officine Galileo, to achieve required performance (total accuracy: 53 (mu) rad, 3(sigma) ). The defined centroiding algorithm will be then implemented on the AFC. In this paper, the SRU architecture, characteristics, and main technological areas are described. Expected performance is reported.

This paper presents the concept and implementation aspects of the Pointing, Acquisition and Tracking Subsystem (PAT) which is developed in the frame of the SOLACOS (Solid State Laser Communications in Space) program.

We propose an architecture for the duplex transmission of microwave/RF signals over a fiber-optic link. The link employs only one laser. The forward channel is implemented through laser intensity modulation, while the reverse channel uses an external modulator. We evaluate insertion gain, linear dynamic range and phase noise degradation for each of the two channels.

In this paper, we study the optoelectronic functions of a 3D camera based on a laser rangefinder for an autonomous planetary rover. The main specifications to be satisfied are a range of 50 cm to 20 m with an accuracy of 1 cm or 1 percent and a continuous measurement rate of 0.1 ms. A critical analysis enables us to select two physical principles based on the measurement of the time taken for the propagation of light: pulsed and phase-shift rangefinding techniques. From the intrinsic properties of these both techniques, we study the influence of restrictive factors like the time of acquisition of a complete image and the comparison of the signal to noise ratio of these different kinds of devices.

We describe recent advancements in laser light scattering hardware including intelligent single card correlators, active quench/active reset avalanche photodiodes, laser diodes, and fiber optics which were used by or developed for a NASA Advanced Technology Development program. We then preview a space shuttle experiment which will employ aspects of these hardware developments.

The hardware development of an integrated backscatter fiber optic system for doing dynamic light scattering (DLS) measurements in the constrained microgravity environment of a space shuttle or space station is described. The small size and light-weight system easily fits in a Spacelab Glovebox and ensures maximum safety for the orbiter crew. It offers high precision particle size measurements from dispersions of colloidal particles within seconds. It needs neither onboard optical alignment nor index matching fluid, and it offers sample mixing and shear melting capabilities to study aggregation (flocculation and coagulation) phenomena under both quiescent and controlled agitation conditions. The system can easily be adapted for other microgravity experiments requiring the use of DLS.

Time averaged static light scattering (SLS) intensity collections on suspensions with defined particle dimensions and concentrations were achieved by utilization of the Zeiss SG 100 goniometer scatterometer. The bidirectional straylight distribution functions (BSDF) of suspensions containing 36 mg/ml lysozyme with NaCl-buffer and latex standards with diameters of 90 nm, 270 nm, 310 nm, 500 nm and 3100 nm in various concentrations are given. The measurements clearly reveal absence of complex Mie light intensity distributions to which presence is often referred to. Presented data collection achieved with commercial equipment would enable extraction of instrument designs appropriate for operation of dynamic light scattering (DLS) in reduced gravity environments. Optoelectronics and fiber optics of the multiangle Coulter N4MD submicron analyzer are concisely described and analyzed. DLS is introduced concerning baseline determination and applied evaluation procedures. Consequences of analysis yielded potential advancements to two optomechanics (optrode; lens protein analyzer LPA and goniometer device; DIMINIGON-A). Their operation is demonstrated in linkage with a 288 real time correlator (ALV-Langen). Experimental results achieved during aggregation and crystallization of lysozyme together with detection of intercalation between DNA and doxorubicin (antitumor therapeutics) are included. Results of viscosity measurements under low shear rates as function of the protein concentrations will be presented.

The optical system of the critical point facility (CPF) was developed to investigate the behavior of fluids around their critical point. The fluids can be analyzed in four measurement channels: the visual, the interferometer and the small (SALS) and large (WALS) angle light scattering channels. During upgrading of CPF-IML1 for the IML2 Mission, special effort was made to improve the S/N ratio at minimum input signals and in consequence the absolute sensitivity of the small angel light scattering channel. The S/N ratio of the SALS channel was limited mainly by stray light from the surfaces and bulk materials of the optical components of the system and by the detector electronics. In the following the term 'scattered light' is used for the light scattered by the fluid and which has to be measured, and 'stray light' is the contribution of all other light sources disturbing the measurement. After the description of the optical system of CPF, modifications performed to improve the dynamic range of the SALS channel are described. The most important stray light sources are identified and ways to reduce their contribution to the noise are discussed. Modifications to improve the dynamic range of the SALS detector unit are described. Finally the consequences of the system upgrading activities on the optical performance of CPF are discussed.

ALICE is an instrument developed for optical analysis of fluids near their critical point in Manned Space Station. A first model is used since July 1992 in the MIR Station. Two optical subsystems contain light source, imagery and measurement tools. Between them, in a thermostat, two sample cell units are experimented. The six available diagnostics, in coherent and incoherent light, are presented with the most significant results.

Stereo imaging velocimetry is the quantitative measurement of three- dimensional flow fields using two sensors recording data from different vantage points. The system described in this paper, under development at NASA Lewis Research Center in Cleveland, Ohio, USA, uses two CCD cameras placed perpendicular to one another, laser disk recorders, an image processing substation and a 586-based computer to record data at standard NTSC video rates (30 Hertz) and reduce it offline. The flow itself is marked with seed particles, hence the fluid must be transparent. The velocimeter tracks the motion of the particles, and from these we deduce a multipoint (500 or more), quantitative map of the flow. Conceptually, the software portion of the velocimeter can be divided into distinct modules. These modules are: camera calibration, particle finding (image segmentation) and centroid location, particle overlap decomposition, particle tracking, and stereo matching. We will discuss our approach to each module in this paper, and give our currently achieved speed and accuracy for each where available.

