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An analytic description of the off-axis PSF of grazing incidence optics is complicated. A convenient measure of the image spread is the so-called RMS blur radius, defined as the radius within which 68% of the photons fall. The expected ROSAT XMA RMS blur circle radius increases rapidly with off axis angle.
The XMA off-axis point response as a function of energy is shown in Fig. 2.6 where the widths of the mirror and PSPC detector components contributing to the PSF are compared (for four energies). The figure illustrates how the mirror effect dominates over the PSPC effect outside of about 14'.
The change in mirror blur with increasing off-axis angle was parameterized as a Gaussian with
For a complete parameterization of the XMA + PSPC off-axis PSF see CAL/ROS/93-015 [Hasinger et al.1994] and Sect. 4.1.2.