Picture shows the solar array deployment test of the XMM spacecraft in ESA ESTEC.

The scientific objectives of this mission, for high-throughput X-ray spectroscopy, call for a powerful imaging instrument with the largest possible collecting area, for high-quality spectral measurements of faint sources down to 2x10 -15 erg/cm2/s, together with fast low -and medium- resolution spectroscopy of brighter objects.
An assembly of 51 mirrors, carefully sized, formed and nested one inside another, makes the most sensitive X-ray telescope ever built. XMM will carry three identical telescopes of this kind.

This astrophysics mission is a Cornerstone in ESA’s long-term Space Science Programme Horizon-2000