Infrared systems are becoming essential part of the diagnostics of Fluid Physics Facilities thanks to their capability to visualize thermal flows on fluid surfaces. In fact, one of the main objectives of the research in this field is to monitor changes in the surface temperature of a liquid following the application of different stimuli. The only way to achieve this without disturbing the fluid under test is to use nonintrusive infrared measurement techniques. The scientific community demands systems able to provide real time temperature maps in the range from 0 degree(s)C to 80 degree(s)C with a frame rate of five images per second at least and typical resolution in the order of 0.2 degree(s)C. A 100 X 100 mm² field of view is generally observed with a geometrical resolution as fine as 1 mm. These performances allow one to follow the evolution of the complex phenomena related to the heat convection and mass transport effects at the surface of fluids. In weightlessness condition, convection driven by surface tension is the only form of thermal convection because of the zeroing of buoyancy forces; observation of the free liquid interface may therefore provide important results for its interpretation. This paper deals with the use of Infrared Scanners to reconstruct the temperature distribution across the surface of fluids subjected to thermal gradients. The technical problems involved in this application are discussed. Finally, a review of the results achieved so far and the future possibilities are presented.

NASA's Advanced Technology Development (ATD) program is sponsoring the development of a new generation of surface light scattering hardware. This instrument is designed to noninvasively measure the surface response function of liquids over a wide range of operating conditions while automatically compensating for a sloshing surface. The surface response function can be used to compute surface tension, properties of monolayers present, viscosity, surface tension gradient and surface temperature. The instrument uses optical and electronic building blocks developed for the laser light scattering program at NASA Lewis along with several unique surface light scattering components. The emphasis of this paper is the compensation for bulk surface motion (slosh). Some data processing background information is also included.

The present paper deals with the conceptual definition of optical diagnostics around an experiment cell with fluid in a facility for observation of fluid physical phenomena in a microgravity environment. The concept comprises video observation of an experiment cell interior, interferometry and IR thermography along two directions and tracer observation with light sheets along three directions for tracer particles observation, in a FOV of 10 X 10 cm² maximum. The optics will be arranged in various exchangeable modules to attain flexibility for instrument upgrading and ease of maintenance. The selected optical techniques for the diagnostics are reviewed.

The Point Diffraction Interferometer (PDI) is a self-referencing interferometer. A reconfigurable PDI based optical system provided with the automatic control of the alignment has been developed to study fluids in microgravity. It will be flown on board of the IML2 Space Shuttle mission as a part of the Bubble Drop and Particle Unit (BDPU). Main features of the developed system are the automatic control of the alignment and the possibility of changing configuration (from a PDI scheme into a Dark Field mode of operation). To minimize the payload specialist manipulations the system is completely automatic, i.e. the alignment of the system during the start-up operations and the change of configuration are made automatically. System performances have been evaluated during laboratory tests on the apparatus and will be presented here. Future improvements of the PDI apparatus have also been investigated. They mainly concern the laser source, the alignment search procedure and the mechanism of the positioning of the diagnostic tools on the test beam.

In this paper, aberrations in reconstruction are analyzed, and aberration elimination method are discussed, and the feasibility of direct reconstruction of lensless Fourier transform x-ray hologram with visible light is investigated.

The use of interferometric fiber-optic sensors is discussed in relation to the measurement of physical parameters such as vibration and pressure. The performance of fiber-optic sensors incorporated into networks is presented along with calculations to show how this performance would be affected if the network were connectorised to enable it to be reconfigured.

Flow motion detection and analysis of 2-D and 3-D velocity fields in a fluid is a very important task in many applications, for example the study of fluidodynamics phenomena in microgravity conditions, i.e. in space environment. An extremely interesting technique useful to analyse fluids flow without introducing significative disturbances employs the change in optical density of some specific dyes when lighted with ultraviolet light. In this paper a complete processing system is described that uses the dye method to analyze fluid flow by means of digital image processing techniques.

High throughput and/or high resolution imaging telescopes for x-ray energies up to 8 keV are part of several space based astronomic missions to study small and faint cosmic x-ray objects. High throughput telescopes are applied for spectroscopy missions, high resolution telescopes to detect and analyze small X-ray sources. Depending on the goal and the constraints of the mission some of the various parameters such as resolution, throughput, number of nested shells or weight etc. are optimized. The production technology has to match to the mission goals and constraints to obtain an optimum balance between scientific performance, production time and costs. The entire production process of XMM mirror shells at Carl Zeiss and Medialario (Italy) respectively will be presented in this paper. This technology will be compared with the ones of other x-ray telescopes such as EINSTEIN, EXOSAT, ROSAT, JET-X, AND AXAF; and EUV telescopes such as CDS and EUVE regarding potentials and limitations of the manufacturing processes and optical performances.

