[fitswcs] NAXIS vs. WCSDIM

[Doug Mink]

I have been dealing with WCS's independent of images, Doug Tody's case I,
for quite some time.  Sometimes I need to fit a WCS to a set of pixel values
in a separate file and create an output file which can be used for pixel to
sky and sky to pixel conversions.  Since no image is really necessary, I have
included in my WCSTools package a progam to create a header-only FITS file,
with NAXIS=w and BITPIX=0.  This works as well as having a separate keyword,
but it requires software that doesn't crash on a heretofore illegal BITPIX
value.

As for cases with NAXISn=1, I don't see any big problem, though I would prefer
that the degenerate axes be defined after the real ones.  Most of my image
software, because it is tested against radio data as well as optical/IR,
looks for the first two axes with NAXISn > 1, and uses those axis for 2-D WCS
determination.  Because so much radio data exists with degenerate axes and one
of the purposes of the WCS standard is to be able to match radio and optical
images, any general purpose WCS software will have to be able to deal with
this data no matter what is decided, so I need to see stronger reasons for
creating a new keyword.  In the dimensionally reduced case, it seems to me
that leaving NAXIScut=1 is the best way to note the change; if the WCS depends
on all three axes, you need all of the CRVAL's to compute the WCS for a given
pixel.

-Doug Mink
 (who is too busy finishing up WCSTools 3.0(.5) and writing an ADASS poster
  about it to join in more fully)