The PSF of the X-ray telescopes depends on the source off-axis angle, i.e., its distance from the centre of the field of view (FOV). It also depends slightly on the source azimuth within the FOV. In Fig. 8 the dependence of the shape of the XMM-Newton X-ray PSF on the position within the FOV is presented. One can see that the PSF at large off-axis angles is elongated due to off-axis aberration (astigmatism). The reader is also referred to Figs. 17, 18 and 19 for additional impressions of the off-axis PSF.
For the two EPIC MOS cameras, the PSF is also affected at a level of a few times (integral relative intensity) by scattering off the RGA rib structures. This contribution, however, is negligible in the vast majority of cases.
Figs. 9 and 10 show the off-axis angle dependence of the radius at which 90% of the total energy is encircled (W90). As the off-axis angle increases the energy dependence of the PSF changes as focusing of high energy photons is no longer confined to the inner shells. A substantial fraction of these high energy photons are redistributed into the wings of the PSF by X-ray scattering; hence W90 is larger at high energies than at soft energies for large off-axis angles. Note that these data are based on early ray-trace simulations at three discrete energies, and assume a fractional encircled energy of 100% at a radial distance of 5 arcmin, independent of the off-axis angle. For a more precise estimate based on the currently implemented PSF calibration, the user may use the SAS task calview to obtain the encircled energy fraction at a given energy and off-axis angle (note that the "Accuracy Level" should be set to "ELLBETA" through the task interface http://xmm-tools.cosmos.esa.int/external/sas/current/doc/calview/node3.html). Also note that encircled energy correction of point sources is performed by default through the respective SAS tasks (see Section 3.2.3).
Users are referred to Read et al. 2011, A&A, 534, 34 for a comprehensive description of the EPIC PSF, as well as for an empirical characterization as a function of off-axis angle and energy.