Each of the three mirror modules of the XMM Wolter-I-type telescope is composed of 58 nested mirror shells. The smallest gap between two adjacent mirror shells measures only 1.6 mm. The fixation and alignment concept has been qualified with the Optical Demonstration Model ODM, which consists of four CFRP mirrors. It has the same entrance diameter (700 mm) as the flight model. The 3D topology of the mirrors was determined for spatial wavelengths ranging from 3 micrometers to 550 mm by three measuring devices. The optical quality of the mirror shells as a function of x-ray energy was predicted by a code developed at Zeiss. During integration roundness and alignment of the mirrors were monitored by a full-aperture Hartmann-type test with visible light. The entire model and single mirrors have been x-ray tested at the MPE long beam facility at photon energies up to 8 KeV. The results of x-ray tests and performance predictions are compared and discussed.

A wide range of space-borne measurement applications require very lightweight, reflective telescopes with wide wavelength coverage, capable of near diffraction limited image quality operating over wide temperature ranges and gradients. These desired features are becoming increasingly more important for future space hardware, especially for 'LIGHTSAT' and 'MICROSAT' applications. A silicon carbide (SiC) based telescope is an extremely attractive emerging technology which offers (1) the lightweight and stiffness features of beryllium, (2) the optical performance of glass to diffraction limited visible resolution, (3) superior optical/thermal stability to cryogenic temperatures, and (4) the cost advantages and rapid, commercial fabrication processes of aluminum. This paper describes SSG, Inc. hardware status in very lightweight SiC athermal telescope systems (mirrors and structure) for a wide range of space applications including NASA scientific and remote sensing missions. Status on several SiC mirror and telescope hardware is presented, including a 50 cm very lightweight scan mirror for a Geostationary Operational Environmental Satellite, or 'GOES'-type mission; a 20 cm aperture diameter, 4-mirror off-axis optical system weighing less than 2 kg; and a 50 cm on-axis, 30-mirror re-imaging telescope weighing less than 16 kg. Specifically, data is presented on the optical/thermal stability performance of SiC mirror and telescope hardware to cryogenic temperatures, as well as environmental qualification test results. For example, our test results show less than 0.25 wave peak-valley (p-v) in visible wavefront change (current noise floor), and no hysteresis from ambient to less than 100 degree(s)K temperature for mirrors and complete telescope assemblies.

The Coronal Diagnostic Spectrometer (CDS) is one of the key instruments of the ESA cornerstone mission SOHO, scheduled for launch in 1995. It is designed to study the solar corona in the EUV. After successful completion of an engineering model in 1991 the flight model fabrication was started end of 1992. In August 1993 the optical alignment of the CDS flight mirror (FM) module was completed. The paper reports about the fabrication and achieved quality of the primary and secondary mirrors as well as the alignment of the mirror system. The predicted optical quality (half energy width) of the whole telescope is HEW <EQ 3 arcsecs for spatial wavelength >= 10 mm and <EQ 6 arcsec for (lambda) >= 3 mm (2).

A new material designated C/SiC is the basis for a current ultra-lightweight scan mirror (ULSM) ESA/ESTEC technology development contract for the METEOSAT Second Generation (MSG) Program (Ref. {1}). It consists of random- oriented carbon fibers as 'greenbody' which is shaped to the designed configuration, and is infiltrated in the liquid phase with Si resulting in a SiC enriched surface layer, with a CVD grown SiC optical polishing layer. The reflective coating finally is silver under a dedicated protection layer. The requirements for this mirror are rather stringent: during 7 years in a geostationary orbit of life it will permantly be exposed to centrifugal forces induced by the satellite spinning at 100 rpm. Due to high image quality requirements for the imager - designated SEVIRI (Spinning Enhanced Visible IR Imager) - the MTF degradation caused by the scan mirror alone are defined to <EQ 3%, resulting in rather high surface quality requirements. The main contribution is the spin induced mirror tip deflection which turned out to be the design driver. The technologically most challenging requirement is to develop a scan mirror with 800 X 500 mm² effective aperture and a central cutout - the telescope is a Gregory concept - to withstand tight environmental requirements at a mass which was specified as < 6 kg. The ESA contract covers a baseline concept with the subsequently presented design of a monolithic C/SiC mirror, mated to a CFRP structure by isostatic mounts. At the end of this technology program, a fully tested 1:1 scale mirror with silver and protection coating will be rendered to ESA. Since this contract is a technology contract, applications beyond the MSG scan mirror are borne in mind, including higher optical surface qualities such as (lambda) /30 or curved surfaces, also aspheric mirrors of possibly even larger diameters. The manufacturing and optical process implement subcontractors which are not listed among the authors, i.e. SGL Carbon/Meitingen, FRG, for the Si infiltration; Schunk Kohlenstofftechnik/GieBen, FRG, for the CVD SiC coating; REOSC Optique, Ballainvilliers, France for the optical polishing, and Jenoptik, Jena, FRG, for the silver and protection coating.

SILEX is the acronym for Satellite InterLink EXperiment. By this experiment ESA (European Space Agency) starts the optical communication technique in space. Similar to the usual RF-communication technique the optical technique requires antennas for transmitting and receiving signals. Such antennas are telescopes. For Silex a two mirror telescope of an aperture of 250 mm was specified. To gain the benefits of optical communication such a telescope must fulfil extreme optical performances, especially concerning the wavefront quality which is strongly governed by the stability of the telescope structure. Thus the structure of SILEX telescope must guarantee a stability of +/- 2 microns over 320 mm in length. This figure must be maintained for 10 years under extreme environmental conditions, this especially concerns temperature, irradiation, ageing and above all launch loads. Looking at this area the glass ceramic ZERODUR was a very promising material to be used as a structural material provided one overcomes the justified concern on its mechanical reliability due to the fact that it is a brittle material similar to glass. This contribution presents solutions of the basic problems in structural design, the means of material and process qualification, and final qualification against launch loads of the critical structural item.

