The RECT option permits a polar model of the specified psf to be constructed. The syntax is, RECT(psf, xbin[,ybin]) where psf is the name of an instrument psf (eg. XRT_PSPC).
xbin controls the placement of psf model bin boundaries in the x-axis direction. It can be specified as either a single value indicating bin width, or as a bin-boundary specification (values separated by colons). ybin may also be supplied. If omitted, the same bin boundaries are used in the y-axis as in the x-axis. Some examples are,
Bins 10 arcmin wide in both x and y axes.
Bin boundaries are -RECT(PWFC,10.0)
-60, -60 
-30 and so on to 60 
RECT(PWFC,-60:-30:-10:0:10:30:60)