XMM gathering X-ray radiation

XMM's three parallel X-ray telescopes simultaneously scan the same area of the sky for cosmic sources. In addition, one can see three more, smaller entrance windows. The one at the bottom right is the Optical Monitor, a 30 cm optical/UV telescope observing the same part of the sky as the X-ray telescopes, and at the top two redundant star trackers are mounted.

Photo courtesy of Dornier Satellitensysteme GmbH.