The instrument SUMER (solar ultraviolet measurements of emitted radiation) is designed to investigate structures and associated dynamical processes occurring in the solar atmosphere from the chromosphere through the transition region to the inner corona, over a temperature range from 10⁴ to 2 multiplied by 10⁶ K and above. The observations will be performed, on board SOHO (solar and heliospheric observatory) scheduled for launch in November 1995, by a scanning, normal-incidence telescope/spectrometer system in the wavelength range from 500 to 1610 angstrom. Spatial resolution requirements compatible with the pointing stability of SOHO are less than 1000 km corresponding to about 1-arcsec angular resolution. Doppler observations of EUV line shifts and broadenings should permit solar plasma velocity measurements down to 1 km s^-1. We report here on some specific features of this instrument related to its pointing as well as its spatial and spectral resolution capabilities.

The coronal diagnostic spectrometer is designed to probe the solar atmosphere through the detection of spectral emission lines in the extreme ultraviolet wavelength range 15.0 - 80.0 nm. By observing the intensities of selected lines and line profiles, it is possible to derive temperature, density, flow, and abundance information for the plasmas in the solar atmosphere. Spatial resolution down to a few arcseconds and temporal resolution of seconds, allows such studies to be made within the fine-scale structure of the solar corona. Furthermore, coverage of a large wavelength band provides the capability for simultaneously observing the properties of plasma across the wide temperature ranges of the solar atmosphere. The CDS design makes use of a Wolter-Schwarzschild II telescope which simultaneously illuminates two spectrometer systems, one operating in normal incidence the other in grazing incidence. In this paper we describe the salient features of the design of the CDS instrument and discuss the performance characteristics of CDS as established through pre-delivery test and calibration activities.

Optical characteristics in the wavelength range 15 - 75 nm of the EUV imaging telescope to be launched soon on the SOHO mission are discussed. Bandpasses and photometric sensitivity of the multilayered optics telescope have been measured by a dedicated synchrotron light source at Orsay, France.

The SUMER (solar ultraviolet measurements of emitted radiation) instrument on the SOHO (solar and heliospheric observatory) satellite is sensitive to the state of polarization of the incident radiation primarily due to two optical elements, the scan mirror and the holographic grating. The angle of incidence of light striking the scan mirror varies from roughly 73.3 to 81.6 degrees (with respect to the mirror normal), which causes the mirror reflectance to be sensitive to the state of polarization of the incident radiation. Therefore, the measurement and characterization of this polarization sensitivity as a function of wavelength was performed using the engineering model optics (scan mirror and grating) and synchrotron radiation, which is nearly 100% linearly polarized, from the SUPERACO (Super Anneau de Collisions d'Orsay) positron storage ring in Orsay, France. The polarization sensitivity or modulation factor of the SUMER instrument was found to be between 0.4 to 0.6, depending on the wavelength and the angle of incidence of light striking the scan mirror, and agrees with the calculated polarization properties based on the measured optical constants for silicon carbide (SiC).

A new technique has been developed for improving the spectral resolution of spectrographs mounting discrete array detectors. The basic idea is to acquire various spectroscopic images each spectrally displaced on the detector by a fraction of the pixel and to apply suitable numerical procedures in order to extract a spectral profile with subpixel resolution. This technique has been applied to a vacuum spectrograph adopting the Johnson-Onaka configuration. The dispersion element is a concave toroidal grating which can rotate around a pivot axis displaced from its vertex, in order to keep always a good spectral focus on the detector. The latter is a multi-anode microchannel-plate array (MAMA), operating in photon counting mode. Several stigmatic images of the HI Ly(alpha) line at 1216 angstrom emitted by a D₂ lamp have been acquired for various rotation of the grating. The results of the application of this new technique and the numerical algorithm are presented and discussed in terms of potentialities and limitations due to signal to noise (S/N) ratio and intrinsic spectral broadening of the signals.

Until recently, spectrometer designs for use at extreme ultraviolet wavelengths have been limited to grazing incidence Rowland spectrometers and transmission gratings. In principle, the new class of variable line-space spectrometers offer substantial advantages over these classic designs. The Extreme Ultraviolet Explorer Satellite has three variable line-space spectrometers as part of its scientific complement; this is the first use of this class of device. The results obtained have been quite noteworthy. The in-orbit performance of these spectrometers are demonstrated with examples of the scientific results obtained in this mission.

A high resolution XUV imaging spectrometer is designed for Japanese solar mission Solar-B. A spherical varied line-space (SVLS) grating is chosen so as to obtain high spectral and spatial resolution within the spectral range of 25 to 29 nm. The spectral image focusing properties are compared with those with toroidal uniform line-space (TULS) grating design and the SVLS design is found to be superior to the TULS design in off-plane spectral images and also in insensitiveness to optical misalignment.

