The Earth's environment and specifically the means to monitor and preserve it are being increasingly recognised as matters of major concern. Mankind is now appreciating that his activities may be disturbing the delicate balance which determines his environment on all scales from the local to the global. Linked to these environmental concerns is the realisation that the Earth has limited resources on which increasing demands are being placed. Thus better and more organised information about the behaviour of the environment and factors influencing the Earth's natural resources is essential if valid political decisions are to be made. This can only be achieved effectively on the basis of a better understanding of the Earth when viewed as a single system in which the physical, chemical and biological interactions between the atmosphere, the oceans, the land and ice regions, and the Solid Earth are all taken into account.

Astronomical adaptive optics improves the angular resolution of both solar and stellar telescopes. In addition to giving astronomers the ability to resolve the objects under study better, adaptive optics will : (i) enhance the contrast of pointlike sources against the sky and telescope (thermal) background, (ii) increase the spectroscopic resolution of most spectrographs, and (iii) improve the sensitivity of optical interferometers. We review the status of a number of efforts underway at various observatories around the world to implement adaptive optics on both large solar and stellar telescopes.

The paper presents the status of the COME-ON (CGE Observatoire de Meudon ESO ONERA) experiment. This instrument, developed and tested by several European laboratories, is an adaptive optical system with a 19 actuators deformable mirror and a Hartmann Shack type wavefront sensor. The wavefront sensing is performed at visible wavelengths; a special computer drives the deformable mirror which should achieve diffraction limited infrared imagery with large optical telescope. The different components and their individual characteristics are described. The results of the tests of some components are given: 19 actuators deformable mirror, tip-tilt mirror. The expected performances are summarized and possible applications of the instrument to astronomical sources are presented. The isoplanaticity aspect, the required temporal bandwidth and reference source brightness is discussed. The conclusions of the experiment will be used for the design of adaptive optics for the ESO Very Large Telescope.

In this high-resolution technique, both wavefront phase and speckle image are recorded simultaneously. The image processing leads to a high signal-to-noise ratio than in speckle interferometry.

Correlation functions of the Zernike-polynomial expansion coefficients of turbulence induced aberrated wavefronts are presented and experimentally confirmed using laboratory-generated Kolrnogorov turbulence. These functions are convenient for characterizing the anisoplanatic error for adaptive optics. The isoplanatic patch-size of such systems is evaluable according to their specific working parameters. We find, for example, with a 4 m telescope working at 3.5 μm and sensing the wavefront phase fluctuation on 20 Zernike polynomials, an isoplanatic patch about 22 arcseconds wide.

An optical wavefront propagating through the atmosphere will be perturbed by local variations in the refractive index of the atmospheric gases. When accumulated over long optical path distances they will impart a spatial and temporally random distortion to the wavefront. These distortions have a characteristic spatially coherence length r0, and an atmospheric decorrelation time τo. In directed energy applications, atmospheric distortions can reduce the peak target energy densities of large diameter laser beams by orders of magnitude. The problem is not solved through the use of larger apertures; once the aperture size increases beyond one or two τ0 the far-field spot remains constant in size. Hence, for large aperture systems, the overall performance is set by the spatial coherence of the atmosphere and not by the systems' exit pupil. An adaptive optics (AO) system can compensate for the degrading effects of the atmosphere and significantly restore diffraction limited performance. A segmented or deformable mirror with appropriate control signals can "pre-distort" the outgoing beam to cancel the atmospheric aberration. To be effective, the system must measure and correct wavefront variations over spatial resolution elements on the order of one ro and generate the compensation in a time less than τo. ITC is currently producing its third generation of segmented mirrors for atmospheric compensation. Advanced systems for short wavelength operation with five hundred segments have been tested and proven as part of integrated adaptive optical systems, see Figure 1. These systems have excellent optical figure, wavefront fitting, and dynamic performance as well as low cost. The segmented mirrors' inherent modularity has eliminated the classic optical fabrication problems associated with large optics. Using TTC's third generation technology it is now practical to produce segmented wavefront correctors with more than 10,000 segments for compensation of short wavelength systems with apertures of 4 m and beyond.

The Very Large Telescope program (VLT) is based on an array of four 8 m telescopes which can work in individual or combined modes. This program will, and has already initiated the development of a number of new technologies in a wide variety of domains. Some of them are listed here below. - Development of thin zero expansion glass mirrors. - Development of metal mirror technology. - Active support of deformable mirrors. - Replication of optical surfaces. - Wide band optical coatings. - Very large brushless torque motors. - Adaptive optical systems.

