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 satellit e. The most important feature of the AXAF instrument was the high-reso lution spectra (lambda/Delta lambda as high as 2000) that could be obt ained from astronomical objects over the lambda range 1.2 to 100 Angst rom. The crystals are formed into narrow cylindrical facets and assemb led into a pseudotoroidal surface. The spectral resolution is degraded by a number of effects, which we describe. The primary effect over wh ich we have control is the figure of each cylindrical segment. We meas ure deviations from the nominal cylindrical shape, which we call Delta theta(slope), using a laboratory x-ray spectrometer. We present spect rometer data for several crystal samples, evaluated at different energ ies, We compare these results with similar tests performed at optical wavelengths. Our results indicate that the Delta theta(slope) error ca n be controlled, and that curved crystals with the desired resolving p ower can be fabricated.