XMM-Newton Users Handbook

3.5.7 OM distortion

The XMM-Newton OM optics, filters and (primarily) the detector system result in a certain amount of image distortion. This effect can be corrected with a “distortion map”, by comparing the expected position with the measured position for a large number of stars in the OM field of view. A U-filter analysis has been performed on the G153 pointing with 813 stars. The effect of applying this correction is shown in Fig. 111. A positional r.m.s. accuracy of $0.5-1.5$ arcsec is obtained. The distortion map has been entered into the appropriate CCF file and is used in SAS.

Figure 111: Positional error of sources in a field of view, where the distortion map has been applied. The histogram was made using sources from the 3C273 field corrected for distortion