The Far Ultraviolet Spectroscopic Explorer (FUSE) is an astrophysics satellite currently being designed to provide high spectral resolving power ((lambda) /(Delta) (lambda) approximately equals 30,000) observations in the interval 853 - 1248 angstrom. It consists of four co-aligned normal incidence mirrors which illuminate high density, holographically ruled diffraction gratings on spherical substrates. High resolution spectra are formed on a pair of microchannel plate detectors with KBr photocathodes and delay line anodes. A separate low dispersion channel ((lambda) /(Delta) (lambda) on the order of magnitude 500) permits observations in the extended wavelength interval 800 - 1600 angstrom with long slit spatial imaging better than 5 arcseconds. The utility of observing in the prime FUSE spectral region is due to the vastly greater number of important transitions of atoms and ions in the ultraviolet compared to the visible. This makes FUSE a powerful instrument for measuring physical parameters such as temperature, pressure, density, and abundance in a wide variety of astrophysical sites. Among the many problems FUSE will address are the deuterium abundance in the universe; the character and distribution of the components of the hot interstellar medium such as O VI which must be understood to model galactic evolution; and physical processes in the atmospheres of planets and planetary systems.

The Far Ultraviolet Spectrograph Explorer (FUSE) is a delta class explorer mission that will perform 30,000 resolution spectroscopy from 900 - 1248 angstroms with unprecedented sensitivity. The design presented here derives from requirements necessary to meet the science objectives and technical and programmatic considerations. Meeting these various requirements has resulted in a design that is completely different from the Phase A concept, which proved to be infeasible within the FUSE constraints. We present the design philosophy, current design parameters for the instrument, and raytrace analyses of the instrument spectral and imaging performance.

The performance of the engineering prototype Stellar X-Ray Polarimeter (SXRP) has been evaluated. One hundred percent polarized monochromatic x rays at 2.6 keV and 9.7 keV were used to measure the response of the instrument in the energy bands of the graphite and lithium polarizing elements, respectively. On-line analysis showed that the respective depths of modulation are 96% ad 70% as expected. Irradiating SXRP with broadband unpolarized x rays in the energy band 2 - 17 keV demonstrated that the level of spurious modulation inherent in the instrument is less than 3%. Up-to-date results are presented and compared to the predictions of Monte Carlo simulations.

The Ultraviolet Coronagraph Spectrometer (UVCS) of the Solar and Heliospheric (SOHO) mission has been developed for spectroscopic and polarimetric determinations of temperatures, densities and flow velocities in the extended solar corona. The instrument consists of two ultraviolet (UV) spectrometers and a visible light (VL) polarimeter. The grating of the UV channel optimized for the HI Lyman (alpha) (Ly-(alpha) ) line ((lambda) 1236 angstrom) and that for the OVI doublet ((lambda) (lambda) 1032/1037 angstrom) have been characterized.

The use of x-ray polarimeters for the study of cosmic sources has been severely limited by the lack of launch opportunities. Thus far, the most significant x-ray-polarimetry experiment was performed by a device aboard the Orbiting Solar Observatory (OSO)-8 satellite in the 1970s. The next polarimetry experiment will be the Stellar X-Ray Polarimeter (SXRP), to be flown on the Russian Spectrum-X satellite in the next few years. Here we describe a simple experiment designed as a dedicated x-ray-polarimetry mission to operate in the 10 - 20 keV band and to complement scientifically the SXRP.

Multilayer coatings make it possible to create soft x-ray (0.25 keV) optics with large graze angles which are necessary for designing efficient polarimeters. If the detector is placed out of focus, then stellar images will take the shape of the front aperture, which would show intensity variations if the source is polarized. Two basic designs are considered. The first uses a single parabolic optic with graze angles from 25 degree(s) to 40 degree(s) and the second uses a Cassegrain telescope with angles centered at 45 degree(s). The former gives higher reflectivity at the expense of wide-field imaging, which may be sacrificed when the source region is not crowded, while the second telescope allows for off-axis use as a spectropolarimeter but has reduced throughput. A simple imaging proportional counter would be used as a detector and the overall sensitivity of the first design would be high enough to measure the polarization of the bright BL Lac object, PKS 2155-304, to an accuracy of 3% during a rocket flight. A small satellite would be capable of measuring several hundred polarizations with an uncertainty of 1% per year.

