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 -infinity -> -60, -60 -> -30 and so on to 60 ->infinityRECT(PWFC,10.0)