A cooled optically servo-controlled infrared Fabry-Perot spectrometer has been constructed. The control system maintains both alignment and spacing of a tunable infrared etalon. An annular portion of the IR etalon is coated for control use at visible wavelengths. Correct spacing is determined by reference to an associated visible etalon of fixed and slightly different spacing. The spacing of the IR etalon then varies approximately linearly with the output wavelength of a visible control monochromator. The full range of the mono-chromator, typically 500 to 700 nm, corresponds to a change in etalon spacing which can be as little as a small fraction of an IR order or as much as several orders. The movable plate of the IR etalon is positioned by an electromagnetic drive system with a mechanical range of about a millimeter. The instrument is planned to accommodate up to three IR etalons which can be scanned synchronously by tuning the output of a single control mono-chromator. A single-etalon version of the spectrometer has been operated at the Cassegrain focus of the KPNO 1.3m telescope.

We describe here the application of capacitance micrometry to the control of piezo scanned Fabry-Perot etalons using a closed loop feedback system. This system removes the problem of the non-linear response and hysteresis associated with piezo-electric transducers and allows precise control of both etalon gap and parallelism.

The servo-control techniques described in paper I have been utilised in the construction of a number of major astronomical instruments. We review three different etalon types which form the basis of a number of these instruments, and describe in some detail a stellar radial velocity spectrometer with a sensitivity of 10 ms -1, and a very high resolution Fabry-Perot (FP) module for use with the echelle spectrograph being constructed for the 4.2m Wm. Herschel telescope.

A number of Fabry-Perot coating designs have been developed specifically for infrared astronomy. These have been optimized for uniform reflectance over five contiguous spectral ranges of particular astronomical interest between 1 and 5 microns. Wavelength coverage in individual designs is given by (maximum ?/minimumλ) > 1.4, over which reflective finesse is maintained near 35. Consideration has been made for the peculiar requirements of Fabry-Perot coatings. Visual alignment and testing of the Fabry-Perot has been made convenient by bringing the reflective finesse of the coating at 0.6328 μ to the same value as the main infrared reflection band. For example, a calcium fluoride FabryPerot can be aligned and tested at the 0.6328 p Hehe laser line, with the assurance of a corresponding finesse and spectral resolving power over its infrared (eg. 3.6-5.1 μ) range. In addition, the FabryPerot can be used for infrared spectroscopy while being simultaneously monitored and/or actively controlled at the laser wavelength. The potential for the coating itself to degrade the effective surface flatness of the Fabry-Perot through either mechanical stress or thickness non-uniformities is minimized by reducing to a minimum the number and thickness of coating layers. Several of these coatings have been fabricated and are actively in use at a number of observatories.

The paper briefly describes a high resolution (Rz 50, 000) pressure scanned Fahry-Perot spectro-meter with photon counting detection. The spectrometer, which is designed for the study of emission line profiles in gaseous nebulae, has been used at f/13 Cassegrain focii of 1 metre telescopes in Kavalur and Nainital, India. The pressure variation is done in a step wise fashion; the pressure and photon counts are simultaneously recorded at each step on a paper printer and a digital cassette tape. The spectrometer along with its electronic data acquisition system is schematically given and some interesting results obtained in the velocity mapping of planetary nebulae NGC 3242 and NGC 6720 are presented.

A scanning Perot-Fabry interferometer, comparable with the english TAURUS system, has been developed by Marseille Observatory to work at the Cassegrain focus of the 3.6m CFH Te lescope, using a focal reducer and a photon counting detector. This system is named CIGALE from "Cinematique des Galaxies") and has been used successfully at CFHT in december 1982 and april 1983. Promising results have been obtained about the velocity field of galaxies, but thereare many others interesting applications with such a performing sytem,

A nickel-mesh Fabry- Perot interferometer for far infrared spectroscopic study of astronomical objects had been built and flight-tested. The interferometer was used in conjunction with a liquid helium-cooled far infrared photometer with four pass-band filters in the region 30-200 μm, which serve as prefilters. The photometer alongwith a Germanium bolometer detector was maintained at a temperature of 1.6 K by pumping down on the liquid helium, while the Fabry- Perot itself was at room temperature. In this setup the instru-ment had a spectral resolving power of 100-200 in the range 30-200 μm. Higher resolut ions may be obtained by using higher orders of the Fabry- Perot, with narrow-band prefilters. The main advantage of this instrument is that it can be used with any multiband photometer to cover a wide spectral range in the far infrared where numerous collisionally excited forbidden emission lines from atoms and ions exist. An attempt was made to measure the forbidden emission line of the singly-ionized carbon at 157 μ m in the extragalactic H II region, 30 Doradus, using this instrument at the Coude focus of a 32 cm telescope on board a Caravelle aircraft flying at 37000 ft. The details of the instrument and its flight performance are presented.

A common-user Cooled Grating Array Spectrometer has been built for use at the cassegrain focus of the 3.8m UK Infrared Telescope. The instrument which is designed to give reso-lutions (R = λ/tλ) of around 500 over the 1 - 5 micron spectral range uses a seven element Indium Antimonide array. The exit apertures of the system are defined by the array elements so that seven almost adjacent resolution elements are sampled simultaneously. A pair of gold coated replica gratings on copper substrates are mounted back to back to give high efficiency over the whole wavelength range. The spectrometer is mounted in a commercially available cryostat with the optics cooled to liquid nitrogen temperature and the array and associated electronics cooled to solid nitrogen temperature. A number of room temperature Fabry Perot etalons are available which can be mounted in front of the spectro-meter to increase the resolution up to 10 km s between 2 and 5 microns. The grating spectrometer and Fabry-Perot system have both been successfuly commissioned on the Telescope. Sensitivity and other test data will be presented along with a description of the spectrometer, array, electronics and Fabry-Perot system.

A fiber linked spectrograph is described which was built for use at astronomical telescopes with apertures of less than 100 cm^12,13. The link between the telescope and the spectrograph was made by means of two single step index glass-on-glass fibers with g core diameter of 6,3 μm. The transmission of a 10 m fiber link lies between 65 % at 4000 Å and 98 % at 8000 Å. Reciprocal dispersions between 27 Å/mm and 370 Å/mm can be realized. The best resolution at 27 Å/mm is 0.5 Å The spectrograph is equipped with an intensified TV ca-mera guiding system for remote operation.

Optical fibres are now regularly being used at the AAO for multiple object spectroscopy. The first prototype FOCAP system, built in late 1981, comprised a three metre bundle of 24 polymer-clad silica (PCS) fibres with core diameters of 200 microns. This linked an aperture plate at the auxiliary Cassegrain focus with the main intermediate dispersion spectrograph (known as the RGO spectrograph). Using the Image Photon Counting System as a detector, it was then possible to simultaneously record the spectra of up to 25 separate objects within the 12 arcmin field of the auxiliary focus. This system had a throughput of 40-50%, the major source of loss being focal ratio degradation (FRD). Intended as a trial, this first FOCAP bundle was used successfully in various observing programs on a total of 12 nights in 1982. Following on from the experience gained from this observing, new FOCAP bundles were developed using new all-silica (AS) type fibre which showed much less FRD. Bundles of 50 fibres were built, employing a faster f/6 collimator to collect more of the fibre output beam. With these improvements, the FOCAP system transmission was increased to 60-70%. A total of 15 nights were scheduled in the first three quarters of 1983 to use this system in a wide variety of multi-object spectroscopic observing programs. As an example of the substantial savings in telescope time that are possible with this system, during a recent run of three nights, spectra were obtained for a total of 400 objects with an average exposure of two hours. Other developments that are planned or in progress include a fibre feed to the low--dispersion faint object red spectrograph (FORS) and an aperture plate system to access the full 40 arcmin field available at the AAT's Cassegrain focus. This paper reports on this work and outlines possible future developments.

