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 arcsec is obtained. The distortion map has been entered into the appropriate CCF file and is used in SAS.