We obtained monochromatic emission line images with a prototype model of the Reflection Grating Spectrometer for XMM, at the MPE Panter long beam test facility in Munich. We concentrate on the interpretation and analysis of the distribution of dispersed light from single gratings. We present the outline of an exact first order scalar diffraction calculation of the effects of scattering on a grating on the angular profile of the dispersed radiation. Using the resulting predicted scattering profile, we extract the core of the measured profiles for individual gratings, and find good agreement between the shape of these cores and the shape predicted for the long-spatial wavelength slope distribution on the gratings, obtained from interferometry. The widths of the cores meet the specifications for the flatness of the grating substrates.

A prototype array consisting of eight diffraction gratings has been fabricated for the XMM Reflection Grating Spectrometer. A component of the full spectrometer is an array of approximately 200 diffraction gratings. The diffraction gratings were produced using lightweight silicon carbide substrates and a replication technique. The prototype array was developed and assembled using the same tolerances as the flight arrays which have typical tolerances of 3 micrometers in translation and sub-arc seconds in rotation. The metrology applied during inspection and assembly included precision linear measurements, full aperture figure measurements, and angular interferometry.

Hard x-ray (> 10 keV) concentrators are of key importance to overcome the sensitivity limitations of the current generation of (imaging and non imaging) direct viewing telescopes for hard x-ray astronomy. We studied the feasibility of a hard (> 10 keV) x-ray concentrator based on the Bragg diffraction technique. Apart from its unprecedented sensitivity in the band from 10 to approximately 100 keV, this concentrator has an intrinsic capability to measure the polarization of x rays in the low energy part of its operative range. In this report we discuss this property and the expected sensitivity of the instrument as a polarimeter.

Supermirrors are multilayer structures where the thickness of the layers down through the structure changes so that wide-band reflection occurs. The principles were developed in the mid-70s and have been used extensively for neutron optics. Absorption in the upper layers limits the attainable reflectivity for x rays. For hard x rays (>= 15 keV), the absorption, however, is low enough that it is possible to design supermirrors with 10 - 70% reflectivity in a band approximately equals 3 times the width of the total reflection regime. Supermirrors of W/Si and Ni/C have been successfully fabricated and characterized. The measured x-ray reflectivities are well accounted for by the standard dynamical theories of multilayer reflection. Hard x ray applications that could benefit from x-ray supermirror coatings include focusing and imaging instrumentation for astrophysics, collimating and focusing devices for synchrotron radiation, and particle filtering in plasma diagnostics.

Good theoretical designs of far ultraviolet polarizers have been reported using a MgF₂/Al/MgF₂ three layer structure on a thick Al layer as a substrate. The thicknesses were determined to induce transmission and absorption of p-polarized light. In these designs Al optical constants were used from films produced in ultrahigh vacuum (UHV:10^-10 torr). Reflectance values for polarizers fabricated in a conventional high vacuum (p approximately 10^-6 torr) using the UHV design parameters differed dramatically from the design predictions. Al is a highly reactive material and is oxidized even in a high vacuum chamber. In order to solve the problem other metals have been studied. It is found that a larger reflectance difference is closely related to higher amplitude and larger phase difference of Fresnel reflection coefficients between two polarizations at the boundary of MgF₂/metal. It is also found that for one material a larger angle of incidence from the surface normal brings larger amplitude and phase difference. Be and Mo are found good materials to replace Al. Polarizers designed for 121.6 nm with Be at 60 degree(s) and with Mo at 70 degree(s) are shown as examples.

The observations of the emission from laser-produced plasmas of carbon, aluminum and tungsten have been performed by means of a vacuum normal incidence stigmatic spectrograph. The detector used is a Peltier-cooled CCD, optimized for the vacuum ultraviolet spectral range. The results presented include the spectrum of the plasmas, in the range from 380 to 800 angstrom, the spectral and spatial profiles along the expanding plasmas and the absolute measurements of the plasma brightness. The measurements show an outstanding capability of this type of detector, in terms of sensitivity, resolution and dynamic range, with respect to traditional devices such as photographic film, intensified linear arrays, and CCD.

