Efficiencies for diffraction of 45-275-eV x rays into orders by interferometrically formed, electrodeposited, gold transmission gratings have been measured on the 4 degrees beam line at the Stanford Synchrotron Radiation Project (SSRP). Anomalous dispersion affects the observed efficiency since the gold is partially transmitting to x rays. Model calculations which include anomalous dispersion are in good agreement with observations. With a suitable choice of material and thickness, a grating can be optimized for a given wavelength range by reducing the zero order transmission and enhancing the higher orders. Even orders are suppressed for a grating with equal slit and wire sizes.
Efficiency and resolution measurements are reported of an x-ray transmission grating to be flown on board the European X-ray Observatory Satellite (EXOSAT). By optimizing the thickness of the grating bars, very high efficiencies can be obtained over a limited energy range. An absolute efficiency of 20% at 7.1 A decreasing to 10% above 15 A has been achieved, including all effects of support structure and mounting necessary to withstand space qualification. This means that 10% of the incoming radiation at 7.1 A results in the minus first spectral order and 10% in the plus first order. The measured resolution at the short wavelength end of the range (lambda > 20 A) is determined by the mirror resolution; at the longer wavelengths (300 A), the effects of coma and astigmatism dominate. The measured resolution at 304 A for the EXOSAT-mirror-grating system is 4.5 A in agreement with theoretical predictions.
The proposed grating spectrometer for the Advanced X -Ray Astrophysics Facility (AXAF) covers the wavelength region between 2 and 140 A. The wavelength resolution AA = 0.05 A. The effective sensitive area as a function of wavelength is discussed. To illustrate the expected performance of the spectrometer some simulated spectra of a few interesting astrophysical objects are presented. IntroductionThe Transmission Grating Spectrometer described hereafter utilizes the high resolution mirror assembly as the objective, a transmission grating closely behind the mirror assembly and a high resolution detector, to read -out the dispersed image in the focal plane. Astrophysical requirements determine the desired wavelength range (up to 140 A) and resolution ( 100). The design of the grating has been optimized for efficiency and complementarity (low energy response) to the other spectrometers on board (see other AXAF instrument papers, this conference). The trade -offs involved in the optimization regarding dynamic range, spectral resolution and efficiency will be discussed. Scientific aimsThe very detection of X -ray sources has provided evidence for the existence of very hot (T > 107 K) astrophysical plasmas. Present -day X -ray astronomy has focussed on the identification and the variability studies of such sources and in particular X -ray spectroscopy is of major relevance to study the physical processes that release the energy and maintain the hot plasmas. With a resolving power R > 100 powerful temperature and density diagnostics become available. Coronal and shock-or photoionized, hot plasmas are observed in many sources both inside and outside the Galaxy. Measurements of emission -line intensities in such plasmas allow the determination of physical quantities such as: gas temperature, density, ionization state, elemental abundances and velocity. The detection of absorption features provides similar information in cases where bright compact X -ray sources are embedded in cooler, extended gas clouds, which may be ionized by the X -rays. Since the ultimate spectral resolution of transmission gratings can only be achieved for point sources, prime candidates for study in our Galaxy mainly comprise stellar coronae, white dwarf atmospheres, X -ray binaries and cataclysmic variables. Extragalactic spectroscopy will, for the first time come within reach, in particular in relation to the study of active galactic nuclei (AGN). The X -ray spectra of optically thin plasmas in the temperature range 105 -107 K, such as stellar coronae, reveal a wealth of narrow emission lines which provide in highresolution measurements excellent diagnostics of temperature, density and velocity, allowing accurate studies of structure, energy balance and heating rates. Typical 5a detection of the strong lines of the brightest of these sources can be achieved in a few minutes observation. The throughput of the AXAF telescope is sufficiently large to allow detailed spectroscopic studies of a number of relatively bright extragalactic X -ray sources wi...
A method of fabricating free-standing transmission gratings with line densities of the order of 1000 ~/mm is described. The technique involves a combination of two well-known procedures: application of photoresist and electroplating for the production of fine metal grids, and holographic (interferographic) manufacturing of dispersion gratings. Results of preliminary tests are mentioned.
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