[fitswcs] Keywords for other FITS image representations

[William Pence]

Here is another draft of the WCS keyword naming convention
that tries to capture the latest consensus of this group.
I've changed the notation used in the table, and fiddled with
a couple of the keyword names themselves, so please review it 
carefully to see if you have any objections.

In particular, a few things to consider are:

1. leading zeros are never explicitly represented for the
   axis number (i and j), the column number (n) 
   EXCEPT for the the rotation matrix
   in the BINTABLE and Pixel List representations, where
   leading zeros are required to fill the 2 or 3-digit fields.
   Leading zeros for the projection parameter, kk, are only 
   required for the pixel list representation.

2. The version number (A-Z) is optional in all cases EXCEPT
   for BINTABLE vector represention where it must be present
   to separate the axis number from the column number.  I suggest
   that the underscore character be used to represent the 'main'
   set of WCS keywords in the BINTABLE vector representaion.
   (In the other 2 representations, the 'main' set of WCS keywords
   has no version code at all).

3. In most of the BINTABLE Vector keywords, the column number n
   is restricted to the range 1 - 99 if the image has more than
   9 axes.  If it has 9 or less axes, then the image my be 
   any table column over the entire legal range 1 - 999.   

4. For simplicity in the vast majority of cases, and for better
   backward compatibility, it is suggested that the following
   keywords be used when the same value applies to all axes/columns in
   the image/table: EQUINOX, EQSYS (was RADECSYS), MJD-OBS, LNGPOLE
   (was LONGPOLE),  LATPOLE, and PARVALkk (was PROJPkk).    
   The more complicated forms of these keywords,
   as shown in the table, only need to be used when different values
   have to be applied to different axes of the image.
  
5. Currently, the maximum number of allowed projection parameters
   is 99.  Is this sufficient?  The root name could be
   changed to support up to 999 parameters.

6. The CDELTi keyword is no longer written at all (is that
   correct?).

----------------------------------------------------------

Keyword               Primary or   BINTABLE    Table   
Description           IMAGE Ext.   Vector      Pixel List
------------          ----------   --------    ----------
 Axis Type            CTYPEiv      CTYivn      CTYPnv

 Axis Units           CUNITiv      CUNivn      CUNInv

 Reference Pixel      CRPIXiv      CRPivn      CRPXnv

 Reference Value      CRVALiv      CRVivn      CRVLnv

 Rotation Matrix      CDi_jv       Ciijjvn     Cnnnmmmv
                                or Cijvn     

 Coord. Epoch         EQUINOXv     EQUINOXv    EQUINOXv
                      EQNOXiv      EQNivn      EQNXnv

 Reference Frame      EQSYSv       EQSYSv      EQSYSv
  (was RADECSYS)      EQSYSiv      EQSivn      EQSYnv

 MJD of Obs.          MJD-OBSv     MJD-OBSv    MJD-OBSv
                      MJDOBiv      MJDivn      MJDOnv

 Coord. Rotation      LNGPOLEv     LNGPOLEv    LNGPOLEv
  (was LONGPOLE)      LNGPLiv      LNGivn      LNGPnv

 Coord. Rotation      LATPOLE      LATPOLE     LATPOLE
                      LATPLiv      LATivn      LATPnv
 
 Parameter Value      PARVALk      PARVALk     PARVALk
  (was PROJPk)        PVk_iv       Pkkivn      PVkknv

----------------------------------------------------------

Notes:

i,j   = axis numbers,   1 - 9,  10 - 99
ii,jj = axis numbers,  01 - 09, 10 - 99 
k     = projection parmeter number,  1 - 9,  10 - 99
kk    = projection parmeter number, 01 - 09, 10 - 99
n     = table column numbers, 1 - 999, no leading zeros
nnn,mmm   = table column numbers, 001 - 999, 
v     = version letter code,  A - Z, (or underscore in the
        case of BINTABLE vector).


____________________________________________________________________
Dr. William Pence                          pence at tetra.gsfc.nasa.gov
NASA/GSFC Code 662         HEASARC         +1-301-286-4599 (voice)     
Greenbelt MD 20771                         +1-301-286-1684 (fax)