The versatility and importance of fibre optics in astronomy has received rapidly growing attention in the Last 5 years, particularly for spectroscopic applications in which the lightness and flexibility of fibres is preferred to the use of clumsy and expensive reflective or refractive optics. The majority of fibre systems developed at various observatories can be classed into one of two catagories, from each of which prototype systems have been developed and tested at ESO. It is proposed here to provide a detailed description of each of these instruments under the separate titles: I : "Coupling of the 3.6 m telescope to a remote spectrograph via a 40 m fibre link". I: "Multiple object spectroscopy at the 3.6 m telescope using OPTOPUS". I. COUPLING OF THE 3.6 M TELESCOPE TO A REMOTE SPECTROGRAPH VIA A 40 M FIBRE LINK

Optical fibres are already being used in routine applications for multi-object spectroscopy^l,2,3 and remote instrument feeds^4,5. Although astronomers using these systems have found obvious advantages in their use, there are conflicting reports on the transmission efficiencies and focal ratio preservation properties of the various fibres available. The whole issue is complicated by the large number of fibre manufacturers, the great variety of fibres produced by them and the frequent change in the availability of particular fibres as manufacturers "improve" or rationalize their range of products. The author has attempted to choose the fibres most suitable for astronomy and to measure the properties of those fibres relevant to their use with astronomical instruments.

Single silica fibers of the type developed for communications are useful in the design of astronomical spectrographs. They can be used to couple light from many objects over a wide field to a single spectrograph, or to eliminate wavelength errors from image motion and gravitational flexure. For many applications it will be advantageous to use small lenses to make the most efficient coupling into and out of a fiber. If the telescope pupil is imaged on the fiber core, an efficient coupling can be made that preserves image size over a certain range. This method is demonstrated by a laboratory test using sapphire spheres as coupling lenses.

This paper first looks at the general constraints imposed on a cryogenically cooled infrared spectrometer designed for astronomical work in the 1 μm - 5 μm region. Limitations imposed by telescope diameter, detector size and type of object are discussed, which can be different from those usually experienced with visible instruments. The large background fluxes present at infrared wavelengths can make the conventional Resolution-Luminosity product an inappropriate measure of performance. The need to cool such spectrometers is also reviewed. A working cooled grating spectrometer is described and we present a new design for a cooled array spectrometer, using a grating, capable of utilizing a 32 or 128 element linear InSb array. This instrument is being built at Imperial College and the IAC, Tenerife for use on the 2.5 m INT and 4.2 m telescopes at the N.H.O., La Palma.

Speckle interferometry was first introduced by Labeyrie (1970) in the visible as a method of restoring diffraction-limited information from large telescopes despite the presence of a turbulent atmosphere. It has been extensively used in the visible to resolve small sources and measure binary separations (Bonneau and Foy, 1980; McAllister and Hendry, 1982; Beddoes et al, 1976; Morgan et al, 1978).

The UCSD IR astronomy group has built an array grating spectrophotometer suitable for ground based 10 pm astronomical spectroscopy. The spectrophotometer employs a 128 element linear Si:Bi array with on-focal-plane analog multiplexers and discrete integrating capacitors. The system is designed to give thermal photon background limited noise performance and readout rates rapid enough to allow readout synchronous with standard IR chopping (roughly 30 Hz, corresponding to data rates of approximately 5 x 104 bits per second).

An improved 4 - 18 micron array camera system has been developed at NASA Goddard Space Flight Center for astronomical photometry, using an Aerojet ElectroSystems Co. 16 x 16 Si:Bi (Bismuth doped Silicon) accumulation mode charge injection device (AMCID) with 256 active pixels, obtained from NASA/Ames Research Center as part of a new scientific collaboration between the Ames and Goddard infrared array research groups. An astronomical observing program using this device has been carried out as a collaboration between NASA Goddard Space Flight Center (Infrared and Radio Astronomy Branch and Micro Electronics Branch) , the Harvard-Smithsonian Center for Astrophysics, and Steward Observatory of the University of Arizona, and NASA Ames Research Center. The 16 x 16 device had sufficiently good sensitivity, uniformity and noise characteristics to be used for successful observations at the Steward Observatory Mt. Lemmon 60 and 61-inch telescopes in May 1983, and at the NASA Infrared Telescope Facility (IRTF) at Mauna Kea in August 1983. Initial results indicate that this detector has sensitivity and noise characteristics comparable to other devices from the same generation of Aerojet arrays. Si:Bi AMCID detector array characteristics and performance have been discussed in general by Parry (1980) , McCreight and Goebel (1981) , and by Parry (1983). For a discussion of earlier array camera work at Goddard see Arens et al. (1981), Lamb et al. (1983) , and references therein.

A digital CCD camera system is described. The image sensor is a Fairchild CCD 222 incorporated in a C3000 camera. Data acquisition is done by an LSI11/23 microcomputer.

A description of the Galileo/Institute for Astronomy charge-coupled device (CCD) imaging system and its initial operation has been presented previously. Originally designed to operate a Texas Instruments (TI) 500 x 500 backside illuminated three-phase CCD, the system has been modified to allow use of either the 500 x 500 sensor or the TI 800 x 800 virtual phase CCD. We discuss the modifications for and the operation of the virtual phase CCD and current system performance with each type of sensor. We also describe our implementation of various techniques discovered at the Jet Propulsion Laboratory (JPL) that improve imager performance. These techniques include tri-level clocking of the virtual phase CCD to elim-inate spurious charge generation in the serial register, the use of ultraviolet light flood with the backside thinned 500 x 500 three-phase device to dramatically improve the quantum efficiency in the blue, and the practical elimination of deferred charge in the three-phase device. Results of astronomical observations with each sensor are presented.

The 2D-Frutti instrument is a spectroscopic detector used at the cassegrain spectrograph of the Palomar 5-meter telescope. A Fairchild 380 x 488 element CCD array is used to detect photon event scintillations in a high-gain image intensifier. The diodes are inspected serially at a 15 MHz rate. As little as 1/8 of the CCD vertical format can be inspected in a minimum frame time of 1 msec. The event centers are located to 0.25 diode horizontally and 0.5 diode vertically. Up to 600 FWHM resolution elements are produced in the horizontal direction. Coincidence corrections are no more than a few percent for count rates up to 105/ sec when the array is uniformly illuminated.