The efficiency of beveled edge type avalanche photodiodes has been determined for soft x rays in the range 50 eV to 300 eV. An efficiency of over 80% is measured for energies below the Si L absorption edge at 100 eV. The measured efficiency is described by a model which accounts for absorption in an oxide overlayer and recombination at the front surface of the diode. These results are very encouraging for soft x-ray/EUV applications involving both laser-produced plasma sources and synchrotron radiation.

X-ray spectra at 6 keV from niobium based superconducting tunnel junctions with highly transmissive tunnel barriers are presented. Signals from the two films can clearly by discriminated by their different temporal and pulse height characteristics. Levels of tunneled charge as high as 2.7 X 10⁶ electrons at 5.9 keV are observed. The best energy resolution obtained at T equals 1.2 K is 53 eV FWHM including electronic noise, for a 50 X 50 micrometers ² device in a configuration where the x-ray source is collimated to selectively illuminate the center part of the device. Non-linearity is observed which appears dependent on film volume, implying that self recombination may play an important role in these devices. The energy resolution is found to degrade with increasing magnetic field. The spectra from the polycrystalline top film appear significantly degraded if magnetic flux is deliberately trapped in the device.

A method of designing spectrometers having stigmatic image focusing properties by using spherical varied line-space (SVLS) grating is developed and is compared with toroidal uniform line-space (TULS) grating design. Helium emission sensors using concave grating spectrometers are designed to detect 30.4 nm and 58.4 nm EUV emissions. The SVLS design is found to be superior to the TULS design not only theoretically in stigmatic image focusing properties, but also practically in the ease of fabrication.

Low energy monochromatic x rays can be ]used to detect the attenuation differences between fibrous, fat, and cancerous tissues in mammography, evaluation of computer processor chips, as well as quality control for micro components. At 17 keV, carcinomas, fibrous and fatty tissues attenuate incident radiation to different degrees which permits non invasive detection of cancerous tissue. A sliced transmission multilayer grating is presented that produces monochromatic x-ray radiation at 17 keV. The sliced multilayer utilizes the coincidence of Bragg and diffraction orders to achieve higher efficiency and resolution than a conventional grating. A classical multilayer consisting of low and high atomic number materials along with a monomaterial multilayer in which the density of the material is varied to simulate two different layers are presented.

It's well known that just synchrotron radiation (SR) polarization characteristics determine the accuracy of the spectroradiometric scale based on SR sources. It's very effective way to use a SR source in "big bunch" mode to diminish the SR polarization characteristic influence on the total errors of secondary standards calibrated against the primary standard based on SR sources. The other way is connected with special SR source mode when SR ~-polarization component has the "plane angular distribution at the medium plane.

Chemical elements are characterized by the nature of the interaction between their electrons and incoming photons. The uniqueness of a spectral signature of each chemical element is used in spectroscopy for element detection and characterization. We report the design of the detection and chemical composition analysis system, TOMOSCOP, which is based on tomographic spectroscopic imaging at x-ray energies within the 15 - 30 keV range. The detection of the presence of an element or a group of elements and measurements of their concentration in a sample are based on calibration attenuation measurements conducted on known samples. The TOMOSCOP system utilizes x-ray filter spectroscopy for positive identification and concentration measurement of an element within a sample. Tomography combined with spectroscopy provides the unique detection capability of the TOMOSCOP system in which elements and compounds (substances) present within the test volume are localized and detected.

We have designed and constructed a compact spectrometer dedicated to in-situ characterization of thin films during deposition, using soft x-ray emission spectroscopy. It consists of a Rowland-circle mounted spherical grating and entrance slit, or slit array to enhance throughput. A 2D position-sensitive detector (microchannel plate stack and resistive anode) is mounted tangent to the image of the slit(s) on the Rowland circle. The instrument covers an energy range of 240 - 700 eV using a 300 1/mm grating in the first order. Thus, the spectrometer simultaneously records K emission for low-Z elements C through F, while L emission for 3D metals can be recorded in first or higher orders. The resolution is approximately 300, allowing chemical analysis. Both detector and grating are housed in a vacuum chamber that is turbomolecularly pumped to a pressure below 10^-6 Torr. The instrument can be attached to any process chamber using a standard UHV flange. The slit extends into the process chamber separated from the housing by a valve. This valve can be closed, or in one of two open positions where thin foils serve as vacuum windows to protect the detector and grating, and as filters to reduce background counts from UV light. The spectrometer has successfully monitored a variety of processes in situ, including growth of optical TiN films by reactive magnetron sputter deposition, synchrotron radiation induced CVD of metallic films, and hot-filament CVD growth of diamond.

