The usual mode of operation is to use a psf which is constant across the area being in searched. For certain instruments, however, this is inappropriate - in these cases the PSFCON parameter should be set false forcing re-evaluation of the psf at every point. In order that cpu time is kept to a minimum, it is recommended that the psf model facility is used. In this case, the default of PSFCON is changed to FALSE.
When PSFCON is false, PSS provides the option to vary the size of the source box across the field by providing more than one value for PSFPIX. A table of enclosed energy is printed as a function of off-axis angle. From these numbers a triplet of psf radii can be chosen. PSS will then interpolate to decide the box size to use anywhere on the image.