The CCD detector and data handling system now in regular use at Lick Observatory is described. At the Shane 3 meter telescope, a transmission grating ("grism") system has been installed on the automated cassegrain spectrograph. A valuable feature of this setup is the ability to go from spectroscopic to direct imaging mode by merely sliding the grism out of the light beam and opening the spectrograph slit jaws. Using a commercial gas expansion refrigerator, a very compact CCD (charge coupled device) cooling system has been developed. The dewar is a cylinder 15 cm in diameter, 6 cm high. The CCD temperature is held at -130°C. This cooler has been used at the telescope since December 1982. The data acquisition computer, an LSI 11/23 with 256 Kbyte RAM, 160 Mbyte Winchester disk, and color video display provides for FITS format magnetic tape storage as well as preliminary analysis of images and spectra. UNIX-based interrupt-driven software used on this computer allows data taking and data analysis tasks to be carried out concurrently. The 3 meter telescope spectrographic system uses a 500 x 500 pixel thinned Texas Instruments (TI) CCD. An English Electric (GEC) P8600 385 x 576 pixel CCD has been used for direct imaging observations at the Anna Nickel 1 meter telescope. An interactive, easy-to-use and well documented software system "VISTA" has been developed for image processing of CCD data with the VAX 11/780 computer. The language used is FORTRAN 77, running under the VAX VMS monitor. VISTA is also used on the LSI 11/23. Design information and operational experience for these detector systems is presented, together with some results to illustrate the quality of the data being obtained, and the present limitations of our CCD detectors and data system.

This paper presents some results of work carried out with the Cambridge, England, CCD System that demonstrate its abilities to work at the faintest levels yet achieved at a telescope. Future applications of this method are also discussed in relation to Next Generation Telescopes and larger CCDs. Recent results of our programme to extend our wavelength coverage into the near IR (1 - 5 microns) are also described.

The large field Lallemand electronographic camera is an ideal receptor for bidimensional photometry with large instruments. With the C.F.H. 3,6 m Telescope, the angular field is 10' at the Cassegrain F/8 focus ; in such a field, there is enough stars in the range 14 c V , 16 to start a decent electrographic sequence ; this sequence extends up to V = 21 and permits to measure stars up to V - 24 - 25.

Three types of electronographic cameras have been designed for different scientific objectives at the CFH Telescope. Two of them are already intensively used and the third one is being completed. The prime focus camera and the cassegrain focus camera have already reached the expected limiting performances such as image quality better than 0.8 arcsec on long exposures, limiting magnitude higher than 26 for accurate photometry, and slitless spectroscopy of quasars fainter than 23rd magnitude. The main purpose of the third camera will be as a detector for the "long slit spectrograph" mounted at the cassegrain focus.

The impact on ground-based IR astronomy of the new generation of infrared detector arrays which incorporate charge storage and multiplexed readout schemes on the focal plane array itself is considered. Plans for proposed instrumentation for UKIRT which will use such detector arrays are outlined. The new instruments will include a 1-5μm area mapping photometer and a low resolution 1-5pm cooled grating spectrometer. Preliminary work on the evaluation of certain types of arrays is described.

We have built a low noise spot scanner for use in testing the performance of infrared detector arrays for NASA's IR detector technology development program and the University of California's MICRO program. The scanner provides a convenient low noise detector test environment and a wide range of test conditions including versatile temperature control of the detector, ambient background, and blackbody source temperature and control of spot size, color, and brightness.

In 1981 Steward Observatory developed and put into use an intensified photon-counting Reticon detector system for the Cassegrain spectrograph on the observatory's 2.3 meter telescope. During initial design of the detector, particular emphasis was placed on selecting image intensifier components that would provide the highest possible quantum efficiency in the blue spectral region as well as provide a superior pulse-height distribution, good uniformity of response, high resolution from center to edge, and excellent geometrical stability. The Reticon camera electronics and discriminator electronics are the products of a cooperative project between Steward Observatory and the Smithsonian Astrophysical Observatory, and represent a newer technology version of the design pioneered by Shectman and Hiltner. The paper presents a description of the detector components, a summary of laboratory evaluation tests, and a summary of the detector's performance at the telescope. Of special significance is a discussion of the detector's true counting efficiency.

An intensified reticon scanner has been developed at Kitt Peak National Observatory for medium resolution astronomical spectroscopy. In this system, a dual reticon (2 x 936 pixels) is used to measure the light emitted at the output phosphor of an image intensifier. While the gain of the image tube system was sufficient to render the random readout noise of the reticon detector insignificant, the overall sensitivity of the instrument was initially limited by variations in the fixed-pattern noise associated with the reticon readout. The elimination of these variations required modifications in the reticon clock drivers and in the data handling techniques used. The stabilization of the fixed-pattern noise was accomplished by thermally controlling the environment surrounding the FET switches used to produce the clock signals that read out the array, and by filtering the switched output signals. The clock voltages were adjusted not to minimize the amplitude of the fixed-pattern noise, but rather to minimize the variation in the fixed-pattern noise observed, as small variations in the clock voltages were introduced. The resulting large fixed noise pattern is removed from the data by the subtraction of a bias scan - a readout of the array taken with no input signal. The bias scans and all data scans are 1024 elements in length. Each portion of the dual reticon is clocked as though it were this long. The overscan pixels are then used to measure exactly any small variations in the fixed-pattern noise which can then be subtracted from the data.

An IDARSS (Intensified Diode Array Rapid Scan Spectrometer) system has been developed for astronomical spectroscopic observations. The image detector is a Reticon of 1024 pixels in a linear array coupled with a microchannel intensifier through fiber optics, and is cooled down to about -40°C to reduce the thermal noise. The output from the detector is digitized and accumulated in a local memory, and is later sent to the data aquisition system including a mini-computer. The detector system was installed at the coude spectrograph of the 1.88 m reflector of the Okayama Astrophysical Observatory. Various kinds of observations are executed with this system. Observations of radial velocity variation of the Delta Scuti type variable 20 CVn are briefly reported.

We present an interactive program for reducing the data collected from spectrophotometers based especially on solid state detectors. A description of the program and of the characteristics of the principal available routines is done.

A 25mm. diameter, S25 photocathode, 10- pixel position sensitive photomatrix tube has been recently tested at the focus of the 2.12 m. telescope of the Observatorio Astronomico Nacional at San Pedro Martir in Baja, California. Direct imaging, high resolution spec-troscopy with a REOSC echelle spectrograph covering the 3200 - 6600Å range and high resolution spectroscopy with a scanning Fabry-Perot interferometer have been carried out on faint objects. Spectra of 13th to 16th magnitude quasars and galaxies have been observed with the MEPSICRON detector and satisfactory S/N ratios have been achieved in a few minutes exposure. Limiting total count rates, dark noise, linear spatial and time resolution were carefully measured in an astronomical setting. We present these results and briefly discuss their implication for fast, high resolution, large area imaging and spectroscopy of very faint astronomical sources.

We have previously reported on the astronomical performance of an uncooled, intensified, silicon vidicon for spectrophotometry-'2 and also on the performance of a dry ice cooled version of a similar detector for direct imaging application3 In this report, we discuss the performance of the cooled tube for spectrophotometric applications.

The Mosaicked Optical Self-scanned Array Imaging Camera (MOSAIC), is a 50 cm2 active area detector system encompassing 5.76 million picture elements. This camera is being developed for use in astronomical instrumentation requiring the use of large area imaging detectors with high resolution photon counting capability in the space ultraviolet. This paper gives a descriptive outline of the MOSAIC camera system including: the 100 mm diameter microchannel plate intensifier, the 3x3 array of 800x800 pixel charge coupled devices, the signal processing and buffer storage subsystem, the electronic support and control subsystem, and the data processing subsystem. Performance characteristics of the camera and its subsystems are presented, based on system analysis.

Some properties(linearity, dynamic range, detective quantum efficiency, behaviour at high spatial frequencies, flat-field) of our one dimensional photon counting detector system are discussed. Some spectra taken with the detector system are presented.

