When calculating source extraction radii based on encircled
energy fraction the task assumes that the behaviour of the PSF is
symmetric about the optical axis. Furthermore the effect of boresight
offsets of the X-ray telescopes are ignored.
A extra feature which may be useful is the facility to dump the
region descriptions of all sources to a FITS file where it could be
picked up by the IAS.
PSF characteristics depend on photon energy. Ideally the task
would (maybe optionally) attempt to calculate a mean photon energy for
each source from the information in the source list.
