XMM-Newton Users Handbook

3.3.1.1 EPIC MOS chip geometry

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.

Figure 20: The layout of the MOS cameras is presented in this figure for MOS1 (above) and MOS2 (bottom). The images (in detector coordinates [DETX,DETY]) are extracted from an exposure taken with the calibration (CAL_CLOSED) filter in Rev.#80 (Obs.#0124700101). The number of the CCD where each photon has fallen is included in the CCDNR column of the calibrated event list files. The MOS cameras are orthogonally oriented. Hence, the RGS dispersion direction is aligned with the DETY direction in the MOS1 camera, while it is aligned along the DETX direction in the MOS2 camera. Users are warned that scientific observations are performed with MOS1 CCD3 and CCD6 switched off due to a hardware failure, probably due to a micrometeorite impact (see Sect. 3.3).

Figure 21: The layout of the EPIC MOS cameras as presented in SAS (cntd. from previous figure): MOS2.