The operation and performance of a Digicon-based autoguider system recently developed will be discussed. The photosensor is a twelve-channel, photon-counting Digicon specifically developed by SAI for this application. The design and implementation of the system will be discussed with an emphasis on the objective of developing a general purpose auto-guider system for a wide range of applications. Data will be presented on the Digicon performance, i.e., quantum efficiency, background count rate, and pulse counting performance. In addition, overall system guiding accuracy, dynamic range, and sensitivity will be detailed.

The Infrared Astronomical Satellite (IRAS) was launched on January 25, 1983, and has provided astronomers with their first extended views of the universe in the 8 to 120 micron wavelength region. The infrared telescope system is one of several United States contribu-tions to this international project sponsored by the U.S.A., The Netherlands, and the U.K. This telescope employs four bands of detectors cooled to 2.4 K to achieve a noise equivalent flux density in the 10^-18 to 10^-19 watts/cm 2 range and will operate for approximately eleven months before depletion of its supply of superfluid helium. This paper describes major problems, early flight results and technical lessons learned during the course of the telescope's development and flight operations.

The Infrared Astronomical Satellite (IRAS) launched in January of 1983 utilizes a super-fluid helium cooled IR telescope and detector Focal Plane Array (FPA) to perform an all-sky survey in four wavelength bands from 8 to 118 micrometers. Temperatures at which different portions of the electronics must operate range from 2.5 to 280 K. The FPA consists of 62 infrared channels and 8 visible channels operating at 2.5 K. The IR detectors are grouped in eight 7 or 8 channel staggered linear suharrays with shared bias voltage; the visible detectors are grouped in two 4 channel skewed arrays, also with shared bias. Each IR channel detector is DC coupled to a TIA preamplifier through a very low power thermally isolated JFET source follower operating at approximately 65 K within the FPA housing. The TIA preamplifiers and bias supplies are located outside the telescope dewar and operate at approximately 280 K. The signal outputs from the preamplifiers are further processed by additional DC coupled gain amplifiers, filters and multiplexers prior to being digitized by a 16 bit analog to digital converter. The visible channel detectors are AC coupled to TIA preamplifiers and signal chain electronics using MOSFET source followers operating at approximately 2.5K within the FPA housing. This paper describes the detectors, preamplifiers and processing electronics, the system characterization test methods and results, and finally the performance of the detectors and electronics during the first month of on-orbit operation of the IRAS telescope.

The Infrared Astronomical Satellite (IRAS) operates in a circular sun-synchronous orbit at 900 km. This orbit carries it through the radiation zones of the South Atlantic Anomaly and the polar regions. Penetrating radiation, mainly protons and electrons, produce spurious detector pulses which can be large compared to the outputs generated by infrared sources. To preserve the infrared (IR) sensitivity of the main IRAS instrument, pulse circumvention circuitry discriminates between signals from IR sources and those due to penetrating charged particles. This paper describes the pulse circumvention concept used and its implementation in the IRAS survey instrument. An analysis is given of the operation of the circuit and of the optimization of its parameters for maximum IR sensitivity. Early flight data validates the operation of the system. In the South Atlantic Anomaly a typical reduction in pulse generated system noise of two orders of magnitude is being obtained with this circuit.

The Infrared Astronomical Satellite (IRAS) is a satellite which is mapping the celestial sphere for infrared (IR) sources. One of the critical electronic circuits in the instrument is the circumvention circuit which eliminates the unwanted charged particle pulses from the IR signal. The circumvention circuit was designed to allow IRAS to function throughout its orbit and into part of the South Atlantic Anomaly (SAA), through most of the north and south polar horns, and in the presence of cosmic x-rays. This paper describes the operation of the circumvention circuit along with preflight and in-orbit testing. Ground testing of the brassboard circuit, using a simulated preamplifier output, showed the circuit would perform the circumvention function as designed. The initial testing of the IRAS flight circumvention, circuit and remaining electronics was done using ten prototype detectors and preamplifiers. This testing showed that the system noise exceeded the 0.5 millivolt detection threshold set for the circumvention circuit. Increasing the threshold to 2 millivolts was the only change required to the flight circumvention circuit. When all the flight detectors and preamplifiers became available the circuit was tested using a gamma source to simulate charged particle sources. With the low energy deposited in the detectors (20 keV average) the noise was reduced by up to 5 times with the circumvention circuit turned on. In-orbit results show the circumvention circuit decreases the unwanted charged particle background noise up to two orders of magnitude. The difference in the results with the circumvention off and on are so great that the science team has recommended that no data be taken with the circumvention circuit off. Figures are presented showing in-flight results with and without the circumvention circuit.

The result of a design study on a Focal Plane Instrument for the 6.5 to 175 nm wavelength range is described. The instrument is meant to be used in combination with a grazing incidence telescope consisting of a sector out of a full revolution configuration mirror system, type Wolter II. The presented design is based on the principles of grazing incidence reflection and conical diffraction. The absolute efficiency of a grating in a conical mount is high. The combination of a plane grating with some mirrors for imaging therefore results in an arrangement with a satisfactory instrumental transmission. The outcome of efficiency calculations is presented. The spectral image can be made stigmatic over the entire wavelength range in a field of view with sufficiently large dimensions. Simultaneously, radiation from 240 spatial pixels of 1 arcsec x 1 arcsec is analyzed in four wavelength bands.With a change of the angle of incidence on the grating it is possible to choose these bands, with some restrictions, anywhere in the mentioned wavelength range. The spectral resolution varies from 0.003 nm to 0.0075 nm. For detection micro-channel arrays are chosen.

A balloon-borne 75 cm aperture Cassegrain telescope has been developed for far-infrared astronomical observations. This telescope uses a three axis stabilised platform which is designed to orient the optic axis with an accuracy of R',0.5 min of arc. A star sensor is used for the guidance; the axis of the star-tracker can be offset by angles upto ±4 deg. along two axes for pointing the telescope towards dark fields. The light weight mirrors (primary %40 Kgm) of the telescope are made in epoxy resin backed by aluminium blanks through a copying process. The secondary mirror is chopped at 20 Hz and the signals from the infrared detector are analysed by a phase sensitive detector (PSD). The gain of the PSD can be changed during the flight to provide an overall dynamic range of 1.5 x 105. The zero level of the PSD output and the relative phases of the chopper and PSD can be adjusted during the flight for optimum performance. The telescope was flown at an altitude of 32 km from Hyderabad on December 10, 1980 for photometric observations in 80um to 120um wave-length band. An aperture of 4 min of arc was used and a sensitivity of 100 Jy was expected. The flight performance of the telescope can be summarised by the following: (i) The r.m.s. pointing accuracy was r%)0.7 min of arc, (ii) stars upto mv%3.9 were used for guidance, (iii) the offset signal due to background infrared emission was less than 70000 Jy and (iv) the sensitivity deteriorated during the orientation mode to a value about 2000 Jy.

The Diffuse Infrared Background Experiment (DIRBE) is a cryogenically-cooled 10-band photometer with a large field-of-view (0.886 x 0.886 degrees2) which scans by rotation of the Cosmic Background Explorer about a spin axis. In-orbit calibration requires that the DIRBE detect and measure with precision the signatures of compact sources as they transit the field-of-view. Analysis of the conceptual optical design revealed that response of the 10 bands would vary significantly as a function of source position in the field-of-view, caused by anamorphic pupil distortion and field separation. The optical design reported in this paper is the result of changes which greatly improve the response uniformity and radiometric accuracy of the DIRBE.

