The OM telescope consists of modified Ritchey Chrétien optics (see schematic in Fig. 100). 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.