The OM telescope consists of an 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
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://www.mssl.ucl.ac.uk/www_astro/xmm/om/om.html
Spectral resolution can be 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.