Optical instrumentation for space applications has to cope with a wide range of environmental loads on ground, during launch and in space. These loads in connection with long lifetimes in space and often demanding optical performance requirements are driving elements in the design of space optics. This paper will give an overview of the various environmental loads to be taken into account for space optics design. The associated degradation effects and their potential influences on the instrument performance budgets will be described. The consequences of these considerations for the optical instrument design concept and the selection of materials and technologies will be discussed.

Samples of 20 types of optical glass (both normal and radiation hardened crowns and flints, and fused silica), were subjected to both gamma (Co60) and proton radiation in air to simulate the dose expected over a 10 year lifetime in geostationary orbit. The samples were all 2 mm thick optically flat disks and were characterized before and after irradiation by spectral transmittance (250-800 nm) and wavefront error (approximately 630 nm) measurements. The geometry of the gamma irradiation was such that the outer area of each sample disk was covered by an annular lead shield giving a dose step of 50% of the nominal between the center and edge. This unique configuration gave rise to 'aperture imprinting' in some of the samples after Co60 irradiation, which was manifested as both a visible change in the coloration densities and a clear step in the measured interferograms from center to edge. The spectral transmission properties were much as expected from previously published work, with significant coloration and transmission loss in the near UV and the visible for the non cerium doped samples. The results of the tests are presented in summary form, conclusions are drawn with regard to the selection of optical glasses for application in space borne instrumentation, and recommendations for future work in this field are made.

Optical components which are being used for space instrumentation have been exposed to space conditions to examine degradation effects. The samples have been mounted into the SESAM (Surface Effects Sample Monitor) instrument which was integrated on the ASTRO-SPAS Satellite. Exposition occurred during the STS-51 mission. It was found out that especially supersmooth glass samples made of BK7 show damage effects which increase the roughness from i.e. 0.05 nm to 0.19 nm.

Results on spatial dark current fluctuations in Thomson TH7863 CCD image sensor are presented. The fluctuations have been investigated after irradiations by ⁹⁰Sr and neutrons (1 MeV) sources. Both ionization and bulk displacement damage are considered. Measured data are compared to predicted values calculated using extreme value statistical analysis. An excellent agreement is found for neutron-induced dark current fluctuations. In contrast, the case of dose-induced dark current fluctuations is more difficult to interpret owing to the lack of an adequate model for interface- state buildup. However, particle-induced damage appears to be the most important contribution to the observed fluctuations. Extension of these results to other types of radiation is discussed in order to assess the radiation response of the TH7863 for a given space flight environment.

The Coronal Diagnostic Spectrometer (CDS) to be flown on the Solar Heliospheric Observatory (SOHO) satellite makes use of both grazing and normal incidence optics operating in the extreme ultraviolet wavelength range from 15 to 80 nm. Its optical components include a Wolter Schwartzschild type 2 telescope and grazing and normal incidence reflective diffraction gratings. The instrument is thus highly sensitive to contamination both from particulate debris of a kind readily controlled by use of clean rooms, but also from offgassed and outgassed molecular species arising from materials used in clean room construction and from components within the instrument itself. We describe the contamination control protocol used to constrain the contamination, both particulate and molecular, to levels that will maintain the instrument performance for the expected duration of the SOHO mission. We also describe and report some results from a parallel activity in which we have screened materials for inclusion in CDS construction, by making use of a purpose built ultra high vacuum system to provide a contamination source and surface science diagnostics. In this apparatus material samples are held at elevated temperature to expel, in a controlled manner, outgas products which are allowed to contaminate previously characterized gold coated witness mirrors. The outgas products being monitored by residual gas analysis and a quartz crystal microbalance. The contaminated witness mirror surfaces are chemically characterized without breaking vacuum using X-ray photoelectron spectroscopy (XPS), and the change in reflectivity at ultraviolet wavelengths is also measured in the same apparatus.

Optical components for space optics - especially coated optical elements which represent the external surfaces of optical space instrumentation - have to work under harsh operation conditions like thermal loads, irradiation by photons, electrons and protons, as well as in atomic oxygen environments at low earth orbits. Additionally they have to withstand other cross contamination coming from the spacecraft. Therefore, the stability against these influences is a decisive factor for the application performance of optical coatings in space-borne devices. Some very recent results, based on the Surface Effects Sample Monitor (SESAM) flight experiment carried out aboard the ORFEUS-Shuttle Pallet Satellite (SPAS), STS-51, Discovery, are presented here along with laboratory experiments in an UHV-surface analysis system. The topics include ground simulation of selective and complex particle bombardment of optical coating analyzed by XPS as well as the verification of these results by flight experiments in combination with optical measurements (transmission, scattering).