An all-reflective wide-angle flat-field telescope (WAFFT) designed and built at Goddard Space Flight Center demonstrates the remarkedly improved wide-angle imaging capability which can be achieved with a design based on a recently announced class of unobscured 3-mirror optical systems. Astronomy and earth observation missions in space dictate the necessity or preference for wide-angle all-reflective systems which can provide UV through IR wavelength coverage and tolerate the space environment. Our initial prototype unit has been designed to meet imaging requirements suitable for monitoring the ultraviolet sky from space. The unobscured f/4, 36 mm e.f.l. system achieves a full 20° x 30° field of view with resolution over a flat focal surface that is well matched for use with advanced ultraviolet image array detectors. Aspects of the design and fabrication approach, which have especially important bearing on the system solution, are reviewed; and test results are compared with the analytic performance predictions. Other possible applications of the WAFFT class of imaging system are briefly discussed. The exceptional wide-angle, high quality resolution, and very wide spectral coverage of the WAFFT-type optical system could make it a very important tool for future space research.

In response to recommendations of the Astronomy Survey Committee of the National Academy of Sciences, NASA formed a science definition team in 1982 to establish science goals for an extreme ultraviolet (EUV) mission in the explorer class. Originally, it was intended that the primary goal of the FUSE mission would be to obtain high and moderate resolution spectroscopic data from approximately 912-1250A. However, a significant conclusion reached by the team was that it is possible to design and build instrumentation to cover the spectral region from approximately 100-1250Å, especially if a glancing incidence telescope optimized for the EUV is used. In this paper, we discuss the optical design and technology trades to be considered in the selection of a telescope for FUSE. A discussion of the advantages and disadvantages of both normal incidence and glancing incidence optical designs is given in terms of maximum glancing angle, collecting area, and physical length. Throughput limitations for normal incidence systems are explored in view of new optical coating technology, i.e., SiC coatings and layered synthetic microstructures (LSMs). Factors contributing to degradation of image quality in terms of rms blur circle radius and field curvature are discussed, and in particular a comparison between Wolter and Wolter-Schwarzschild Type II designs is presented. A number of selected telescopes satisfying the design constraints are compared. Although surface height differences between a Wolter and a Wolter-Schwarzschild may be small, either could be fabricated with the aid of computer controlled polishing presently being developed for cylindrical components.

We describe an all reflecting externally and internally occulting solar coronagraph which can be used for observations of the sun in the hydrogen Lyman a line. In the particular design considered and tested, a multiple occulting disk assembly is integrated with a Gregorian telescope in such a way that (i) scattered and diffracted light is minimized, and (ii) a large effective collecting area (16 cm2) as compared to the total telescope length (95 cm) is obtained. The telescope is suncentered pointed and has a large field of view (2.3 degrees diameter). Its image can be used to feed a multiple slit spectrograph, a camera using a Lyman a interference filter or other instrumentation which can make use of the availability of a full coronal image.

Large Space Borne Telescopes are limited in their Ultra Violet resolution by their large and medium scale (20-30 cm) surface irregularities (ripples). We present the principle, instrumentation and first results of a new interferometric method using a Michelson-Twyman configuration (diverging light beams) that allows to engrave on deep U.V. photoresists a phase compensating plate which permit to recover, partly or entirely, the diffraction limited resolution of the telescope. In our recording procedure there is no intermediary step between the "aberrated" mirror and its corrector : the aberrated mirror directly records its own corrector with all the interferometric quality available from a 257 nm U.V. coherent source. The corrector that we obtain is a phase compensating plate in reflection with highs and lows on its surface perfectly conjugated in negative, with the primary mirror surface irregularities.

The NASA Office of Space Sciences and Applications is sponsoring a series of space shuttle missions named Astro-n which are dedicated to a variety of astronomical projects. These missions will be the first in which the Space Shuttle crew will be interactively operating an astronomical observatory which is fixed in the payload bay. The operation of an instrument in such an environment requires a considerable amount of software both on board and on the ground. The Spacelab Experiment Computer provides generalized interactive telemetry display and command processing services through the Experiment Computer Operating System. The experimenter must then design a software package for his Dedicated Experiment Processor which can provide complete control of his instrument as well as communicate with the Experiment Computer. In this paper we describe the design considerations and operational features of the Ultraviolet Imaging Telescope Dedicated Experiment Processor and Instrument Ground Support Equipment software.

Many space astronomy instruments have used near-normal incidence reflecting mirror surfaces of aluminium protected by a thin coating of magnesium fluoride to give high reflectance at ultraviolet wavelengths. This type of coating has excellent performance for wavelengths longward of 1200 Å, but the extreme ultraviolet (EUV) reflectance at < 1200 Å is very low because the MgF2 overcoating becomes strongly absorbing in this wavelength region. This limitation, together with the related MgF2 detector window transmission limit, has resulted in a relative shortage of observational data in the astrophysically important spectral range 912 Å-1200 Å. While grazing incidence techniques can provide higher EUV reflectance values than normal incidence systems, they are less suitable for complex multi-element spectroscopic instruments. To obtain a high normal incidence reflectance in the spectral range Å< 1000 Å it will be necessary to use alternative reflecting surfaces without protective overcoatings. Possible mirror coating materials are discussed in this paper and a novel technique is proposed for producing high reflectance EUV mirrors, without absorbing overcoatings, for use in the space environment.

Sputtered multilayer coatings allow the use of normal incidence optics in the extreme ultraviolet (EUV) region below 500 R. Multilayer mirrors can be tailored to provide images at strong EUV lines in the sun and stars, in many cases making more efficient use of the telescope aperture than grazing incidence optics. Alternatively, the bandpass can be broadened at the expense of peak effective area, by varying the multilayer structure over the mirror surface. Such mirrors can also serve as optical elements in spectrographs for investigation of specific emission and absorption line complexes, and are self-filtering in that they reject nearby geocoronal and cosmic resonance line backgrounds. Current efforts at the Lockheed Palo Alto Research Laboratory in the design, fabrication, and testing of EUV multilayer mirrrors are discussed. This program includes the design and fabrication of normal incidence EUV multilayer mirrors, and the deposition of multilayers on lacquer-coated substrates.

An alternative to a monolithic space Michelson interferometer is a device composed of three separate spacecraft. Such a device consists of two telescopes which collect light from a source and transmit it to a third spacecraft positioned so that interference fringes are detectable in an onboard interferometer. The contrast of the fringes is then measured as a function of separation of the telescopes. A multiple spacecraft design allows extremely large interferometer baselines. The resulting high angular resolution permits fundamental astrophysical measurements different from those possible with foreseeable monolithic devices. We describe an orbital analysis and assessment of performance for a particular device design, SAMSI, Spacecraft Array for Michelson Spatial Interferometry. The device we consider includes two one-meter telescopes in orbits which are identical except for slightly differing inclinations. The telescopes achieve separations as large as 10 kilometers and relay starlight to a central station which has a one-meter optical delay line in one interferometer arm. Our four key findings are as follows: 1) a 1000 kilometer altitude, zero mean inclination orbit affords natural scanning of the 10 km baseline with departures from optical pathlength equality which are well within the corrective capacity of the optical delay line; 2) electric propulsion is completely adequate to provide the required spacecraft motions (principally those needed for repointing); 3) all necessary technology is already in a high state of development; and 4) resolution and magnitude limits of 10-5 arcsecond and my = 15 to 20 are achievable.

