Not only the shape of the X-ray PSF, but also the effective area of the mirrors is a function of off-axis angle within the mirrors' FOV. With increasing off-axis angle, less of the photons entering the telescopes actually reach the focal plane. This effect is called “vignetting”. The vignetting of the XMM-Newton telescopes, which is reflected by the decline of the X-ray telescope's effective area as a function of off-axis angle, is displayed for a few energies for the X-ray telescope in Fig. 13.
The off-axis vignetting in the two telescopes with an RGA has a dependence on angle with respect to the RGAs' dispersion direction. A source at an off-axis position perpendicular to the dispersion direction will be vignetted by a different amount to one at a position parallel to the dispersion direction. This azimuthal dependency of the vignetting is shown in Fig. 14. Readers are referred to Mateos et al. 2009, A&A, 496, 879 for a discussion on the calibration of this azimuthal dependency.
European Space Agency - XMM-Newton Science Operations Centre