A Video-Polarimeter enables imaging of a field exclusively with the light which is polarized. A polarization selector isolates at will the six Stokes parameters. A CCD detector array feeds the images on a video system and on a memory. An interactive image processor computes combinations of these images to produce final images reproducing the scene in degree of circular polarization, in degree of linear polarization and in the azimuth of the linear polarization. Application for bidimensional polarization measurements in laboratory are illustrated. The Video-Polarimeter allows also open air field inspection and enables detection of artificial objects added in the field of view. Through a telescope, the instrument produces polarization images of astrophysical objects such as planetary surfaces. Adapted to a monochromatic solar telescope, the design allows analysis of the photospheric active centers by producing magnetic, velocity and temperature gradient images.

This paper deals with the design of a three spherical lenses corrector, able to produce wide flat field at the prime focus with focus ratio F/2.2 - 3.5m of a R-C telescope. The system is corrected over a spectral range extending from 300nm to 1100 nm. The corrector has the following characteristics: reduced size of the optics; all spherical surfaces; resolution better than 1 arcsec within 30 arcmin flat field ; possibility to extend the field up to 1 degree with reduced resolution. The image quality of the corrector is shown by means of Spot Diagrams, Point Spread Function and Encircled Energy Function for different off - axis values and spectral ranges. The paper also includes an analysis of possible alignment errors performed by means of computed interferograms.

An advanced two dimensional photon counting system employing a Resistive Anode Image Converter (RANICON) has been recently developed and tested at the STScI for use on ground based telescopes. It can presently obtain two dimensional images over a 25 mm diameter active area with 35 - 40 μm FWHM overall spatial resolution and ≤ms time resolution. The entire near UV, optical and near IR range can be covered with the use of bialkali, S-20 and GaAs photocathodes at overall sensitivities of 10 - 20 %. Its basic operating characteristics and results of recent field trials will be presented and discussed with special emphasis on its applications to time resolved imaging.

A system study has been started to elaborate and define methods of moving optical telescopes of the 2-m class continuously during astronomical observations. When operating several such telescopes together in an interferometer configuration, this mobility permits us to suppress costly delay lines and may also yield faster and better coverage of the u, v plane. The motion of the telescopes is however subject to the strict requirements of mechanical and thermal stability necessary for interferometric work. This implies stiff and vibration-free structures as well as metrology systems for speed and position control. The metrology systems may present a particular bottleneck as they have to measure distances up to several hundred metres with a precision of a fraction of a micron. All these considerations lead to a solution in which a telescope transporter unit would be equipped with two sets of tripod supports. They are alternately placed on counterparts in the ground and can move the telescope in x and y directions. During their operating cycle, the tripod supports are rigidly connected to the foundations, thus providing the pedestal of the telescope with the required stiffness for astronomical and interferometric observations. Overall speed and position control is derived from locally-installed, short distance measuring systems.

We describe here a proposed orbiting interferometer covering the UV, visible and near IR spectral ranges. With a 6-meter baseline and a collecting area equivalent to about a 1.4 meter diameter full aperture, this instrument will offer significant improvements in resolution over the Hubble Space Telescope, and complement the new generation of ground-based interferometers with much better limiting magnitude and spectral coverage. On the other hand, it has been designed as a considerably less ambitious project (one launch) than other current proposals. We believe that this concept is feasible given current technological capabilities, yet would serve to prove the concepts necessary for the much larger systems that must eventually be flown. The interferometer is of the Fizeau type. It therefore has a much larger field (for guiding) and better UV throughput (only 4 surfaces) than phased arrays. Optimized aperture configurations and ideas for the cophasing and coalignment systems are presented. The interferometer would be placed in a geosynchronous or 6-pm sunsynchronous orbit to minimize thermal and mechanical disturbances and to maximize observing efficiency.