The Explosive Transient Camera (ETC) is a wide field (-3isteradns) electronic camera array which can detect coincident optical flashes with durations of ~10 to 10 seconds. It is anticipated (but not yet conclusively demonstrated) that simultaneous optical flashes will be associated with certain classes of gamma ray bursts (GRBs). For the ETC, each array element is a 20° x 30° FOV, cooled CCD detector, developed at MIT. An optical transient as faint as B = +11 (1 second duration) can be detected with S/N > 20, and its position determined to an accuracy of + 10 arc seconds. Thus, candidate events ~10 times fainter than the archived event (plate taken in 1928) reported for the 19 November 1978 gamma ray burst (GRB) by Schaefer (1981) should be detectable in real time. In addition to detecting GRBs, the ETC is expected to catalog large numbers of flare stars, as well as potentially new classes of astronomical transients. The coordinates established by the ETC will be immediately transmitted (--1 second delay) to the Rapidly Moving Telescope (RMT) under development at NASA/GSFC (Teegarden, Cline and von Rosenvinge 1982), which can further refine the position of a flash and follow its (presumed) subsequent decline. Communications to other rapid response radio or IR instruments will also be provided for, as well as time comparisons to gamma ray events detected by the International GRB Satellite Network (Hurley 1981). A prototype camera element was first tested in April 1982 to establish sky background levels and spurious event rejection schemes. A 1/2 steradian test version of the ETC is planned for operation in early 1984. Expansion to the full 3 steradian complement of 16 detectors at each of two sites is planned during the 1984-1985 period.

A large area (200 cm2), broad bandwidth (0.1-70 keV), imaging gas scintillation proportional counter (IGSPC) has been constructed for use in X-ray astronomy. The IGSPC consists of a high pressure xenon gas scintillation proportional counter (GSPC) coupled to a multi-wire proportional counter (MWPC) via a calcium fluoride window. The MWPC, filled with a mixture of argon, methane, and tetrakis (dimethylamino) ethylene, detects the UV photons emitted by the xenon gas in the GSPC. The detector has a measured energy resolution of 8.0% (FWHM) and 4.3% (FWHM) at 5.9 keV and 22.1 keV, respectively. The predicted spatial resolution of the detector is <1 mm (FWHM) between 3-22 keV and 37-60 keV. A method to determine the three-dimensional location of detected X-rays is described. In addition, we discuss a combination of discrimination schemes designed to reduce the non-X-ray background in the IGSPC by more than two orders of magnitude.

Any serious attempt to establish the value of low energy γ-ray astronomy as an observational science must ensure that new telescopes are capable of detecting a significant number of celestial sources. Furthermore, the instruments should combine the high sensitivity with a fine angular resolution imaging capability. Since it is impossible to focus γ-ray photons, the only way to manipulate them is to cast a shadow onto a suitable detector. A single pinhole, in an opaque material, above a position sensitive detector, will cast an image of the sky onto this plane. However, the small effective aperture of a pinhole leads to a very low sensitivity instrument and, since γ-ray telescopes are constrained to operate in a regime of low signal to noise, a larger aperture is required. The optimum solution under these conditions, is to construct the absorbing mask from an array of pinholes, such that the open fraction is one half. This is the principle of the coded aperture mask technique. In this paper we describe a telescope, for operation on a dedicated spacecraft, which has the capability of locating point sources of "Y -radiation with a precision of one or two arc minutes. The sensitivity of this instrument which operates over the range 10 keV - 3 MeV, is such that, for observations lasting one day, one will be able to detect sources having an intensity of lx10uph cm`keV1. With this sensitivity at least 1000 sources should be accessible during the two year observing period.

The performance of scaled down prototypes of the STARLAB detector is described. The performance criteria dealt with include resolution, linearity, final image quality, image stability, and rejection of unwanted events. The performance is such as to make the authors confident that the design goals for the STARLAB detector can be realised.

The design and basic performance characteristics of the Faint Object Spectrograph (FOS), one of five instruments built for use on the Space Telescope observatory, is summarized briefly. The results of the recently completed instrument-level calibration are presented with special emphasis on issues affecting plans for FOS astronomical observations. Examples include such fundamental characteristics as: limiting magnitudes (system sensitivity and noise figures), spectral coverage and resolution, scattered light properties, and instrumental polarization and modulation efficiencies. Also gated toward intended users, a rather detailed description of FOS operating modes is given. The discussion begins with the difficulties anticipated during target acquisition and their hoped-for resolution. Both the "normal" spectroscopic operating modes of the FOS and its "exotic" features (e.g. spectropolarimetric, time-tagged, and time-resolved modes) are presented. The paper concludes with an overview of the activities to assure proper alignment and operation of the FOS within the entire Space Telescope system (orbital and ground-based).

We report some of the laboratory measured performance parameters of the HRS. We then describe three aspects of performance of particular importance to astronomers: (1) the capability of detecting very weak features against a continuum, (2) the capability of producing reliable line profiles, and (3) the capability of assigning accurate wavelengths to spectral features. Specific technical descriptions of the performance of the HRS detectors (Eck and Beaver, this volume), gratings (Bottema et al., this volume), and control and data-handling system (Becker, this volume) are reported separately.

The High-Resolution Spectrograph (HRS) covers the 105 nm to 320 nm wavelength range at nominal spectral resolutions 103, 104 and 105. It employs one ruled, plane, first-order grating (600 grooves/mm), four holographic, plane, first-order gratings (3600, 4320, 4960 and 6000 grooves/mm), one r/2 echelle (316 grooves/mm) and two ruled, multi-partite, concave cross dispersers (88 and 198 grooves/mm). These gratings are all replicas. The absolute efficiencies of the first-order gratings were measured by the Goddard Space Flight Center (GSFC) before integration into the HRS. The spectral resolution was derived from line-width measurements in the Pt spectrum during pre-flight calibration of the complete instrument at the Ball Aerospace Systems Division (BASD). Relative scattered-light levels were determined from gaseous absorption spectra. In some of the holographic gratings spurious images were observed parallel to the spectra. In the echelle the effective blaze angle in the UV was derived from relative-efficiency measurements and found to be about 62.8°. The spectral resolution and the scattered-light background were measured by the same techniques as above. Ghosts were detected at 0.45 interorder distances. Their relative intensities are less than 0.1% at 150 nm.

The Flight Software for the High Resolution Spectrograph (HRS) for the Space Telescope (ST) has several diverse requirements. The Flight Software must be able to process observation descriptions and command the HRS through 12 to 24 hours of sustained autonomous operation, as well as being able to respond to near real-time interaction with astronomers on the ground. The HRS Flight Software target acquisition algorithms can begin execution after the ST is pointed at the target's nominal position. Using a plane mirror, or a low-order diffraction grating as a plane mirror, a star-presence check is conducted using a frame of data acquired through the HRS instrument. If the flux does not fall within user-prescribed limits, a search in an outward-growing spiral pattern is generated by requesting small-angle maneuvers of the ST. Star field maps of the successive fields-of-view may be generated and downlinked, and any target found can be precisely centered in either the large (200 micron square) aperture or the small (50 micron square) aperture. Once a target has been acquired, and the appropriate grating on the grating carrousel has been rotated into position, science data can be taken through either of two detectors. Incoming frames of science data, at rates up to 5 per second, are checked for data quality and co-added and stored on-board. The growing accumulation of data are subjected to exposure control, and the detector deflections which accompany each request for a new frame of science data are corrected for doppler shifts. When an observation is completed, all co-added on-board data are dumped to the ground or to an on-board tape recorder, before the Flight Software automatically begins the next observation.