In space optics, optical components are required more and more insensitive to extremely severe environmental conditions, with guaranted time of life and, concerning coatings, very severe spectral specifications. For most applications, coatings made with oxyde layers are not anymore convenient, because of their low compactness. Absorption and desorption of water in atmospheric or vacuum conditions induce stress in the stack which can involve incertainty in the stability in life. Indexes of materials are higher in atmospheric pressure than in vacuum, which makes shift the spectral curve. High and low index material which are quarter wave during evaporation in vacuum are not anymore quarterwave after beeing put in atmospheric conditions, which can completely affect the shape of the spectral curve. This phenomenon is particularely sensitive for bandpass interference filters which cannot be manufactured by classic thermal evaporation of oxyde materials. Less hard coatings can be used if protected by bonding but these kinds of components are sensitive to climatic conditions, involving changes in wavefront. Moreover, the resistivity of the cement to irradiations is not fully convenient. So it was necessary to develop new kinds of hard coatings with very high compactness, deposited on external faces of single substrates. In the contex of spatial programs, the coating Laboratory of MATRA has developped such components. Based on materials which are not oxydes, these coatings are perfectly compact so that their spectral curve is completely insensitive to air vacuum transition. Moreover, their hardness make them cleanable and resistive to adhesion and abrasion test. These qualities has been obtained by the choice of materials and the optimisation of their conditions of evaporation, the procedure including only classic thermal evaporation, without any kind of assisted ion gun. Many kinds of coatings can be made by these procedures which provide components of very high stability in terms of time of life and during all environmental conditions

A synthesis method is outlined for the design of broadband antireflection coatings for use in spaceborne infrared optics. The Golden Section optimization routine is used to make a search, using designated non-absorptive dielectric thin film combinations, for the coating design which fulfills the required spectral requirements using the least number of layers and different materials. Three examples are given of coatings designed by this method: (i) 1 micrometers to 12 micrometers antireflection coating on Zinc Sulphide using Zinc Sulphide and Yttrium Fluoride thin film materials. (ii) 2 micrometers to 14 micrometers antireflection coating on Germanium using Germanium and Yttrium Fluoride thin film materials. (iii) 6 micrometers to 17 micrometers antireflection coating on Germanium using Lead Telluride, Zinc Selenide and Barium Fluoride. The measured spectral performance of the manufactured 6 micrometers to 17 micrometers coating on Germanium is given. This is the antireflection coating for the Germanium optics in the NASA Cassini Orbiter CIRS instrument.

The High Resolution Dynamics Limb Sounder is described, with particular reference to the atmospheric measurements to be made and the rationale behind the measurement strategy. The demands this strategy places on the filters to be used in the instrument and the designs to which this leads to are detailed. A second set of filters at an intermediate image plane to reduce 'Ghost Imaging' is discussed together with their required spectral properties. The spectral characteristics of the primary and secondary filters in each channel are combined together with the spectral response of the detectors and other optical elements to obtain the system response weighted appropriately for the Planck function and atmospheric limb absorption. This method is used to demonstrate whether the out-of-band spectral blocking requirement for a channel is being met, and an example calculation is demonstrated showing how the blocking is built up for a representative channel. Finally, the techniques used to produce filters of the necessary submillimeter sizes together with the testing methods and procedures used to assess the environmental durability and establish space flight quality are discussed.

The calculation of the BRDF (Bi-Directional-Reflection-Distribution-Function) from profile measurements was performed theoretically and verified by measurements on a BK7 sample. The assumptions on the surface topography and approximations done are highlighted.

The ARS-scatterometer at the IOF Jena which has been designed and built in cooperation with the Carl Zeiss JENA GmbH is capable of performing accurate measurements of the BRDF over a range of approximately eleven orders of magnitude (from 10⁵ to 10^-6 sr^-1) in BRDF. These measurements can be carried out for surfaces made of any dielectric, metallic or semiconductor material. A He-Ne-laser is used in conjunction with a mirror beam focusing optics. The eight motorized degrees of freedom of the instruments are computer controlled and interfaced to the user by a sophisticated software running under MicroSoft Windows. It is shown, that the scatterometer is sensitive to a rms-roughness better than 1 nm.

This paper presents an overview of activities carried out in support of both NASA and European Space Agency programs for the development of a laser for space-based wind measurement. For the NASA Laser Atmospheric Wind Sounder (LAWS) program, a Phase One Laser Transmitter Study was undertaken, leading to the recommendation of an e-beam sustained CO₂ laser. For the European Space Agency (ESA) program for Development of a CO₂ Laser for Spaceborne Doppler Wind Lidar, some of the most significant results obtained to date are: (1) Demonstration of sealed lifetests using an e-beam sustained test laser to 10⁷ pulses using a gas phase catalysis technique, (2) Optimization of multimode laser head efficiency to 17% has been accomplished, with a corresponding 10% single transverse mode efficiency, (3) Demonstration of foil lifetimes consistently in excess of 10⁹ pulses at higher temperature and pressure pulse levels than expected in the breadboard laser, (4) Design, procurement, assembly, and initial testing of a compact breadboard laser. As well as laser performance, particular attention has been paid to the sever vibration requirements, thermal and gas flow aspects.

The CNES (Centre National d'Etudes Spatiales) established the laser system characteristics for Mars-Earth spatial communications using the Pulse Position Modulation (PPM) technique. Today, diode-pumped solid-state lasers are under intense research and development. Indeed, in comparison with flash-lamp pumped laser, they offer significant advantages in terms of efficiency, compactness, lifetime and high beam quality. We have demonstrated that gain- switch operation is preferable to Q-switching technique to control and to obtain a good pulse width and amplitude reproducibility. The pulse width requirement and the laser-diode pumping scheme lead to a preferred configuration based on a microchip laser oscillator coupled to an amplifier. Experiments were performed with several microchips of Nd:YAG and Nd:YVO₄ crystals pumped by fiber-coupled laser-diode. The design of the transversally-pumped amplifier is based on a modified multipass 1:1 confocally reimaging longitudinally-pumped amplifier proposed by Plaessmann et al.

