[fitsbits] Primary & Alternate WCS Keyword Order

[Mark Calabretta]

On Fri 2012/06/29 10:55:06 +0200, Lucio Chiappetti wrote
in a message to: FITS List <fitsbits at nrao.edu>

>What Bill did for the verifier (excerpt below) appears consistent with the 
>LETTER of the standard, which talks only of the position of WCSAXES and 
>not of WCSAXESa

That's only because WCSAXES is introduced in Paper I before alternate
representations are introduced, so it should follow that what applies
to WCSAXES also applies to WCSAXESa.

The only thing special about the primary representation is that

 "Alternate axis descriptions are optional, but may only
  be specified if a primary axis description is specified."

I think we should keep it that way.

>(a) do any amendment to the standard (releasing the positioning 
>requirement),

I would favour deprecating WCSAXESa.  After all, its use is already
optional and its function can be achieved easily by other means.

Failing that, I would favour changing the standard to recommend (not
mandate) that WCSAXESa precede all other WCS keywords for alternate 'a'.
Note that in the WCS papers, 'a' always means blank as well as A-Z, so
this statement would also apply to the primary representation.

Regards, Mark