Select an initial trial source box radius - (should be large)
Select the centre of the background box with a cursor
Select the background box radius or radii - (should be large)
The procedure then displays information about the source and
background boxes.
The background subtracted counts are then calculated along
with the optimum source box radius (if not turned off).
This radius together with the fraction of the PSF contained
within it, is displayed on the screen. A dashed line
representing the source box selected by the procedure is
drawn on the screen.
Finally, the data is sorted within both boxes.
The background box may be either a circle away from the source box or
be an annulus enclosing the source box entirely. The boxes shouldn't
overlap, unless the source box is totally enclosed within the bckgnd
box.
