[fitswcs] coordinates by table lookup

[William Pence]

In the latest (08 July 2002) draft of the WCS III paper on spectral
coordinates, it is required (in section 6.1) that the separate coordinate
lookup extension must be contained within the same physical FITS file as the
data which reference it.  This seems overly restrictive to me, especially
since our data systems seem to be growing more distributed and
hierarchical.  I suggest that another optional parameter, PSi_3 be defined
to give the full or partial URL of the FITS file that contains the look up
table.  If this parameter is not present, then the lookup table would be
assumed to be in the current file, just as in the current draft.

Examples of the value of this new parameter are:

 PS1_3 = 'wcsfile.fits'                    - in the same directory
 PS1_3 = 'subdir1/subdir2/wcsfile.fits'    - in a subdirectory
 PS1_3 = '/local/user/wcsfile.fits'        - absolute path
 PS1_3 = 'ftp://nrao.edu/pub/wcsfile.fits' - URL

If this proposal is deemed to be to extreme to fully implement now, then I'd
at least suggest that this sentence:

"The specified coordinate table is found in the same FITS file as the data
which reference it."

be made less restrictive, and just state that in the absence of other
conventions which might be defined in the future, it will be assumed that
the coordinate table will be found in the same file as the data that
reference it.

Bill Pence