The X-ray Mirror Assembly.

At X-ray wavelengths metals have refractive indices of $n<1$ (Culhane & Sanford 1981, page 38). Light incident in a vacuum on a surface with a refractive index less than one will be totally internally reflected at some angle $\theta > \theta_{{\rm c}}$ where

$${\rm sin}\theta_{{\rm c}}=n$$ (8)

Thus X-rays can be totally internally reflected in a vacuum at a vacuum-metal interface, although $n$ is sufficiently close to 1 that reflection is only possible at grazing incidence. For metals of low atomic number the reflectivity is high but only for a narrow range of angles, for metals of high atomic number the reflectivity declines more slowly as the incident angle increases. For this reason X-ray mirrors are usually gold-plated, for which $\theta_{{\rm c}}\sim 89^{\circ}$ at X-ray energies of $\sim 1$keV.

The grazing angles associated with X-ray reflection allow mirrors to be nested one inside another to increase the collecting area. ROSAT employed four nested mirror shells with a maximum aperture of 83.5cm and a focal length of 240cm. The mirrors were of type Wolter-I which consists of a paraboloid section and a hyperboloid section, resulting in two reflections of each photon before detection at the focal plane. The mirrors were constructed of a glass ceramic coated in gold.

The effective area of the mirror assembly is described by a function of off-axis angle and energy of the incident photon. This is a result of the following. The reflectivity of gold decreases with increasing energy. Off-axis photons are reflected at a shallower angle and thus the projected area of the mirrors decrease. Additionally the reflectivity of the gold surface is reduced and also the photons suffer a higher degree of obstruction from the supporting struts which stabilise the mirror assembly. The result of this is an energy dependent decrease in effective area as a function of off-axis angle, known as vignetting. Further to this is a small degree of scattering from irregularities in the mirror surface, which is a function of energy.