XMM-Newton Users Handbook

3.5.1 OM telescope

The OM telescope consists of an $f/12.7$ modified Ritchey Chrétien optics (see schematic in Fig. 82). From the primary mirror incoming light is reflected onto a secondary, from where it is reflected onto a rotatable $45^\circ$ flat mirror located behind the primary, which can direct the beam onto one of two detector assemblies operated in cold redundancy. See more details of the telescope parts at: http://mssls7.mssl.ucl.ac.uk/main.html Spectral resolution is achieved by the use of narrow-band filters contained within the Optical Monitor's filter wheel; there are also two grisms in the filter wheel for low resolution spectroscopy.

Figure 82: The light path in XMM-Newton's optical/UV telescope, OM.