One of the principal science instruments to be on board the Space Telescope for its first mission will be the Faint Object Spectrograph (FOS). True to its name, this instrument is designed to produce spectrographic measurements data from extremely faint astronomical objects. The photodetector utilized by FOS is a digicon offering a linear array of 512 photon-counting channels. This detector has been frequently described (ref 1,2), but most relevant here is the fact that a digicon is able to achieve photon-counting sensitivity by the utilization of a very high operating voltage, in this case 22,500 volts. No digicon operating at this voltage has yet been used on any long duration space missions. Previously described space applications have been of only short duration aboard sounding rockets (ref 3). The task of packaging such detectors for long duration space use is not to be taken lightly. Indeed, insulation failures at far lower voltages have cut short numerous missions in the early days of space flight, and even today one occasionally hears of a new example. In the present case, the inherent packaging problem posed by the 22.5 kV potential and the space environment is aggravated by the particular requirements of the FOS program and by the extreme sensitivity of the digicon and its attendent preamplifiers and counting circuits which will respond to charge impulses as small as about 5 x 10^-16 coulombs. In attacking this problem, one totally false start was made. The errors were exposed by environmental testing and a second design effort was begun. This time the effort began with the fundamentals. A conceptual design phase attempted to select materials which were compatible with each other and with all of the environmental requirements. In parallel a mechanical/structural design effort developed numerous concepts of the encapsulated detector assembly and how it would interface with the rest of the FOS assembly. This effort was supported by computer-aided electric field analysis and mapping which provided identification of trouble spots in the design. Further design iterations sought to alleviate high stresses. Following this conceptual design phase was the development phase which produced detailed engineering drawings and procedures, and validated same through building and testing various non-flight detectors. While the design proved to be satisfactory in terms of insulation, minute surface discharges in view of the photo-cathode were an unacceptable source of noise on the order of 0.01 counts per second per channel. This was cured by an application to the affected surfaces of Cr203 which served to prevent the build-up of static charges. The resulting dark count was about 0.001 count per second per channel at -100C. This value is to be expected for the FOS trialkali photocathode.

We describe a new technique called "Differential Speckle Interferometry" (DSI) which uses simultaneous narrow band images of astronomical objects to study their structure. Simultaneous specklegrams of red supergiant and giant stars taken in the hydrogen lines and in the nearby continuum allow us to reconstruct the image of the extended chromospheres of these stars at resolutions of 100 nanoradians and better. We describe the instrumentation, analysis techniques, and results related to DSI.

Speckle Interferometry has now been shown capable of yielding diffraction limited information on objects as faint as visual magnitude 16. Research in progress at Steward Observatiry is aimed at improving (a) the resolution, (by using the Multiple Mirror Telescope with its 6.9 meter baseline), (b) the accuracy of the derived results (by implementing better recording devices and reduction algorithms), and (c) the efficiency and speed with which the information can be provided (by means of high-speed digital signal-processing hardware). The instrumentation proposed here will improve patial resolution at visible wave-lengths to approximately 15 milliarc-seconds (75 x 10 ' radians, the best possible for any existing telescope), reduce detector induced image distortion to less than 1% and increase the throughput to essentially real-time complex Fourier transform amplitude and phase integrations at the telescope.

The specification and performance of the Imperial College hardwired real-time auto-correlator are described in detail. The application of the autocorrelator to the technique of stellar speckle interferometry is illustrated and results obtained are discussed. Emphasis is given to developments of the system prompted by our experience in observing faint and extended objects.

We describe in this paper the auxiliary optics developped for high angular observations in polarized light. These optics and their related data processing system - based on microprocessors - are implemented on the stellar interferometers at CERGA. Our observationnal program concerns the detection of Rayleigh scattering in circumstellar enveloppes around single and binary stars. We also plan to observe magnetic stars with the 1.5 meter aperture interferometer when it will be operationnal. We consider a simple geometrical model for a stellar magnetic field and show how the visibility functions of the star should change in transverse and longitudinal components of a Zeeman pattern. We discuss feasability and the requirements on sensitivity.

Reports from Mauna Kea observers and measurements of imagery, including the CFHT "trail plates" have strengthened the necessity for instrumentation which is capable of optimizing superb seeing conditions. Motivated by such experiences, a Very High Resolution (VHR) Camera has been constructed at CFHT. The principles of operation and preliminary results obtained will be discussed. This first experiment has pointed the way to achievable improvements in resolution. Plans for upgrading the device are now in progress.

The satellite HIPPARCOS of the European Space Agency aims to build a catalog of the astro-metric parameters of 100,000 stars. The satellite carries a special telescope which scans the sky ; it images the stars on a modulating grid and the angular distance between two stars is deduced from the dephasage between the two modulated signals observed after the grid. The study showed an effect induced by the wavelength dependency of the diffraction. This effect, which occurs even when the system is all-reflective (and so when there is not classical chromatism) has been called "CHROMATICITY". The theoretical reasons of this aberration are explained and numerical values of the HIPPARCOS telescope CHROMATICITY are computed ; the accuracy of the results is then discussed.

As part of the Roque de los Muchachos Observatory on La Palma in the Canary Islands, the Royal Greenwich Observatory is building the 4.2m William Herschel Telescope. It is scheduled for completion in 1986 when it will be commissioned with a complete suite of new instruments. The instruments will be equipped with modern electronic detectors developed from those currently being commissioned on the La Palma 2.5m Isaac Newton and lm telescopes. The number of instruments and detectors in use or available for use at any one time, together with the problems of handling large quantities of high-speed data, require an integrated system approach to be adopted. Particular features of this are the use of a large semiconductor memory external to the instrumentation computer, and the adoption of distributed processing and networking concepts. Emphasis is on achieving operational flexibility to match the anticipated high-quality optical performance of the telescope and excellent seeing conditions at the site.

This paper describes a new microprocessor system, to be known as the "4MS" (4.2m Microprocessor System), for instrument control which is primarily intended for the instrumentation being developed for the new observatory in La Palma, Canary Islands. The Royal Greenwich Observatory Electronics Department intend to supply this system to interested Observatory and University groups throughout the U.K. and Europe. The system to be described is an integrated hardware and software package utilising a simple and highly reliable technique for the generation of "target" software without the process of target compilation. Traditionally the transfer of software from a development system to a target system has been a process subject to errors where fault finding can be a very time consuming process, this step has been virtually eliminated. The hardware is based on a series of Eurocard modules compatible with the G-64 bus and the hardware forms both the development system and the target system. The software is based on the language polyFORTH and is supplied as a TURN-KEY system having a system console and disk drives. In this day and age, cost can be a significant factor in instrument design, this package eases this problem because the cost of a separate development system has been dramatically reduced.

The entrance slit width of a diffraction-grating spectrometer often limits the light-grasp or etendue of the instrument. The etendue may be increased by the use of a wide grating or mosaic, or one with a large blaze angle. However, those methods are not the only ones available. Further developments may use two or more gratings in arrangements other than a mosaic. These arrangements may use the different gratings in separate beams or consecutively in the same beam.