The influence of sub surface damage under smooth optical surfaces on the scattering properties was investigated. Usually this sub surface damage is filled and covered by a polishing layer. Therefore sub surface damage does not contribute to the micro roughness of the surface. Three glasses, SF3, BK7, and SUPRASIL, with different Knoop hardness and related different sub surface damage density were chosen for the measurements. Three samples of each glass were polished with increasing polishing time in order to reduce the layer which contains the sub surface damage. Beside the extensive measurements of the scatter behavior the samples were also investigated by means of microscopy (Nomarski, darkfield, cross polarization) and optical profilometry. The stray light was detected in the case of reflection (back scatter), transmission (forward scatter) and total reflection. In the case of totalreflection the scattered light behind the reflection surface was investigated. The detected scatter light was integrated over the measurement range and the resulting value was compared with the polishing time. Additional investigations were carried out to determine the influence on the light polarization.

The Objective lens 'Apo-Orthometar 5.6/175' and the Filter Plate are parts of the High Resolution Stereo Camera for metrical and spectral Mars surface mapping intended for application in the Russian Space Mission Mars 94. The Mars surface will be mapped using three CCD line sensor assemblies with three CCD lines each (attached behind the filter stripes). These line sensor assemblies are adjustable independently so that for stereo, photo-metrical, and nadir channels the image 'plane' can be adapted optimally also that residual focus differences remain inside the depth of the focus limit (also taking into account a field of view (FOV) of +/- 20 degree(s)). The Lens resolution is adapted to the sensor characteristics. Due to the wide spectral range (blue to near infrared) an apochromatic lens correction is required. Additional filter stripe thickness adaptation of blue and IR channel and optimum line sensor assemblies adjustment cause a very good resolution of the spectral channels. The following requirements for application in space had to be considered in the lens and filter design: Low mass, low thermal focal shift , adapted material, mechanic-dynamical stability, and applicability in space. The optical and mechanical design, manufacturing, mechanical-dynamical tests, thermal vacuum tests, optical parameters verification and consideration of tests results shall be described.

A test system for performance verification of diffractive optical components was designed. The principle of the test methods is to measure the light intensity distribution in the near-field by scanning it with a pinhole detector while illuminating the element with a plane wave. Contrary to other performance verification methods, the scanning method allows high flexibility in the characterization of diffractive optical elements and allows also the determination of the diffraction efficiency.

In the framework of a contract with the Indian Space Research Organization (ISRO), MATRA DEFENSE - DOD/UAO have developed, produced and tested 36 types LISS 1 - LISS 2 lenses and 12 LISS 3 lenses equipped with their interferential filters. These lenses are intended to form the optical systems of multispectral imaging sensors aboard Indian earth observation satellites IRS 1A, 1B, 1C, and 1D. It should be noted that the multispectrum cameras of the IRS 1A - 1B satellite have been in operation for two years and have given very satisfactory results according to ISRO. Each of these multispectrum LISS 3 cameras consists of lenses, each working in a different spectral bandwidth (B2: 520 - 590 nm; B3: 620 - 680 nm; B4: 770 - 860 nm; B5: 1550 - 1700 nm). In order to superimpose the images of each spectral band without digital processing, the image formats (60 mm) of the lenses are registered better that 2 micrometers and remain as such throughout all the environmental tests. Similarly, due to the absence of precise thermal control aboard the satellite, the lenses are as athermal as possible.

The main goal of the present paper is the study of opportunities of solar energy conversion into electricity in space by the systems including holographic concentrators. A maximal efficiency of such systems was analyzed. The emphasis is made on the analysis of dichromated gelatin emulsion layers to be used for hologram recording.

Spectralon has been proposed as a material for diffuse calibration panels for several satellite-based earth-observing instruments. This paper is an interim report on the ongoing flight qualification testing of this material, including tests of the stability of the material under exposure to atomic oxygen, proton fluences, and UV/VUV radiation which match those of the polar orbit environment. No measurable degradation of optical properties were observed following atomic oxygen exposure or proton bombardment. Under initial UV/VUV exposure tests, some degradation of the optical properties of the material were observed; this optical degradation has been linked to degradation of organic contaminants. Correlative tests (Gas Chromatography/Mass Spectrometry) indicate that such contamination can be prevented by the adoption of a more rigid production protocol. It is believed that elimination of these contaminants will result in a significant improvement in the optical stability of Spectralon under UV/VUV exposure. The validation of this new production protocol will occur during a second phase of UV/VUV testing.

We realized a digital interferometer for measuring the shape of optical surfaces in an unstable environment. The instrument performs good quality measurements even if it is subjected to severe mechanical vibrations. Interferograms are decoded using a spatial-carrier phase-shifting technique. The system has been mounted over a polishing machine and used for on-line testing of aspheric mirror surfaces.

In this paper we describe a new technique for controlling the shape and frequency of a single mode solid state laser. This technique is based on the injection of a single frequency powerful CW laser in a high power pulsed laser through a phase conjugate mirror. We will expose the principle of this experiment and give the first experimental results obtained up to date.

