psfgen is designed to return the point spread function (PSF) of the EPIC cameras for a given position in the field of view. It allows the position to be specified in any of the common XMM-Newton cordinate systems optionally using an input image to translate between sky (XY) or celestial (RA,DEC) coordinates and telescope coordinates.
Within the XMM-Newton calibration system the PSF has been described by several different models, referred to as 'Accuracy Levels'. The current best model, called ELLBETA, is a 2-dimensional King function which includes instrumental distortions and the 'spokes' (see ref 1.). An alternative model, called MEDIUM, has been produced based on ray-tracing perfromed with the science simulator (SCISIM), while a simpler analytical model, EXTENDED, consists of a 1-dimensional King function. A further two models, LOW and HIGH, are included for completeness but have serious shortcomings and are not recommended.
The task calculates the PSF based on the input parameters and writes the result into an output FITS file. Due to limitations of the PSF implementation within the SAS, the resolution of the output image pixels are fixed to be 1 arcsecond by 1 arcsecond for the EXTENDED and ELLBETA modes and 1.1 x 1.1 arcseconds for MEDIUM mode. Integer rebinning of these images may be performed later by the ftool fimgbin.