XMM-Newton Users Handbook

3.5.4 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. 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 $0''.5$) still occurs.

The on-board measurement of the PSF for different filters is shown in Fig. 105 and Table 19. Measurements

Figure 105: Optics+detector OM PSF in different filters

Table 19: OM on-board PSF FWHM in different lenticular filters

Filter	FWHM ($''$)
V	1.35
B	1.39
U	1.55
UVW1	2.00
UVM2	1.8
UVW2	1.98

are based on the observations of the fields of G153, EXO0748-67, LH9 and PSR 0540-69.3. The FWHM values have been obtained after fitting with a Moffat function plus a look up table. The PSF has a slight dependency on the count rate, which is taken into account by the SAS when data are processed.