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. 110. A positional r.m.s.
accuracy of 
arcsec is obtained. The distortion map has been entered
into the appropriate CCF file and can be used in conjunction with
SAS.
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