[fitsbits] Is this legal FITS?

[Eric Greisen]

On 07/24/2017 02:33 PM, William Pence wrote:
> In this case (with NAXIS = 2), the NAXIS3 keyword is not legal according
> to section 4.4.1.1 of the FITS standard:
>
> "NAXISn keywords. The NAXISn keywords must be present for all values n =
> 1, ..., NAXIS, in increasing order of n, and for no other values of n."
>
> On the other hand, I wouldn't have expected software to choke on such a
> minor infraction, regardless of the value of the NAXIS3 keyword.
>
> -Bill

The first WCS paper that introduces WCSAXES explicitly says

  "Consistent with Hanisch et al. (2001), however, no NAXISj keywords 
may exist for j > NAXIS. ... Accordingly, all axes with axis number 
greater than NAXIS must be one pixel in length implicitly rather than 
explicitly."

This WCSAXES does not allow for this usage.  I would hope that software
would deal with this by ignoring NAXISj for j > NAXis however.

Eric Greisen


>
> On 07/24/2017 03:22 PM, Thompson, William T. (GSFC-671.0)[ADNET SYSTEMS
> INC] wrote:
>> Folks:
>>
>> I was handed a couple of FITS files that our IDL software was having
>> trouble reading.  I was told that these FITS files were produced by
>> DS9.  Looking inside the files, I find the following keywords:
>>
>> SIMPLE  =                    T / Fits standard
>> BITPIX  =                  -32 / number of bits per data pixel
>> NAXIS   =                    2 / number of data axes
>> NAXIS1  =                  360 / length of data axis 1
>> NAXIS2  =                  180 / length of data axis 2
>> NAXIS3  =                    0 / length of data axis
>>