[fitsbits] Primary & Alternate WCS Keyword Order

[Arnold Rots]

An obvious example can be found in radio images that often have single
pixels axes, especially for polarization.
Or, indeed, as Rob suggests, it could be a single Doppler velocity
channel extracted from a cube, where one want to preserve the Doppler
velocity information by adding it as a single-pixel axis.

Such axes may be included in NAXES but not in WCSAXES, or the other
way around, depending on one's preference.

I didn't check it, but I seem to recall that NAXES has to precede all
other coordinate axis information, so I simply assumed that the
requirement for WCSAXES was simply inherited from NAXES and that it
only applied to this specific keyword.

  - Arnold

Rob Seaman wrote:
[ Charset windows-1252 unsupported, converting... ]
> 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
> 
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Arnold H. Rots                                Chandra X-ray Science Center
Smithsonian Astrophysical Observatory                tel:  +1 617 496 7701
60 Garden Street, MS 67                              fax:  +1 617 495 7356
Cambridge, MA 02138                             arots at head.cfa.harvard.edu
USA                                     http://hea-www.harvard.edu/~arots/
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