X-ray Telescopes and Elasticity Theory of Shells

Abstract

Catoptric systems for X-ray focusing cannot benefit from quasi-normal incidence mirrors, which have extremely low reflectance for this spectral range. They require ray deviation angles limited to a few degrees which avoid absorption by the reflective coating and hence are called grazing incidence systems. For high angular resolution, the mirror substrates are preferably in vitroceram or glass materials and the reflective coatings are either a single layer Ir, Au, or Pt, or alternate multilayers such as W+Si. Because of atmospheric absorption, X-ray telescopes must be space-based.

X-ray systems can be designed either as quasi-tubular mirrors extremely accurately aligned on a common axis, or as successive segmented sections of them, or as segments that are not arranged in a common symmetry plane.

In 1952, for the purpose of X-ray microscope objectives, Hans Wolter [29, 30] described three types of grazing incidence two-mirror systems known as Wolter two-mirror systems. All three types are stigmatic with a paraboloid primary mirror and a coaxial and confocal conicoid secondary. Type I is with a convergent primary and a convergent hyperboloid secondary. Type II is with a convergent primary and a divergent hyperboloid secondary. Type III is with a divergent primary and a convergent ellipsoid secondary (Fig. 10.1).

Compared to Types II and III, Wolter Type I is the only design where both mirrors provide a convergence. Given a maximum graze angle to minimize X-ray absorption, the Wolter Type I is the shortest of these long systems and therefore has been extensively utilized as a telescope in X-ray astronomy. We hereafter restrain to the description of this latter form.