Optical aperture synthesis (OAS) may be used to obtain images of much higher resolution than "seeing"-limited observations presently made from the ground. The principles of OAS are essentially those used in radio astronomy and may be applied to space-based or to ground-based observations. The greater spatial resolution obtained would facilitate the imaging of stellar envelopes around Be stars, the study of the internal dynamics of active galaxies, etc. The application of aperture synthesis techniques in space, at visible and at ultra-violet wavelengths, should permit imaging of much fainter sources than would be possible from a terrestrial telescope array. Infra-red observations are best made from the ground. A summary of the conclusion reached by the Space Interferometry Study Team set up by ESA will be presented. A short description of the important parameters relevant to a space mission (attitude control, orbit, structural dynamics, etc) and a comparison to terrestrial atmospheric conditions will be given. Possible instrument configurations will be described and it will be shown that a large field of view may be achieved, so that the instrument may be calibrated on bright stars whilst observing faint sources. Mission concepts for a "monostructure" ~30 metres in size will be examined and a possible strategy for Space Interferometry in the next 20 years considered.

We discuss ways to remap the exit pupils of a ring-shaped array of optical telescopes, to obtain simultaneous spectral and angular informations about the object under study. Such an array, operated as a long baseline interferometer in space, would attain very large li-miting magnitudes in absence of atmospheric turbulence, and open a new imagimg window down to 100 nm wavelengths. We consider this instrument in typical photon-noisy situations, and propose a spatio-temporal technique to integrate the flying fringe pattern recorded by a time-tagged detector at its focal plane. In principle, our method should relaxe the inter-nal stiffness of the array to a few millimeters, making floppy structures or free-flying satellites adequate for very long baseline interferometry in space.

The Faint Object Camera built by the European Space Agency for the Hubble Space Telescope (HST) contains three optical relays capable of reimaging the HST focal plane with high spatial resolution at F/48, F-96, and F/288 focal ratios. Recent simulations and ground calibrations have demonstrated the capability of carrying out high quality astronomical observations down to 15-20 milliarcsecond spatial resolution and limiting visual magnitudes of 29-30. In this paper, the results of these tests and calculations are presented and discussed in the context of the scientific capabilities of the mission. Particular emphasis will be placed on the optical characteristics of the F/288 high resolution apodizer which promise a revolution in our understanding of the structure of very faint nebulosity around bright nearby objects such as planetary systems around young stellar objects and ejecta from evolved mass losing stars.

The mechanical collimator to limit the field of view of the EUV/FUV Imaging Spectrograph Facility will be realized using a series of thin electroformed plates. In the present report the plate number and positions are computed and a procedure to test the instrument using visible light is discussed.

Present Solar telescope projects, on ground or in Space (like the Orbiting Solar Laboratory) are limited in their ambitions to visible wavelengths and to spatial resolutions not better than a tenth of an arcsec. The Solar Ultraviolet Network (SUN) proposal presented in this paper, is an interferometric concept capable of observations with a spatial resolution better than 0.013" (10 km) on the Sun, in the UV range. Based on Stabilized Interferometry principles it consists in 4 telescopes of 20 cm diameter aligned non-redundantly on a 2 m baseline. Despite its size (2.1 x 1.0 x 0.7 m) and its intrinsic complexity, SUN would be perfectly suited for use on the Space Station, when implemented on a pointing platform of performances comparable with the Instrument Pointing System (flown on Spacelab2). The remarkable capabilities of the SUN instrument, resulting from its "compact" non-redundant configuration of telescopes, allow high resolution imaging on a 2 x 2 arcsec2 field (and with a dynamic on the reconstructed images superior to 100 for phase stabilities ≥ λ/10), on the Solar disk (granulation, flares and micro-flares, prominences and filaments), or at the limb and above, across coronal loops.

The limitations of the coherent field of view in Optical Space Interferometry are presented. The size of the field required is inferred from the number of photoevents and from stellar density. Then we examine the limitations of the FOV in both "Michelson" -or planar array-and "Fizeau" -the equivalent of a masked giant telescope- cases in order to assess their maximum size; it is shown that in all cases, the field is too small to include a reference star in the "Michelson" final image field (< a few arcsec) when the input pupil is too diluted; conversely this is not the case for the "Fizeau", but alignment tolerances are extremely severe. The two main limitations in the Michelson type, field curvature effect and pupil geometry conservation, are then derived in terms of tolerances on the optics. The problem of spatial frequency plane coverage is addressed and we finally propose a mission concept that accomodates most of the problems raised in this paper and nevertheless makes use of two off-axis stars for tracking purposes by splitting the fields before recombination: a "Michelson" with flat collectors, making use of fiber optics in the reference channels.

