William Pence writes:
A FITS Region file is a FITS binary table that defines a spatial region
of a 2-dimensional image. The region file is often used to define an
area that is to be included or excluded from certain data processing
operations on the image. The region is specified as the union or
intersection of geometric shapes, such as 'circle' or 'rectangle'. The
REGION table is the FITS equivalent of the ASCII text region file that
has long been used by the ds9 image processing program and its
predecessor, SAOimage.

Another comment:

The sample fits header lists TCRPXn, TCRVLn and TCDLTn as required
keywords for RA and DEC columns. Are these really required? Does
that mean that one cannot simple have a floating point (1E) column
describing the RA and Dec?

I note that X, Y and R are all in pixel space as well, which means
that one cannot define the position and radius in physical units?

Craig Markwardt