Imaging capabilities of normal-incidence x-ray telescopes

Abstract

The enhanced reflectance achieved by recent developments in x-ray multilayer technology has made normal-incidence x-ray/EUV telescopes feasible for many applications of interest. Conventional optical designs with obvious advantages over the somewhat cumbersome grazing incidence designs of Kirkpatrick, Baez, and Wolter can thus be utilized. Preliminary results of actual flight data suggest great promise of scientific achievement from this new technology. It is widely recognized that supersmooth substrates are required since microroughness can decimate the reflectance of the multilayer. However, high x-ray reflectance is a necessary but not sufficient condition for producing high quality images. A second and equally important condition is the ability to concentrate the reflected radiation in a very small region in the focal plane. Optical substrates with satisfactory figure and finish for x-ray/EUV applications have been successfully demonstrated. However, small angle scatter from mid spatial frequency optical fabrication errors will limit the practical resolution attainable from this promising new technology. The surface power spectral density function over the entire range of relevant spatial frequencies is thus required to accurately predict image characteristics. The results of parametric optical performance predictions indicate that subarcsecond resolution is possible provided sufficiently smooth layer interfaces are maintained. However, optical fabrication tolerances imposed on the substrate may require advances over the current state of the art.