The atmosphere of the earth restricts the resolution of conventional astronomical imaging to about 1 arcsec. Much higher resolution can be obtained by speckle methods. The Knox-Thompson method and the speckle masking method (bispectrum or triple correlation processing) yield diffraction-limited images in spite of image degradation by the atmosphere and by telescope aberrations. For example, with a 3.6-m telescope a resolution of 0.03" can be obtained at a wavelength of 400 nm. The limiting magnitude is about 18. We will discuss the theory and applications of speckle masking. High-resolution images and simultaneously the spectrum of each resolution element can be obtained by objective prism speckle spectroscopy and projection speckle spectroscopy methods. Finally, we will discuss the application of speckle masking to coherent arrays of telescopes. For example, observations with the 4x8-m ESO VLT can yield the fantastic angular resolution of about 2milli-arcsec.

New techniques to build and operate CCD mosaics consisting of Thomson THX31157 buttable CCD image sensors have been developed under the convention between ESO-INSU for the development of CCD detectors for astronomy. A split-field microscope is used to align the CCDs with respect to a precision sapphire template thus avoiding cumulative positioning errors in large mosaics. A 2 by 2 prototype CCD mosaic has been aligned with errors of less than 3 μm. A new generation CCD controller, based on commercially available VMEbus boards and newly developed CCD interface boards has been designed, built and successfully tested. The controller is able to read out up to 16 CCDs in parallel. The CCD camera is controlled by a 68020 microcomputer running under the UNIX-like real-time operating system OS-9/V2.2. Rudimentary image pre-processing software has been included in the controller to assemble the single CCD images into one image compatible with IHAP or FITS format. A newly designed cryostat with very high mechanical stability (1 μm) has four built-in preamplifiers to guarantee low noise operation even in noisy environments. Noise levels of less than 5 electrons RMS for the complete mosaic have been measured using A-grade THX31157 CCDs. The results of extensive laboratory testing of the mosaic are presented in this paper.

The last two to three years have seen a revolution take place in Infrared Astronomy. Prior to this time virtually all observations at wavelengths longer than about 1 micron had to be made by either scanning a single detector or by using small arrays of individually wired detectors. This has been as true for spectroscopic instruments as it has for photometric and mapping instruments. The revolution which has taken place has been the availability to astronomers of true imaging detector arrays for use at infrared wavelengths. The main impact of these devices so far has been at the shorter wavelengths using ground based telescopes. In future years this impact will be extended to longer wavelengths and to space based observations. A review of currently available devices and technologies will be presented. Prospects for the near- and mid-term future will be outlined and some desired characteristics for devices will be given.

Si:Ga DVR detector arrays specially designed for broad-band observations in the 10 μm atmospheric window, are currently developed by the LETI/LIR. The originality of these detectors is a large capacitance able to store the huge number of electrons generated by the 10 μm background photons. A prototype array with 32*32 pixels has already been delivered to the Service d'Astrophysique, at Saclay. A 64*64 array with a full well capacity of more than 2.107 electrons will be delivered in the middle of this year. This paper brings up the results of the first set of tests on the 32*32 prototype.

The cryogenic infrared camera, IRCAM, has been operating routinely on the 3.8m UK Infrared Telescope on Mauna Kea, Hawaii for over two years. The camera, which uses a 62 x 58 element Indium Antimonide array from Santa Barbara Research Center, was designed and built at the Royal Observatory, Edinburgh which operates UKIRT on behalf of the UK Science and Engineering Research Council. The system is capable of read noises of about 450 electrons rms and dark currents of about 100 electrons per second. IRCAM achieves background-limited performance throughout the 1 - 5 micron region for all broad-band imaging applications and even for narrow passbands (1% of the wavelength) for wavelengths longer than 2 microns. Performance characteristics including linearity and spatially non-uniform response are discussed. Over the past two years at least 60% of the available time on UKIRT has been allocated for IRCAM observations. Examples are given of recent results which illustrate the power of IR imaging in astrophysics.

This paper presents the CIRCUS infrared camera and recent results in extragalactic astronomy obtained using this 32 x 32 array camera. Observations were carried on starburst galaxies, 3.3μm PAH emission line at large scale in NGC 891 and the twin quasar Q0957+561.