The Berkeley spectrometer aboard the ORFEUS payload achieved a variety of 'firsts' during its inaugural mission in September 1993. The instrument utilizes spherical gratings with mechanically ruled varied line-spacing, and curved microchannel plate detectors with delay- line anode readout systems, to cover the 390 - 1200 angstrom band at a resolution of lambda/5000. The instrument is discussed, and its performance illustrated with calibration and in-flight spectra. Science highlights from the ORFEUS-I mission are presented (oral presentation only). The payload is available for use by guest investigators during the ORFEUS-II mission currently scheduled for late 1996.

Multilayer interference structures (MIS) consisting of alternating diamond-like layers with different parameters were investigated. The structures with periods from 68 Angstrom up to 21 angstrom were obtained by ion-plasma methods. Properties of diamond-like MIS and diamond-like films were studied by x-ray diffraction measurements in the range 1.54 angstrom - 11.8 angstrom and ESCA. The numerical computations of reflecting ability of multilayer diamond-like structures were carried out. The difference between experimental and theoretical results are discussed. It is shown experimentally the feasibility of creating wideband mirrors for the soft x-ray range using diamond-like layers with different electron densities.

Spectrograph designs are presented that utilize a transmission sliced multilayer as a diffraction and focusing element. The multilayer utilizes the coincidence of Bragg and diffraction orders to achieve higher efficiency and resolution than a conventional grating. Traditionally, the lattice spacing in a crystal is used to achieve dispersion in the x-ray region. However, a multilayer as a diffraction element has some significant advantages. The layer thicknesses can be adjusted for any wavelength and for any angle of incidence. The transmission sliced multilayer is a dispersion element that satisfies the desirable properties of spectroscopy: high dispersion, high throughput, and high resolution. The sliced multilayer grating with a variable period can be used for both dispersion and imaging. Designs are studied for 17 keV and 34 keV with applications in mammography and IC chip inspection.

In the framework of past and future x-ray missions the SAX satellite, to be launched in March 1996, stands out for its very wide spectral coverage from 0.1 to 200 keV, with well balanced performances of the low energy and high energy instrumentation. The sensitivity of the scientific payload will allow the exploitation of the full band of SAX also for weak sources (1/20 of 3C273), opening new perspectives in the study of spectral shape and variability of several classes of objects. In this paper we briefly describe the main aspects of the mission, with particular regard to its scientific capabilities.

The low energy gas scintillation proportional counter (LE-GSPC) is an imaging x-ray spectrometer. It is one of the narrow field instruments on board the SAX satellite and covers the lower energy range from 0.1 to 10 keV. The low energy response of the detector is achieved by using a driftless gas cell and a thin multilayer polyimide foil as entrance window. The overall design of the imaging GSPC for space application is described. Using unit level and system level calibration data, acquired at a synchrotron and a long beam x-ray source, the capabilities both in terms of energy and position resolution are discussed. The overall efficiency of the instrument which includes the mirror's effective area, the entrance windows' transparencies, the detector's efficiency and electronics deadtime are reviewed. Background rejection issues and the experiment's consequent sensitivity to the measurement of cosmic x- ray source spectra are addressed.

The scientific instrumentation on board the x-ray astronomy satellite SAX includes a medium energy concentrator/spectrometer (MECS), operating in the energy range 1.3 - 10 keV, which consists of three identical instruments, each composed by a grazing incidence mirror unit with focal length of 1850 mm and by a position sensitive gas scintillation proportional counter. The MECS flight instruments have been calibrated at the X-ray PANTER facility of the Max Planck Institute and the preliminary results are presented in the paper.

The high pressure gas scintillation proportional counter (HPGSPC) is one of the narrow field instruments onboard the Italian Dutch mission for x-ray astronomy SAX. Sensitive in the 4 - 120 keV band the HPGSPC will investigate all categories of astrophysical sources emitting in the not yet well studied hard x-ray domain with a special emphasis on the cyclotron line features that are present in the hard x-ray spectrum of many celestial sources. The on-ground calibration of the flight model of the HPGSPC was carried out at Laben, Italy during October/November 1994. In this paper after describing the flight model of the HPGSPC instrument, we report preliminary results on its spectral capabilities and background rejection efficiency.

