The Optical/Ultraviolet Monitor (OM) is what most people would call a "normal" telescope. It has a 30-centimeter-wide mirror designed to focus optical and ultraviolet light coming from the same direction as the X-rays observed by the other instruments on XMM-Newton. This allowed, for the first time, simultaneous observations of a target in the optical/UV and X-ray regions of the electromagnetic spectrum. This is important because studying different wavelengths gives us a picture of what different mechanisms are at work.
The entire OM instrument. XMM-Newton is the first ever X-ray space observatory with optical and ultraviolet capacity. (Credit: ESA)
The OM has a field-of-view matched to that of the XMM-Newton X-ray cameras. After being focused by two mirrors, the light is directed through a filter. The light then goes through an ultra-compact electronic image intensifier, which amplifies the light signal a million times before it falls on a silicon CCD chip capable of registering 100 frames/second. A 4x magnifier in the filter wheel also allows an option of "zooming in"; on the central region of a field. The filter wheel also contains "grisms", devices that are part prism and part grating, allowing low-resolution spectroscopy.
The OM was designed and constructed at the Mullard Space Science Laboratory (MSSL). The University of California at Santa Barbara (UCSB) is in charge of the digital electronics module (DEM) of the instrument.
One of the two filter wheels positioned in front of the OM's CCD detectors. (Image courtesy of Mullard Space Science Laboratory, UK. and ESA.)