The optical/UV monitor on the X-ray multi mirror mission

A major feature of the planning of X-ray observations has been the extensive coordination of ground based facilities in order to obtain simultaneous observations at optical wavelengths. The Optical/UV Monitor on the ESA X-ray Cornerstone mission XMM will provide simultaneous optical and UV coverage of the X-ray targets viewed by the observatory. The OM has a field of view of 17x17 arcmin, which covers the central field of view of the EPIC X-ray cameras. The instrument consists of a 30 cm modified Ritchey Chretien telesope, which feeds a compact photon counting detector. The detector operates in the blue part of the optical spectrum and in the UV (1600-6000 Angstrom). Because of the low sky background in space, the sensitivity of the OM will be comparable to that of a 4-m telescope at the Earth's surface. It should detect a B=24th magnitude star in a 1000 s observation using unfiltered light. The pixel size of the detector corresponds to 0.5 arcsec on the sky in normal operation. In front of each of the two redundant detectors are filter wheels containing broad band filters, grisms for low resolution spectroscopy and a 4x field expander which will allow high spatial resolution images of the central part of the field of view.