Wolter telescope

Conventional telescope designs require reflection or refraction in a manner that does not work well for X-rays. Visible light optical systems use either lenses or mirrors aligned for nearly normal incidence – that is, the light waves travel nearly perpendicular to the reflecting or refracting surface. Conventional mirror telescopes work poorly with X-rays, since X-rays that strike mirror surfaces nearly perpendicularly are either transmitted or absorbed – not reflected. Lenses for visible light are made of transparent materials with an index of refraction substantially different from 1, but all known X-ray-transparent materials have index of refraction essentially the same as 1, so a long series of X-ray lenses, known as compound refractive lenses, are required in order to achieve focusing without significant attenuation. ==X-ray mirror telescope design==

X-ray mirrors can be built, but only if the angle from the plane of reflection is very low (typically 10 arc-minutes to 2 degrees). These are called "glancing" (or "grazing") incidence mirrors. In 1952, Hans Wolter outlined three ways a telescope could be built using only this kind of mirror.{{cite journal |title=A generalized Schwarzschild mirror system for use at glancing incidence for X-ray imaging |last=Wolter |first=Hans |author-link=Hans Wolter |journal=Annalen der Physik |volume=10 |page=286 |year=1952 |ref=Wolter, Generalized Schwarzschild Mirror System, 1952 |doi=10.1002/andp.19524450410 Wolter's key innovation was that by using two mirrors it is possible to create a telescope with a usably wide field of view. In contrast, a grazing incidence telescope with just one parabolic mirror could focus X-rays, but only very close to the centre of the field of view. The rest of the image would suffer from extreme coma. ==See also==