The Infrared Space Observatory (ISO), a fully approved and funded project of the European Space Agency (ESA), is an astronomical satellite, which will operate at wavelengths from 3-200μm. ISO will provide astronomers with a unique facility of unprecedented sensitivity for a detailed exploration of the universe ranging from objects in the solar system right out to the most distant extragalactic sources. The satellite essentially consists of a large cryostat containing superfluid helium to maintain the telescope and its scientific instruments at temperatures around 2-3K. The telescope has a 60-cm diameter primary mirror and is diffraction-limited at a wavelength of 5μm. A pointing accuracy of a few arc seconds is provided by a three-axis stabilisation. system. ISO carries four instruments, namely: an imaging photo-polarimeter (3-200μm)., a camera (.3-17μm), a short wavelength spectrometer (3-45μm) and a long wavelength spectrometer (45-180μm). ISO will be launched in early 1993 by an Ariane 4 rocket into an elliptical orbit (apogee 70000 km and perigee 1000 km) and will be operational for at least 18 months. In keeping with ISO's role as an observatory, two-thirds of its observing time will be made available to the general astronomical community.

The ISO camera (ISOCAM) is an instrument designed to map selected areas of the sky in the spectral region from 2.5 to 17 μm at various spatial and spectral resolutions, polarization mapping will also be possible. At 10 μm the sensivity limitation ( lmJy in 10 min) will be mainly that imposed by the astronomical background. Spatial resolution will be limited to a few arcseconds by diffraction in the telescope and by the satellite pointing. A very wide range of astrophysical problems can be tackled with ISOCAM. Examples of current interest include : a systematic search for and survey of circumstellar disks or proto-planetary clouds, a probe for dark matter in the form of low mass stars, the nature and distribution of the emitters of the 'unidentified' (PAH?) infrared features, the low mass end of the initial mass function in star-forming regions ; mapping nearby galaxies. ISOCAM will provide imaging capability across a 3 arc min field of view with two arrays of 32x32 infrared detectors. Each array is mounted in one optical channel : the short wavelength channel operates in the 2.5 to 5.5 μm wavelength range with an InSb CID array, made by la Societe Anonyme des Telecommunications ; the long wavelength channel operates from 4 to 17 μm, with a Si:Ga direct read out array (DRO) made by LETI-LIR. Different magnification factors for matching the fixed pixel size to the desired pixel field of view on the sky are provided by four different lenses, which are mounted on a wheel. Choice of 1.5, 3, 6 or 12 arcsec. are possible, thereby determining the spatial resolution of the camera. The spectral range of the observations can be selected in each channel by a set of about 10 fixed band-pass filters and Continous Variable Filters (CVF), all mounted on a wheel. The spectral resolution is about 50 for the CVF and ranges from 2 to 100 for the filters. ISOCAM is being developped by an european consortium of laboratories led by the Service d'Astrophysique of CEN Saclay.

The Infrared Space Observatory Camera (ISOCAM) is one of four instruments attached to the ISO satellite due to be launched by an Ariane 4 rocket in 1993. The camera is designed to operate at 4K and uses two channels to cover the spectral range from 2.5 to 17 μm. Geometrical aberrations and diffraction were studied by a complete ray-tracing of the system including the f/15 Ritchey-Chretien telescope. Curvatures, aspheric coefficients and positions of the various optical components are optimised to reduce spherical as well as off-axis aberrations (coma and astigmatism). Chromatic aberrations are reduced by a suitable choice of infrared materials: Germanium for the 5 to 17 μm range and silicon for the 2.5 to 5 μm. A Monte-Carlo statistical approach was used to evaluate the effect of manufacturing and assembly errors on the 90% encircled energy diameter of the Point Spread Function. Special effects as deformation and possible existence of ghost images in the system are presented. A test facility including a f/15 simulator and a cryogenically cooled camera simulator has been developed in ROE to test the optics at 4K by measuring its PSF. Recent measurements show a good agreement with the computed PSF.

The Infrared Space Observatory's (ISO) photometer experiment ISOPHOT provides 4 sub-systems, as described in LEMKE 1985, which optimizes the different ways of photometric measurements. They allow multiband - multiaperture photopolarimetry, multicolor - polarimetric imaging and spectrophotometry. In order to cover the wide wavelength range 2.5...240μm five different detector materials have to be used. They are arranged according to their application as 2D-arrays, linear arrays and single detectors, a total of 223 detectors are present, see tab. 1.