As well as its three X-ray telescopes, XMM will incorporate one optical/UV telescope which points at the same region of the sky. This will enable XMM to record optical images of the X-ray sources it is studying, so that it can show the star or galaxy from which the X-rays are coming, or whereabouts within a galaxy they originate. This will be done with the Optical Monitor (OM) which is a 30 cm Richey-Chretien telescope encased within a cylindrical tube 2 m in length. The OM focuses light onto a detector via a mirror and filter wheel, which contains various filters and magnifiers. In order to achieve the great sensitivity needed, the OM incorporates a Micro Channel Plate image intensifier and CCD. The intensifier amplifies the signal by a factor of a 100,000 before it reaches the CCD. The OM will detect 24th magnitude stars and galaxies, comparable with what can be done with far bigger telescopes on the ground, due to the dark skies of space. To find out more about the OM, please click here and follow the links to instrumentation.