[fitswcs] WCS with NAXIS=0

[Perry Greenfield]

> 
> William Pence writes:
>  > Is the use of WCS keywords (like CRPIX1 and CRVAL1) now 
> allowed in image
>  > HDUs which have NAXIS = 0?  This question has to do with 
> whether our FITS
>  > verification program (fverify) should issue any warning or 
> error message if
>  > this occurs.
>  > 
> 
> That is what WCSAXES is all about.  See
>       http://www.aoc.nrao.edu/~egreisen
> for links to versions of Paper I and info about its volume and page
> numbers.
> 
Since I think I may have indirectly been the originator of this
question, I'll note that I could not find any text in Paper I
that excluded use of WCS keywords when NAXIS = 0. I did note
WCSAXES, but the same section also says (page 6 section 2.2):

  To provide a solution for this world-coordinate dimensionality
  problme that does not require the use fo degenerate axes, we
  reserve the keyword
 
     WCSAXES (integer-valued)  

  to specify the highest value of the index of any WCS keyword in
  the header (i.e. CRPIXi, PCi_j or CDi_j, CDELTi, CTYPEi, CRVALi,
  or CUNITi). The default value is the larger of NAXIS and the
  largest index of these keywords found in the FITS header...

  There is debate within the community as to whether the official
  definition of FITS (Hanisch et al. 2001) prohibits the occurence
  of WCS-related keywords with indices greater than the value of 
  NAXIS. We make no claims one way or the other, but rather assert
  that in order to accommodate WCS specifications whose dimensionality
  exceeds NAXIS without the use of degenerate coordinate axes, such 
  use must be allowed. Consistent with Hanisch et al. (2001), however
  no NAXISj keywords may exist for j>NAXIS. ...

That seems to suggest that not even WCSAXES is necessary to use
WCS keywords when NAXIS=0, but if that isn't so, please clarify.
This relates to some HST internal convention that uses null
data sets when all the values are constant (the header has a keyword
to indicate what the constant value is, as well as the dimensions
of the array. Naturally the question is whether it is legal to 
retain the WCS keywords in this case. It does appear that it is
as they exist now, and certainly so if WCSAXES is added.

Perry Greenfield