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Grating elements for the AXAF low-energy transmission grating spectrometer

We report on the status of the development of transmission grating elements for the AXAF Low Energy Transmission Grating (LETG). This instrument is supplied by a collaboration of the Space Research Organization in Utrecht/NL and the Max-Planck-Institute of Garching/Germany. We gave special attention of the optical quality of the gratings in order to match the high angular resolution of the AXAF telescope. The gratings have a period of 0.991 micrometers which allows the coverage of a wavelength bandwidth up to 140 angstroms. The variations of the period over the grating area and among different elements are less than 0.025% (RMS). The bar-width is half of the grating period (within tolerances of 50 nm) for the suppression of the even order intensities. The thickness (> 0.4 micrometers ) is adjusted in order to yield the proper phase-grating effect for X-rays at 1 keV which enhances the efficiency by a factor of up to two. The gratings are freestanding held only by a coarse support mesh obscuring 20% of the area. The ability of the gratings to withstand the acoustic noise loads during a launch of the Titan IV (142 dB OAL) has already been proven by test. Since several thousand elements have to be fabricated for AXAF totally, we have developed a new method for the characterization. This method implies the measurement of the transmittance of the grating for polarized light in its resonance domain. We show that all relevant grating parameters can be derived from this measurement. The grating facets will be assembled to 'modules', which, in turn are mounted onto the structure of the LETG. The mounting of the facets to the module require an extreme precision. A special alignment device was developed at MPI which allows the mounting of the elements with an accuracy (regarding rotation and tilt) of a few arcsec.