Rectangular: The selected field will be generally rectangular
in shape. Rectangular does not mean that the region corresponds
to a region seen by a rectangular CCD. It means the following:
For the top and bottom of the region (Decl boundaries), the
boundaries are defined by great circles perpendicular to the
line of RA through the center of the region. For left and
right boundaries (RA boundaries), the region is defined by
non-great circles that are equidistant from the line of RA
through the center of the region. How's that for confusing?
Circular: The selected field will be generally circular in shape.
This is implemented for ASCII style output at this time. Catalog
and IRAF style output will always be rectangular until the
software is fixed.