[fitswcs] Keywords for other FITS image representations

[William Pence]

Mark Calabretta wrote:
>
> Do we need some rule about what version number is implied if "v" is omitted.
> For example, for two versions in one primary header should you write
> 
>    LNGPOLE & LNGPOLEA
> 
> or
> 
>    LNGPOLE & LNGPOLEB?

I think it should be the former: use the letter A for the first alternate set of
WCS parameters.

> >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.
> 
> I believe some others (e.g. Doug) want up to 999 but hopefully only in the
> primary header.

Then we can't use 'PARVALk' for this.  I suggest changing the root name to
'PARVL'.

> G&C allows k = 0 for the zenithal polynomial projection (ZPN, only) since it
> was a natural choice in that case.  k=0 is pretty-well ingrained into WCSLIB
> at this stage, e.g. in the C library PROJP0 corresponds to p[0].  Indexing in
> the FORTRAN library also starts from 0.

OK with me, but does seem a little inconsistent to start with 0 in this one
case.

I modified the following table (and the 'Notes') slightly to take into account
the above comments.

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

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. Equinox       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      PARVLk       PARVLk      PARVLk
  (was PROJPk)        PVk_iv       Pkkivn      PVkknv

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

Notes:

i,j   = axis numbers:   1 - 99, no leading zeros
ii,jj = axis numbers:  01 - 99, with leading zeros
k     = projection parmeter number:  0 - 999, no leading zeros
kk    = projection parmeter number: 00 - 99, with leading zeros
n     = table column numbers: 1 - 999, no leading zeros.  In the
        BINTABLE image representation, if 'i' is greater than 9
        then 'n' is limited to the range 1 - 99.
nnn,mmm  = table column numbers with leading zeros: 001 - 999, 
v     = version code: blank (or underscore in the
        case of BINTABLE vector, to separate the axis number, i,
        from the column number, n) for the primary set of WCS
        parameters; the letter A - Z for up to 26 secondary
        versions.

____________________________________________________________________
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)