Two x-ray spectroheliographs have been designed for obtaining solar images in about 8.42 angstrom and in 1.86 angstrom spectral regions with high spatial and spectral resolution. In both devices unique optical elements-focusing Bragg crystal mirrors are used. Technologies of their production are described here. These mirrors were tested in the x-ray region for their iamging and dispersive properties. These calibration results are presented.

We present a new statistical technique for evaluating the error budget of an optical system. Each parameter evaluated in the error budget is assigned a standard deviation. Randomly generated, Gaussian distributed offsets are then generated for each parameter. The optical system is raytraced a large number of times using the offsets to simulate alignment errors. In this way it is straightforward to simultaneously evaluate the degradation in the optical performance introduced by random misalignments and fabrication errors. The error budget for a system is then defined in terms of the standard deviations assigned to each parameter under consideration.

Several recent experiments were conducted to study the x-ray induced photoemission from solids by polarized x rays for its potential application to stellar x-ray polarimetry. However, it was shown recently that the earlier observed polarization dependence of the secondary photoelectron yield was affected by spurious effects and no polarization dependence could be observed after these spurious effects had been eliminated. Here, we report our first measurement of the primary photoelectrons, i.e., photoelectrons with energy higher than 50 eV. We measured the primary photoelectron yield of a silicon sample as a function of the polarization state of the incident 2.69 keV x-ray beam. We observed that the change of photoelectron yield for different polarization states is especially pronounced around the no-loss photoelectron peaks. This polarization dependency can be explained by the angular distribution of photoelectrons from free atoms excited by polarized x rays. We discuss the experimental results and their implications.

A spectrometric and spectropolarimetric visible light Lyot coronagraph are scheduled for launch in 1995 on the ESA/NASA Solar Heliospheric Observatory (SOHO) mission. The Lyot coronagraph is one of three coronagraph optical trains contained in the NASA sponsored large angle spectrometric coronagraph (LASCO) which will be used to study the emission line, electron, and dust components of the solar corona within a 30 solar diameter field of view. This talk focuses on the spectrometric and polarimetric capabilities of the Lyot optical train which covers the inner solar corona from 1.1 - 3.0 solar radii.

UVSTAR is an EUV spectral imager intended as a facility instrument devoted to solar system and astronomy studies. It covers the wavelength range of 500 to 1250 angstrom, with sufficient spectral resolution to separate emission lines and to form spectrally resolved images of extended plasma sources. Targets include the Io plasma torus at Jupiter, hot stars, planetary nebulae and bright galaxies. UVSTAR consists of a pair of telescopes and concave grating spectrographs that cover the overlapping spectral ranges of 500 - 900 and 850 - 1250 angstrom. The telescopes use two 30 cm diameter off-axis paraboloids having a focal length of 1.5 m. An image of the target is formed at the entrance slits of the two concave grating spectrographs. The gratings provide dispersion and re-image the slits at the detectors, intensified CCDs. The readout format of the detectors can be chosen by computer, and three slit widths are selectable to adapt the instrument to specific tasks. UVSTAR has internal gimbals which allow rotation of +/- 3 degree(s) about each of two axes. Dedicated finding and tracking telescopes will acquire and track the target after rough pointing is achieved by orienting the orbiter. Responsibilities for implementation and utilization of UVSTAR are shared by groups in Italy and the U.S. The first of the five approved UVSTAR flights is scheduled for May 1995.

