[fitsbits] Primary & Alternate WCS Keyword Order

[Mark Calabretta]

On Fri 2012/06/29 12:47:32 MST, "Dick Shaw" wrote
in a message to: fitsbits at nrao.edu

>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.

Once alternate descriptions are introduced in Sect. 2.5 of Paper I,
the WCS papers consistently use the fully qualified keyword name,
e.g. CRPIXja, CTYPEia, LONPOLEa, etc., except when referring
specifically to the primary description.  The only distinction that
is ever made between the primary and alternate descriptions is that
alternate descriptions may only be specified if a primary description
is specified.

Section 8.2 of the standard currently follows the historical
development.  Instead it should introduce alternative WCS descriptions
first, and then introduce and only ever refer to the WCS keywords in
their fully qualified form.

In that vein:

  WCSAXESa -- [integer; default: NAXIS or largest of WCS indexes i or j
  for representation 'a']
  Number of coordinate elements in WCS description 'a'.  This keyword,
  if present, _should_ precede all keywords for WCS description 'a',
  except NAXIS.  Note that the value of WCSAXESa _may_ exceed the number
  of pixel axes for the HDU, and that the default value of WCSAXESa may
  differ for differing 'a'.

Regards, Mark