[fitswcs] Keywords for other FITS image representations

[William Pence]

Here's a draft of the wcs keyword names for the 2 other image
representations (besides an image in the primary array or image
extension).  This is based on my understanding of the current proposal,
which may not be up to date.  In particular, Don just mentioned in
another message this morning that the form of the projection
parameter shown below is not correct.   Please review this, and post
any corrections or comments.

As a reminder to those that may not be familiar with these formats, The
'BINTABLE Vector' case is where one has a vector column which contains
an image in each row of the table.  (In principle, one could have
several vector columns, each of which contains an image, on each row).

The Table pixel list case is commonly used for X-ray data. For each
detected X-ray, the table lists the X and Y pixel location of the
event.  The image is generated by computing the 2-dimensional histogram
of the X and Y columns.  There may be (and often are) multiple sets of
positional columns in the same table, e.g., DETX, DETY give the
position of the event on the detector, and X and Y give the position of
the event projected on to the sky.  (In general, there is no simple
transformation from one to the other because the telescope wobbles
on the sky during the exposure).


Keyword               Primary      BINTABLE      Table   
Description           Array         Vector      Pixel List
------------          -------      --------    ----------
 Axis Type            CTYPEiiv     iCTYnnnv    TCTYPnnn
                                               TCTYnnnv
 Axis Units           CUNITiiv     iCUNnnnv    TCUNInnn
                                               TCUNnnnv
 Reference Pixel      CRPIXiiv     iCRPnnnv    TCRPXnnn
                                               TCRPnnnv  
 Reference Value      CRVALiiv     iCRVnnnv    TCRVLnnn
                                               TCRVnnnv   
 Pixel Increment      CDELTiiv     iCDLnnnv    TCDLTnnn
                                               TCDLnnnv   
 Axis Rotation        CROTAiiv     iCROnnnv    TCROTnnn
                                               TCROnnnv   
 Rotation Matrix      CDii_jjv     ijCDnnnv    TCDnn_mm
                                               TCnn_mmv  
 Coord. Rotation      LONGPOLE     LONGnnnv    LONGnnnv 
                      LONPOLEv
 Coord. Rotation      LATPOLEv     LATnnnv     LATPnnnv    
 Proj. Param.         PROJPkv      kPROnnnv    kPROnnnv      
 Coord. Epoch         EQUINOXv     EQUInnnv    EQUInnnv     
 MJD of Obs.          MJD-OBSv     MJDOnnnv    MJDOnnnv 
 Reference Frame      RADECSYS     RADEnnnv    RADEnnnv 
                      RADESYSv


i,j  = axis numbers,  1 - 9 or 1 - 99
k    = projection parmeter number, 1 - 9
n,m  = table column numbers, 1 - 999
v    = version code, blank or A - Z


Issues regarding the Primary Array keywords
-------------------------------------------

1.  'LONGPOLE' is already 8 characters long, so there is no room
    for a version letter.  Suggest using 'LONPOLEv' for versions.

2.  'RADECSYS' is already 8 characters long, so there is no room
    for a version letter.  Suggest using 'RADESYSk' for versions.


Issues regarding the BINTABLE Vector keywords
---------------------------------------------

1.  It is important to allow 3 digit column numbers, nnn, thus
    axis number must be limited to the range 1 - 9.

2.  If there is only one column in the table that contains images,
    or if there are multiple image columns but they all share the
    same value for a WCS keyword, then the same name of the 
    keyword that is used for primary arrays may be used for the
    following keywords:  LONGPOLE, LATPOLE, PROJP, EQUINOX, MJD-OBS,
    RADECSYS. (e.g., use EQUINOXv instead of EQUInnnv).


Issues regarding the Table Pixel List keywords
----------------------------------------------

1.  It is important to allow 3 digit column numbers, nnn.

2.  The 8-letter keywords TCTYPnnn, TCRPXnnn, etc are already
    in wide use, but do not have room for a version letter.
    Alternate names that include version letters are shown
    below the main keyword name.

3.  If a CD rotation matrix must be specified (usually it is
    not needed), then the X and Y pixel list column must be
    located in columns 1 - 99 of the table, since the TCDnn_mm
    keyword only allows 2 digits for the column numbers.

4.  The TCDnn_mm is 8 characters long, so to allow a version letter
    this would have be be shortened to TCnn_nnv.  In practice, I
    doubt if this would ever be needed.

5.  If there is only one set of columns in the table that contains images,
    or if there are multiple image columns but they all share the
    same value for a WCS keyword, then the same name of the 
    keyword that is used for primary arrays may be used for the
    following keywords:  LONGPOLE, LATPOLE, PROJP, EQUINOX, MJD-OBS,
    RADECSYS. (e.g., use EQUINOXv instead of EQUInnnv).


____________________________________________________________________
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)