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 EPIC MOS cameras as presented in SAS: MOS1 (here) and MOS2 (next figure). The orientation of the DETX/DETY axes are shown, to highlight that the RGS dispersion axes are parallel within spacecraft physical co-ordinates, but the EPIC MOS cameras are orthogonally aligned. The readout node of each CCD is located with a small box. The RAWX and RAWY co-ordinates in each CCD decrease towards the readout. The orientation of the CCD specific RAWX/RAWY coordinate systems is visible e.g. via the SAS task calview under View $\rightarrow$ LinCoord $\rightarrow$ FocalPlane. Note: Since the MOS1 event in XMM-Newton revolution 961 (see § 3.3) and at the time of writing, scientific observations are performed with MOS1 CCD6 switched off.

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