The solar EUV spectrometer SUMER will operate in the wavelength range 500 angstrom to 1600 angstrom on the SOHO spacecraft. It will allow measurements of profiles, Doppler shifts, and intensities of solar extreme ultraviolet lines. Various mechanisms for pointing and focusing of the optical components contribute to the versatility of the instrument. The normal incidence optical design of the telescope and the spectrometer puts very stringent constraints on the cleanliness of the instrument and the mechanical design. The calibration techniques used, including imaging tests in the EUV, wavelength, and radiometric calibrations are outlined, and performance predictions are given.

The Far Ultraviolet Spectroscopic Explorer (FUSE) is an astrophysics satellite currently being designed to provide high spectral resolving power ((lambda) /(Delta) (lambda) approximately equals 30,000) observations in the interval 853 - 1248 angstrom, and moderate resolving power ((lambda) /(Delta) (lambda) > 500) over the extended interval 800 - 1600 angstrom. It consists of four co-aligned normal incidence mirrors which illuminate separate Rowland circle spectrograph channels with holographic gratings and delay line microchannel plate detectors. Two telescope mirrors are made of chemical vapor deposited SiC on a reaction bonded SiC substrate, and the remaining two have an Al+LiF coating on Zerodur substrates. The off-axis parabolic mirrors have stringent reflectivity, imaging, lightweighting, and mounting requirements. Important aspects of the optical and mechanical design are discussed, including the surface accuracy requirements on different spatial scales, from microroughness to full figure errors. Also discussed are the mirror lightweighting requirements.

We designed a new type of polarimeter for polarized hard x rays from stellar objects, utilizing the principle that the direction of the Compton scattered x rays depends on the electric vector of the polarized incident x rays. The characteristics of the new designed polarimeter were investigated by measurements of basic data and computer simulations. From them, it is shown that the polarimeter is capable of observing the interesting astrophysical objects by using a balloon or a satellite. In this paper, we report the characteristics and the expected performance of the new designed hard x-ray polarimeter.

The linear polarization of x rays can be measured by the modulation of the azimuth distribution of scattered photons as derived from the coincident scintillations of a pair of wire- like detectors in a tight package of many. We computed the response of such an experiment by simulations with the MCNP code. We discuss the improvement of the sensitivity, in particular at lower energies, by the use of a combination of fibers of two different scintillators. Possibilities offered by the new scintillating material YAP are also discussed.

In six companion papers we discuss a capability for x-ray tomographic spectrophotometry at three energy ranges to observe foreign objects in various systems using a novel x-ray optical and photometric approach. We describe new types of thin-film x-ray reflecting filters to provide energy-specific optical trains, inserted into existing x-ray interrogation systems. That is complemented by performing topographic imaging at a few, to several, energies in each case. That provides a full topographic and spectrophotometric analysis. Foreign objects can then be detected, localized, discriminated, and classified, so that they may be dealt with by excision, and replacement with benign system elements. We analyze statistical and operational concerns leading to the design of three systems: The first operates at x-ray energies of 1 - 10 keV; it deals with defects in microelectronic integrated circuits. The second operates at x-ray energies of 10 - 30 keV; it deals with the defects in human tissue. The chemical specificity and image resolution of the system will allow identification, localization, and mensuration of tumors without the need of biopsy. The system which we concentrate this discussion on, the third, operates at x- ray energies of 30 - 70 keV; it deals with the presence in transportation systems of explosive devices, and contraband materials and objects in luggage and cargo. We present the analysis of the statistical features of the detection problem in these types of systems, discussing the operational constraints which limits system performance. After considering the multivariate, multisignature, approach to the problem, we discuss the tomographic and spectrophotometric approach to the problem which yields a better solution to the detection problem within the operational constraints.

The diffraction efficiency of holographic gratings depends on the degree and orientation of the linear polarization of the incident light. A test has been run to measure the reflectance efficiency at Kr 1236 angstrom of two holographic gratings in the orders +1, 0, $min1, as a function of the angle of the incident beam polarization plane. To this purpose, a triple-reflection polarizer, optimized for Ly(alpha) (1216 angstrom) has been fabricated and characterized. The test has been run to demonstrate that the coronal resonantly scattered Ly(alpha) can be measured using a holographic grating in the Ly(alpha) channel of the ultraviolet coronagraph spectrograph (UVCS) of the Solar and Heliospheric Observatory (SOHO) without introducing relevant polarization biases inside the instrument.