[fitsbits] WCSAXES usage

[William Thompson]

I know that in my own software, those additional axes will be ignored if there 
is no WCSAXES keyword.

Doug Tody wrote:
> Hi Bill -
> 
> For backwards compatibility I believe we had to define that WCSAXES
> is not required when WCSAXES=NAXIS.  Evidently the standard allows
> the keyword to be omitted when WCSAXES > NAXIS as well.  However I
> suggest that it "should" be provided in this case (although FITS
> does not make such a distinction).  It might be nice to issue an
> informational warning in such a case, even though it is legal usage.
> That is, flag any actual errors, but also issue informational comments
> when there is a departure from suggested usage.
> 
>  	- Doug
> 
> 
> On Thu, 5 Aug 2010, William Pence wrote:
> 
>> I'm in the process of updating the FITS verifier code to deal with
>> changes in version 3 of the FITS Standard, and have a question about the
>> correct usage of the WCSAXES keyword: is this keyword optional in cases
>> where the WCS has more axes than specified by the NAXIS keyword, as in
>> the following fragment of a FITS image header:
>>
>> NAXIS  = 2
>> NAXIS1 = 512
>> NAXIS2 = 512
>> CRPIX1 = 256
>> CRPIX2 = 256
>> CRPIX3 = 1
>> ...
>>
>> Should the FITS verifier issue a warning in this case (where there is no
>> WCSAXES keyword) that the index on the CRPIX3 keyword is greater than
>> the NAXIS keyword value?
>>
>> The FITS Standard states that the default value for the WCSAXES keyword,
>> if it is not physically present, is "NAXIS, or largest of WCS indexes i
>> or j".  This suggests that the above example header is legal, and that
>> the default value of WCSAXES is 3 this case.  If this is the correct
>> interpretation, then the FITS verifier should not issue a warning in
>> this case.
>>
>> Bill
>>
> 
> _______________________________________________
> fitsbits mailing list
> fitsbits at listmgr.cv.nrao.edu
> http://listmgr.cv.nrao.edu/mailman/listinfo/fitsbits
> 

-- 
William Thompson
NASA Goddard Space Flight Center
Code 671
Greenbelt, MD  20771
USA

301-286-2040
William.T.Thompson at nasa.gov