[fitsbits] Primary & Alternate WCS Keyword Order

[Rob Seaman]

On Jun 25, 2012, at 3:46 AM, Lucio Chiappetti wrote:

> The text in 8.2 says:
> 
> WCSAXES – [integer; default: NAXIS, or larger of WCS indexes i or j]. 
> Number of axes in the WCS description. This keyword, if present, must 
> precede all WCS keywords except NAXIS in the HDU. The value of WCSAXES may 
> exceed the number of pixel axes for the HDU.
> 
> Now (I haven't checked the original WCS papers):
> 
>  - I can't image a case where WCSAXES > NAXIS
>  - I (can) hardly imagine WCSAXES < NAXIS (maybe a datacube where X,Y are
>    sky coordinates, and Z is a conventional coordinate, or alike)

Perhaps the thinking was that an image might have been created as a subsection of a higher dimensionality object?  Or on the other hand, a survey data structure (or some complex instrument like a Fabry-Perot spectrograph) might produce multiplex data structures (MEF or separate files) that share a common WCS?  (A bit like Buckaroo Banzai across the 8th dimension.)  Some use cases would help here.

> In general, I think we should avoid as far as possible positional 
> constraints on header keywords.

A major instrument being commissioned has randomly ordered keywords in each header, has randomly ordered HDUs, and submits sequentially acquired exposures in a random order to the archive.  Placing constraints on the key structural keywords is similar to requiring that the primary HDU be the first one in the file.  There may be higher order implications such as the inheritance issues you mentioned.

Rob