Thomas An. - you may also want to give the RADIANCE <pfilt> command line app a try. Here is the man page:
http://radsite.lbl.gov/radiance/man_html/pfilt.1.html
Here are the specific command line options regarding star diffraction patterns:
-h lvl Set intensity considered ``hot'' to lvl. This is the level
above which areas of the image will begin to exhibit star
diffraction patterns (see below). The default is 100
watts/sr/m2.
-n N Set the number of points on star patterns to N. A value of
zero turns star patterns off. The default is 0. (Note that
two passes are required for star patterns.)
-s val Set the spread for star patterns to val. This is the value a
star pattern will have at the edge of the image. The default
is .0001.
-a Average hot spots as well. By default, the areas of the pic-
ture above the hot level are not used in setting the expo-
sure.
kirk