Cooled infrared filters have been used in pressure modulation and filter radiometry to measure the dynamics, temperature distribution and concentrations of atmospheric elements in various satellite radiometers. Invariably such instruments use precision infrared bandpass filters and coatings for spectral selection, often operating at cryogenic temperatures. More recent developments in the use of spectrally-selective cooled detectors in focal plane arrays have simplified the optical layout and reduced the component count of radiometers but have placed additional demands on both the spectral and physical performance requirements of the filters. This paper describes and contrasts the more traditional radiometers using discrete detectors with those which use focal plane detector array technology, with particular emphasis on the function of the filters and coatings in the two cases. Additionally we discuss the special techniques and materials used to fabricate infrared coatings and filters for use in space optics, and give examples of their application in the fabrication of some demanding long wavelength dichroics and filters. We also discuss the effects of the space environment on the stability and durability of high performance infrared filters and materials exposed to low Earth orbit for 69 months on the NASA Long Duration Exposure Facility (LDEF).

The Interferometric Analysis Computer Code is a program developed to evaluate the performances of Fourier Transform Spectrometers. It has been carried out in the frame of the IASI program. It is a stand-alone code which can use as input the optical system data set up by an optical design software. The interference phenomenon is evaluated using the optical data of both interferometer arms by means of real ray-tracing. The mathematical model used to obtain the output signal is based on the concept that, for a monochromatic source, this signal is quite similar to an ideal sine. This allows to calculate three functions describing the difference between the ideal interferogram and the simulated one. These represent the average level of the output irradiance, the modulation and the phase of the oscillating terms as a function of the Optical Path Difference. These functions are quite smooth and then easily representable by fitting. Therefore in order to have a good representation of them it is sufficient a number of points much smaller than those necessary to represent correctly an interferogram. Then a great advantage in terms of computation time is obtained, especially when many signals have to be added to simulate the effect of a detector covering a quite large field of view. Furthermore, the possibility to input in the optical data files different kinds of manufacturing or assembly errors allows to estimate the sensitivity of the optical components respect to these aspects. This makes possible the calculation of an exhaustive tolerance budget.

The aim of the present paper is to present the results of a research, developed in Officine Galileo, on a special coating that is used on cuvettes for thermofluidodynamic experiments in Microgravity as on the BDPU (Bubbles Drops and Particles Unit), which will flight on the IML2 in June 1994, and on the Inex-Mam (Interactive Experiment on Marangoni Migration) module that has been launched with a Maser Sounding Rocket in November 1993. The very stringent optical and thermal requirements and the Microgravity environmental conditions led to the development of a multilayer coating on a quartz covette with a low reflectivity in the visible (< 1.5% at 633 nm and < 4.0% in the range 400-800 nm), a good reflectivity (> 80.0%) in the IR above 3.7 micrometers (this is the upper cut wavelength of Suprasil 300 Quartz, the cuvette materials), and an average transparency > 20.0% in the range of 0.8-3.7 micrometers . These performances allow to reach an extremely low IR emissivity (< 0.1 above 5.0 micrometers ) of the external cuvettes surfaces, but simultaneously allow a partial thermal images acquisition of the fluid surface within the cuvette (in the range 0.8-3.7 micrometers ) and the application of interferometric diagnostic systems in the visible range (PDI: Point Diffraction Interferometer at 633 nm). In the paper the achieved performances in the visible and thermal ranges are discussed with respect to the experiments cuvettes requirements, and a description of the used technology is given.

A direct method for quantitatively controlling organic as well as inorganic contamination on optical surfaces of witness plates of only 1 cm² is suggested. Measurements by X-ray Photoelectron Spectroscopy (XPS) quantified by appropriate software give the quantity and chemical composition of contamination on any interesting surface with a typical detection limit of better than 10^-9 gcm^-2. Results regarding the contamination growth on different optical materials and coatings under atmospheric conditions are presented. The contamination strongly depends on the chemical and structural properties of the surfaces. Therefore, reliable contamination control has to be done by sensitive measurements on either the surfaces of interest or on similarly treated resp. coated witness plates. To utilize the high sensitivity of the XPS-method, the witness plates have to be prepared and handled very carefully. Appropriate procedures for the contamination control of sensible optical surfaces as well as for the general control of clean rooms and fabrication facilities are described.

Absorbing coatings for space-borne optical instruments must accept - besides optical requirements - unusual challenges regarding their chemical stability and their emission of gaseous and condensable materials under space conditions. Consideration of mass balances according to ANSI/ASTM E 595 do not provide enough information for proper choice of the coatings in the vicinity of sensitive optical elements. By in-situ mass spectrometric and XPS measurements we have investigated chemical changes of different black paints and inorganic coatings, their outgassing and the contamination of the surfaces near the coatings. In general, organic black paints are threatened by UV- irradiation and by thermal loads. Changes of their chemical composition under energetic UV-irradiation and the intense emission of contaminating materials were proved. The inorganic coatings investigated do not change their chemical composition and cause only comparatively small contamination on surfaces in their vicinity. For reducing the problem of contamination in optical systems it is still necessary to make available appropriate inorganic coatings with improved absorbing characteristics. However, if lower absorption can be tolerated, the use of inorganic coatings in sensible optical equipment should be considered already now.

