The basic source region shape is a circle. There are three ways to assign the radius, specified via the parameter radiusstyle:
If outunit=`xy', the smaller of the XY-system pixel dimensions in arcseconds provides the conversion factor to convert this radius to sky-image pixel coordinates.
Note that in this case, regardless of the value of outunit, the initial circle must be drawn in the DETXY system, since the PSF is returned in this system. However if outunit=`xy' this involves a reprojection onto another image plane. In theory, locii which appear circular in the DETXY projection plane appear in general as ellipses in another projection plane (and pixellization scheme). At present the effect of this reprojection on the source region contours is ignored and circles are retained. The scalar conversion between circle radii in the two coordinate systems is performed by taking the smaller of the XY-system pixel dimensions in DETXY coordinates as the conversion factor. In most cases the effect of this approximation is expected to be insignificant. This issue may be dealt with more rigorously in a later version of the task.