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The HRI response is basically flat over the entire active area of the detector. The telescope point spread function degrades with off-axis angle and there is a slight vignetting of the telescope so that the effective area decreases. For field angles of less than 20', the mirror point spread function can be represented by a radial Gaussian of the form
where arc seconds (independent of energy), and is the field angle in arc minutes.
Users concerned with sensitivity off-axis will need to estimate the cell size from the off-axis PSF by combining in quadrature from the above expression, , and to give . The cell size is then given by the value , which is the size of a square region where 50% of the source counts will fall. The background rates must also be scaled to the new cell size.
The vignetting correction in Figure 10.18 can be used to scale down the number of counts detected from off-axis sources due to the decrease in telescope area; note that the HRI field of view is only 32' and that the energy dependence of the vignetting correction is small and can be neglected.