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Figure F.14: Plate-scale non-linearity correction in detector pixel for the XRT/PSPC: vertical lines are placed in steps of 10arcmin: at an off-axis angle of 60arcmin ( detector pixel) the off-axis angle has to be reduced by 7.5 detector pixel ( arcsec, cf.Eq.(F.17))
The plate scale varies slightly across the FOV. This is caused by the almost flat detector (PSPC as well as HRI) while the focal surface of the XRT is curved. The resulting distortion effect --shift to larger off-axis angles-- has been simulated using ray-tracing programs. It depends on the apparent off-axis distance d, the apparent off-axis angle and the off-axis distance dependent shift shown in Fig. F.14. The whole correction result in the final detector coordinates , .
SASS source code: package PSPC, bubble CT, subroutine FCOR