The OM telescope produces images on the detector which are nominally diffraction-limited over the field of view (ca. at 400 nm, decreasing with increasing wavelength). However, this is convolved with the somewhat larger detector resolution. The OM onboard software calculates the spacecraft drift from guide stars in the tracking windows and continually corrects the incoming photons from the detector to register them correctly in the accumulating image (“shift & add” in-memory). This can be done only in steps of an integral number of pixels, so some further broadening (on the scale of another ) still occurs.
The on-board measurement of the PSF for different filters is shown in Fig. 105 and Table 19. Measurements
European Space Agency - XMM-Newton Science Operations Centre