Can I use arfgen for spectra extracted from regions of
arbitrary shape?
Yes. However, there are a couple of caveats you need to be aware of:
the PSF correction in arfgen assumes that the source is located at the
center of the extraction region. If your extraction region
is arbitrarily complex, this is probably not the
case. If this happens, the encircled energy correction
in arfgen may be largely
wrong. In order to ensure that the PSF correction
is properly calculated for extraction regions of
arbitrary shape, one needs to specify explicitly the coordinates
of the source centroid, through the parameters:
withsourcepos, sourcecoords,
sourcex, and sourcey, as in the following examples:
You should be aware that arfgen is forced to use the HIGH mode PSF
to calculate the encircled energy correction if the source shape is
other than a simple circle, annulus or rectangle. This mode is
less accurate than the default EXTENDED mode (cf.
the PSF watchout item for more information on this subject).
if your spectrum is extracted from an annulus, make sure that
the center coordinates of the two circles exactly coincide. Otherwise,
arfgen considers these as two separate circles, and
applies again the HIGH mode PSF
