EVALUATION OF CURVED CRYSTALS FOR COSMIC X-RAY SPECTROSCOPY

We describe a spectrometer used to evaluate curved crystals, crystals originally intended for cosmic x-ray spectroscopy on the AXAF satellite. The most important feature of the AXAF instrument was the high-resolution spectra (lambda/Delta lambda as high as 2000) that could be obtained from astronomical objects over the lambda range 1.2 to 100 Angstrom. The crystals are formed into narrow cylindrical facets and assembled into a pseudotoroidal surface. The spectral resolution is degraded by a number of effects, which we describe. The primary effect over which we have control is the figure of each cylindrical segment. We measure deviations from the nominal cylindrical shape, which we call Delta theta(slope), using a laboratory x-ray spectrometer. We present spectrometer data for several crystal samples, evaluated at different energies, We compare these results with similar tests performed at optical wavelengths. Our results indicate that the Delta theta(slope) error can be controlled, and that curved crystals with the desired resolving power can be fabricated.