The straylight behavior of optical components for space instruments is in practice estimated by the valuation of their surface finish (the RMS roughness R_q) and their contamination. Other artifacts like scratches and bulk scatter can contribute too and cause an increase of the straylight level. This paper discussed several test methods to separate these artifacts from roughness scatter, to identify these artifacts and to process them with the optical code ASAP. These tests methods make use of the fact that roughness scatter shows (a) a fractal behavior, (b) must be a superficies radiator and (c) transfers the polarization components s and p independently from each other. The last condition is used to suppress scatter due to roughness. To emphasize the practical importance of these artifacts no specially prepared samples have been used for this paper. All shown measurements have been selected out of regular test series for several space projects. The tests have been carried out with a ZEISS SLG 110 scatterometer, which is described in paper number 2210-51 of this conference¹³, at the optical clean room laboratory of ESTEC.

This paper describes recent progress in the development of more advance laser diode pump source: 'Monolithic Surface Emitting Laser Diode' (MOSEL). In this structure, the linear bar arrays are epitaxially grown on an engraved substrate between etched deflection mirrors adjacent to the laser facets. The laser diode structure grown by MOCVD technique is a single quantum well graded index active region (SQW/GRINSCH). A specific process has been designed to have cleaved laser diode facets. Monolithic elements of 0.1 cm² have been designed and mounted. The quasi-CW operation gives over 100 W of optical power or a QCW power density of 1 KW/cm². A compact assembly of 10 similar elements gives an optical power of 1 KW QCW. CW operation have been also tested up to 150 W CW/cm².

A new type of space optics technology is presented which promises the realization of very large apertures (tens of meters), while packagable into lightweight, small volume containers compatible with conventional launch vehicles. This technology makes use of thin foils of circular shape which are uniformly mass loaded around the perimeter. Once unfurled and set into rapid rotation about the transversal axis, the foil is stretched into a perfectly flat plane by the centrifugal forces acting on the peripheral masses. The simplest applications of this novel technology are optically flat reflectors, using metallized foils of Mylar, Kevlar, or Kapton. Other more complex optical components can be realized by use of binary optics techniques, such as depositing holograms by selective local microscale removal of the reflective surface. Electrostatic techniques, in conjunction with an auxiliary foil, under local, distributed real-time control of the optical parameters, allow implementation of functions like beam steering and focal length adjustments. Gas pressurization allows stronger curvatures and thus smaller focal ratios for non-imaging applications. Limits on aperture are imposed primarily by manufacturing capabilities. Applications of such large optics in space are numerous. They range from military, such as space based lasers, to the civilian ones of power beaming, solar energy collection, and astronomy. This paper examines this simple and innovative concept in detail, discusses deployment and attitude control issues and presents approaches for realization.

The principle of phased array microwave antennas can be applied at optical frequencies. The far-field antenna pattern is found by spatial Fourier transform of the optical field distribution across the subaperture plane. Inertia-free antenna pattern steering can be accomplished by proper phasing of the subaperture waves. A tolerance analysis shows that the required accuracy of phase relationship and subantenna alignment can be obtained in practice when implementing control loops. We develop schemes for both transmit and receive array antennas. Experiments carried out at (lambda) equals 1.06 micrometers demonstrated both modes of operation. Optical array antennas may be applied advantageously for fine pointing in intersatellite data links and in space lidar systems.

The far ultraviolet (FUV) reflectance of an unoxidized thin aluminum film, prepared and maintained under UHV conditions, was measured. Reflectance versus the angle of incidence was measured in two perpendicular planes of incidence. The complex refractive index of aluminum was obtained from those reflectance measurements, taking into account the scattering induced by the surface roughness. The results here shown allow the calculation of reflectance of aluminum coatings and multilayers containing aluminum for application in FUV optical instruments operating outside the atmosphere.

In June 1993, REOSC got the contract for the optical fabrication of a Three Mirror Anastigmat (TMA) for a high resolution space camera of 1-meter focal length, F/4 relative aperture and +/- 2.125 by +/- 0.425 degrees object field angle. After having shown the various advantages of such type of optics, we will discuss the issues of design, fabrication, alignment, and testing of such a system. TMA are a promising type of optical systems that can constitute performant upgrades of present earth observation projects. Preliminary test results will be discussed.

From the local wavefront slope measurement given by a Shack Hartmann wavefront sensor (SH), it is possible to evaluate the decentering aberrations introduced by optical system misalignments. These slopes can be expressed as a function of the centered system aberrations and of the optical axis displacements in the image and pupil planes. After calibration of the wavefront sensor, the system alignment can be, for small aberrations, automatically controlled. The validity of the concept was proved on a representative observation system.

The additional degrees of freedom that arise from segmenting the optical surfaces naturally allow for the development of active control of orbital thermal fluctuations and mechanical stresses as well as atmospheric compensation. The successful performance of segmented optical systems depends on ability to position and control the segments. We briefly review the goal of segment control and our exact analytical solution to adaptive optics segmented mirror control¹. Our completely deterministic control solution eliminates the need to do iterations and eliminates the need to develop iterative control algorithms. We also present diffractive concepts for segment position metrology that exploit and transfer diffractive measurement technology from crystallography and condensed matter physics.

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 1 05 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 0.5 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 1975 . 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 . The HRV (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 1986. 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).