The MOS chip arrays consist of 7 individual identical, front-illuminated chips. The individual CCDs are not co-planar, but offset with respect to each other, following closely the slight curvature of the focal surface of the Wolter telescopes. Technically, this leaves space for the connections to the central CCD. The numbering scheme for the EPIC MOS chip array, the individual chip coordinate frames and the directions of the detector coordinates are displayed in Figs. 20 and 21. Not visible in the figures is that the MOS chips have a frame store region which serves as a data buffer for storage before they are read out through the readout nodes, while the rest of the chip is obtaining the next exposure.
As indicated above, the MOS cameras are mounted on those X-ray telescopes that also carry RGS instruments. Therefore, they receive only 44% of the reflected light.
European Space Agency - XMM-Newton Science Operations Centre