The PSF MODELS are used to increase processing speed. The MODEL option places a grid over the dataset and subsequently computes a single PSF for each grid cell which is to be used for all the enclosed points. For each cell of the grid the PSF is calculated only once, and only if required.
There are three MODEL forms - POLAR(PSF,rbin[,abin]) and RECT(PSF,xbin[,ybin]) have spatial variations while CSPEC(PSF/model,spec[,nbin]) includes energy dependency.
MODEL selection must include a reference to a variable PSF option.
eg. PSF - Choose PSF to use for source model /'XRT_PSPC'/ > polar(xrt_pspc,0.02)
In this example 'polar' is the model, 'xrt_pspc' the variable PSF option and '0.02' the grid cell size.