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sensitivity estimates must take into account
the cell size from the off-axis PSF
(cf. Sect. 5.2.2)
The cell size can be taken as the 50% power radius, i.e.,
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
Fig. 5.12 can be used to scale down the number of counts
detected from off-axis sources due to the decrease in telescope area;
since the HRI field of view is limited, the slight energy dependence
of the vignetting correction can usually be neglected for
feasibility purposes.
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