XMM mirror module close-up view

A close-up view of the XMM mirrors shows the small thickness of the individual shells, leaving open a large fraction of the telescope entrance window, which would be blocked by thick mirrors, as used on several previous missions. The thickness of the mirror shells varies from ca. 0.5 mm (inner shells) to 1 mm (outer shells). The distance between the shells, i.e. the free aperture for incoming radiation onto each shell, ranges from ca. 1.8 mm (inner) to 4 mm (outer).

The innermost part of the mirror modules do not contain any shells, because these would have small surface areas compared to the outer mirrors and thus would contribute only a tiny fraction to the total effective surface area. Instead, the central area is used to give the outer shells a stable fixture.

Photo courtesy of D. de Chambure, XMM Project (ESTEC)