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.
It is important to know the encircled energy fraction at different positions in the FOV, e.g., to use the correct extraction areas for source photons. In Figs. 9 and 10 we show the dependence of the radius at which 90% of the total energy is encircled (W90) on the off-axis angle. Note that 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 large 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.
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.