XMM Users' Handbook

   
Optical/UV point spread function of the OM and tracking

The OM telescope produces images on the detector which are nominally diffraction-limited over the field of view (ca. 0.35'' at 400 nm, decreasing with increasing wavelength). However, this is convolved with the somewhat larger detector resolution noted above (about 1'' generally, decreasing with increasing wavelength). The resulting $\simeq1.2''$ resolution images are optimally sampled by the 0.5'' pixels.

This image quality is much better than that obtained with the X-ray instrumentation, and is significantly degraded by the spacecraft drift. The OM therefore contains software which 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 0.5'') still occurs. The final image quality will have a FWHM of $\simeq1.5''$. Under the nominal $2\times2$ pixel binning (see below), the image is marginally undersampled and the resolution thus determined by the pixel size. Inserting the magnifier (which transmits in the optical region of the spectrum) increases the image scale on the detector by a factor of 4, resulting in $0.125''\times0.125''$ pixels: in this case the detector resolution is effectively increased by a factor of 4 (at the expense of a similar factor reduction in field of view) and the overall resolution is dominated by the telescope diffraction limit.