[fitsbits] Primary & Alternate WCS Keyword Order

[Dick Shaw]

Considering the recent discussion, and Lucio's questions:

> ... I was just trying to understand whether we should make an amendment to 
> the standard at this stage (which will be a couple of lines on a 52 page 
> document, and will introduce a deviation from the A&A published stuff, 
> both Pence et al 2010 and WCS Paper I ... which is "incorporated by 
> reference")
> 
> ... or we could pretend that the standard is clear enough, and issue only 
> some sort of explanatory note
> 
> ... reserving the amendment for the next round (3.1 or 4.0) whenever it 
> will be (not too soon hopefully).

It is clear to me that, at the least, some clarification in the text of the 
standard is called for. After all, if we are confused then what chance does a 
citizen FITS user have of understanding what is required? I agree with the 
apparent consensus that the ordering for WCSAXESa keywords should no longer be 
mandatory.

Let me update my specific suggestion for a change to Sect. 8.2 in the WCSAXES 
paragraph, and suggest that this amendment be considered for the next revision 
(v3.1?):

WCSAXES -- [integer; default: NAXIS or largest of WCS indexes i or j]
Number of axes in the named* WCS description. This keyword, if present, 
_should_ precede all WCS keywords for its named WCS description, except NAXIS, 
in the HDU. The value of WCSAXESa _may_ exceed the number of pixel axes for 
the HDU. The default value applies to any named WCS "a" if WCSAXESa is absent.

[Footnote *]: The "a" suffix refers to the name of the WCS represented by all 
WCS keywords with that suffix (see Sect. 8.2.1.) Its primary function is to 
provide a means by which to specify a particular WCS if multiple versions are 
defined in the HDU. Alternate axis descriptions are optional, but may only be 
specified if a primary axis description is specified.