In the course of a systematic analysis of various design philosophies aimed at achieving high optical throughput low resolution spectrometers for telescopes in the range of 2-4 meters, two distinct optical designs were established. The first is optimised for a telescope prime focus having a fast F/ratio, and is based on a concave grating whose aberrations are corrected by 2 or 3 off-axis lenses. The second system is a focal reducer, utilizing lenses and transmission gratings only. The use of a particular glass type having an extremely low dispersion permits excellent correction of various aberrations over a wide spectral range. Both designs provide a two-dimensionally corrected field typical of present day CCD array formats. These designs are highly suitable for long slit or field spectroscopy and offer the possibility of rapid switching to direct imaging mode.

Two spectrographs were planned for the coude focus of the CFHT, the first having a F/7.4 camera and a large (76cm) plateholder, the second having a faster camera, F/3.7, and a smaller focal surface. The F/7.4 spectrograph began operation in 1981 and its optical design, including proposed additions, is described. It consists simply of a turreted hyperboloidal collimator mirror which illuminates a choice of 30x40cm gratings followed by a spherical camera mirror. When required, a small, high reflectance, diagonal mirror is added to direct light to a CCD or other electronic detectors. High transmission is possible because the spectrograph has few elements, some of which can have high reflectance coatings, and because it is designed for use with image slicers with which it is equipped. The image slicer used with the CCD has five slices and increases the width of the projected entrance aperture on the sky to that of the slit of a F/1.5 spectrograph, i.e.,(7.4)/5. (The gain of an image slicer comes from changing the shape of the entrance aperture whose area is the same as that of the portion of an ordinary slit illuminated by starlight by "trailing".)

An imaging spectrometer is a device for examining the 3 dimensional parameter space (x, y, X) which characterizes a major part of the information of interest about astrophysical sour-ces. There are many ways in which this 3D space may be examined, and the techniques used vary across the electromagnetic spectrum according to the wavelength, and hence the energy of a photon which dictates the telescope diffraction limit and the relevant detector and spectrometer technology. In the visible region of the spectrum the advent of high quantum efficiency, linear, panoramic detectors has led to the development of spectrometers coupled to these detectors, and consequently a large gain in the speed with which the parameter space may be explored. There are three basic types of spectrometer in general use in the visible, based on diffraction gratings, and Fabry-Perot and Michelson interferometers. The last two possess the well known Jacquinot advantage, allowing a much greater throughput of light at a given resolution than a grating of similar size. However, when we are interested in the spectral as well as the spatial information this advantage needs to be reassessed. The use of 2D detectors allows us to access 2 of the three dimensions simultaneously. In the first part of this paper we compare these three types of imaging spectrometer (see Fig. 1) in terms of their ability to obtain both spectral and spatial information, and discuss practical limitations. The second part describes the the new Imaging Fabry Perot being designed for the 4.2 m on La Palma. The final section deals with the conditions under which optimum use is made of the different devices.

The solar five minute oscillation was discovered by Leighton in 1960. Since the fundamental period of the Sun is expected to be about an hour, it was at first difficult to account for higher frequencies; but the cause and structure of these oscillations is now reasonably well understood. Recent results by Grec et al whomade a continuous six day observation of the Sun from the South Pole, show clearly that the oscillations are high 'Q', high-order modes of the whole Sun's oscillation. From these results and others it has been inferred that the structure of the Sun must be quite different from that expected. i.e. the convection zone must be thirty percent deeper and the interior temperature may be different giving a possible explanation of the observed solar neutrino flux. There have also been suggestions by Claverie et al2 that the interior of the Sun is rotating between two and nine times faster than the exterior. This could throw light on the Sun's magnetic field generating mechanism, and perhaps explain other solar periods e.g. the 22 year sunspot cycle. Ando and Osaki3 and Unno et al4 have shown that many stars should be subject to non-radial oscillations of small amplitude in a well-defined temporal frequency range, similar to the solar five minute oscillations. The periods would obviously depend strongly on the size of the star but, in any case, the same kind of discrete power spectrum as observed in integrated sunlight would be expected.

Optical and near infrared spectropolarimetry and imaging polarimetry has recently received considerable attention. A major contributing factor to this renewed interest has been the successful coupling of polarimeters to multi-element detector arrays enabling many spectral and spatial elements to be observed simultaneously. The latest techniques are briefly reviewed and two new polarimeter systems, developed at the Royal Observatory Edinburgh, are described in detail. One system employs a charge coupled device (CCD) and the other uses an image photon counting (Boksenberg) detector. The latter instrument is now in common use at the Anglo-Australian Telescope. Both instruments are computer-controlled and provide on-line data analysis and display. Their performance is illus-trated with recent astronomical data.

To eliminate image motion and blur (caused by "seeing" effects of the Earth's atmosphere) it is necessary to introduce an optical system which produces a real image of the pupil (the primary mirror) followed by a reimaged portion of the field. For the Canada-France-Hawaii telescope (CFHT) at the prime focus, these optics are required to magnify the field from F/3.8 (which is the primary focal ratio of the telescope) to F/17 so that the electronic detector can adequately sample stellar images of 0.25 arcsec diameter. The image motion is removed at the pupil. A simple but wasteful method is to locate a shutter at the pupil which opens only when a guide star is in the mid position of its motion. The guide starlight is taken from the centre of the field before the light reaches the pupil and feeds an autoguider. An alternative is to locate a flat mirror at the pupil which is rapidly deflected to remove the seeing motion. Tests conducted by staff of the CFHT Corporation produced photographs showing 0.25 arcsec resolution using the prime focus corrector followed by an off-the-shelf lens. However this lens produced poor images off the centre of the 4 arcmin field within which the image motions might follow those of the guide star. Custom designed optical systems are described which produce sharp images over a wide spectral region and also provide the option of doing without the prime focus corrector resulting in higher transmission, less stray light and better resolution than in the trial arrangement.

On nights when the seeing is at its best on Mauna Kea, the cores of images produced by the large optical telescopes (UH 2.24-m and CFHT 3.6-m) shrink to sizes < 0.5 arcsec. During these periods, the image cores show translational motion over a patch < 1 arcsec at frequencies < 30 Hz. To remove this translation motion, an Image Stabilizing Instrument System (ISIS) has been built and used on Mauna Kea. The instrument contains a microprocessor-controlled active plane mirror capable of being repositioned every 2 ms, a photon-counting guide probe, and an active shutter to remove moments of excessive image blur. We describe the instrument's design detail, discuss the optimal way to use such an instrument, and show samples of observational data taken with it.

This paper discusses the general problems of designing and calibrating sources of very weak light of known flux and spectral distribution, at levels necessary for measuring the performance of the various types of detector in current use in astronomy. After a brief introduction to ultra-weak-light photometry, the author describes a laboratory low-level light source built at the RGO for use as a primary standard for calibrating CCD's and PM-tubes etc. Alternative secondary standards, for use at observatories, are then discussed; these must be small, simple, portable and reliable, and should be suitable for permanent installation in telescope instruments, e.g. spectrographs and photometers etc. Finally, a few words are added on possible future developments.

A low dispersion spectrograph, employing a collimatorless optical design with a transmission grating and a CCD mounted within an evacuated camera, has recently been commissioned on the Anglo-Australian Telescope. It features high efficiency, through minimising the number of optical elements, minimising vignetting, and optimising the surface coatings for the designed spectral range.