The phoswich detection system (PDS) is one of the four narrow field experiments on board the SAX satellite. The PDS will be dedicated to deep temporal and spectral studies of celestial x- ray sources in the 15 - 300 keV energy band. It also includes a gamma-ray burst monitor. The PDS detector is composed of 4 actively shielded NaI(Tl)/CsI(Na) phoswich scintillators with a total geometric area of 795 cm² and a field of view of 1.3 degrees (FWHM). In this paper we report results of the experiment on-ground calibrations before its integration in the spacecraft and the expected experiment in-flight performance.

The two wide field cameras that will be flown on SAX and which we describe here will be used to image the x-ray sky in the energy range from 1.8 to 30 keV. The field of view is 20 degrees at FWHM, the angular resolution is five arcminutes (FWHM), and the energy resolution is 20% at 6 keV, while the source location accuracy is better than one arcminute. The design is based on the coded mask principle where mask and detector both have equal sizes of about 25.6 multiplied by 25.6 cm² square. The detector is a multi wire proportional counter filled with 2.2 bar xenon gas with 5% carbon dioxide, the position resolution is better than 0.5 mm. The mask consists of a 0.1 mm thick stainless steel plate with a hole pattern. The pattern is based on a so called triadic residue set with elements of 1 mm² of which 33% are transparent for x rays. The sensitivity of the WFCs is a few mCrab in 10⁵ seconds. The principle of image reconstruction is presented and some calibration results. Details on the SAX mission and its instruments can be found elsewhere in these proceedings, as well as a detailed description of the detector.

A large area position sensitive x-ray detector system has been developed and built for the wide field cameras aboard the SAX satellite. This satellite will be launched early 1996. The detector is a sealed multiwire position sensitive proportional counter filled with 2 bar xenon gas. It has a sensitive area of 256 multiplied by 256 mm. The system covers the photon energy range of 1.8 to 30 keV. The position resolution is better than 0.5 mm for energies lower than 10 keV. The background rejection efficiency is better than 95%. The performance of the detector system is described. Pre-flight test and calibration results are presented.

The SOHO ultraviolet coronagraph spectrometer (UVCS/SOHO) is composed of three reflecting telescopes with external and internal occultation and a spectrometer assembly consisting of two toric grating spectrometers and a visible light polarimeter. The UVCS will perform ultraviolet spectroscopy and visible polarimetry to be combined with plasma diagnostic analysis techniques to provide detailed empirical descriptions of the extended solar corona from the coronal base to a heliographic height of 12 R. In this paper, the salient features of the design of the UVCS instrument are described. An overview of the UVCS test and calibration activities is presented. The results from the calibration activity have demonstrated that the UVCS can achieve all its primary scientific observational goals.

SAX is a scientific satellite devoted to x-ray astronomy whose launch is planned for the beginning of 1996, with a minimum operational life of two years. The characteristics of the x- ray detectors on board SAX have been defined in such a way as to have complete energy coverage of x-ray emission from cosmic sources (energy range 0.1 - 300 keV) with moderate angular resolution (1 arcmin). This scientific instrument package will allow spectroscopic and variability studies of the cosmic sources. The SAX scientific instrument package is made up of the following x-ray instruments: (1) An imaging medium energy concentrator spectrometer, energy range 1.3 - 10 keV; (2) An imaging low energy concentrator spectrometer, energy range 0.1 - 10 keV; (3) A high pressure gas scintillator proportional counter, energy range 3.5 - 120 keV; (4) A phoswich detector system, energy range 15 - 300 keV; (5) Two wide field cameras, energy range 2 - 30 keV. Specific design and development problems for these instruments are reported.

The optical performances of the spectrometer for the SOHO/UVCS instrument have been tested. The flight unit of the spectrometer assembly consisting of the structure equipped with the entrance slit assembly, the grating drive mechanism mounting a pair of toroidal grating, and two photon counting cross-delay line detector has been integrated and aligned. Both tests with visible and UV radiation have been performed. Aberration and stray light measurements have shown satisfactory performances of the instrument, practically in compliance with the scientific requirements.

Recent developments in far ultraviolet technology have advanced the state of the art in FUV instrumentation notably in the area of space flight experiments. In this paper, we report on a significant advance in far ultraviolet calibration technology which makes it possible to take full advantage of the improved instrumentation. A new far ultraviolet diffusive source has been developed which is capable of calibrating every pixel of an 8 degree field of view imaging camera to better than 10% absolute accuracy throughout the far ultraviolet wavelength region.

We measured the average anisotropy of the primary charge cloud produced by photoelectron when an x-ray beam linearly polarized is absorbed on a Ne-DME gas mixture by using a micro-gap proportional counter. This average anisotropy is not present when an Fe55 unpolarized x-ray source is used. We discuss the results of our measurement in terms of performances of this detector as an x-ray polarimeter.