Subject: FITS image coordinate keywords

From: pence at tetra.gsfc.nasa.gov (William Pence)
Date: Fri, 4 Feb 94 15:01:44 EST

The HEASARC needs to support FITS image files that can be in a variety
of formats besides a simple primary array.  For instance we also deal
with image data that are expressed as lists of pixels in a table, or as
a 2-d vector element in a binary table.  Along with the image data
itself, we also need to provide the usual coordinate system information
for images in these different formats (analogous to the CTYPEi, CRPIXi,
CRVALi, etc.  keywords in a primary array).  We have developed a
proposal for the FITS coordinate keywords to be used in these different
types of images and are posting it here for further comment and
discussion.  Our local FITS Working Group has provisionally approved
this proposal and we will be using it in some of our data sets.  The
full proposal is available via anonymous ftp from legacy.gsfc.nasa.gov
in the documents/fits/ofwg_recomm/ directory.  The latex file is called
coord_keywords.tex and the corresponding postscript file is called
coord_keywords.ps.

To just briefly describe the proposal, the following table shows
the corresponding keywords to be used for the different types of
images.  Please read the full document for more details and for example
header listings of these different types of images.

--------------------------------------------------------------------
Keyword           Primary    BINTABLE      Table       Referred 
Description        Array      Vector     Pixel List     Table   
--------------------------------------------------------------------
Axis Type         CTYPEi     iCTYPn        TCTYPn       RCTYPi 
Reference Pixel   CRPIXi     iCRPXn        TCRPXn       RCRPXi 
Reference Value   CRVALi     iCRVLn        TCRVLn       RCRVLi 
Pixel Increment   CDELTi     iCDLTn        TCDLTn       RCDLTi 
Axis Rotation     CROTAi     iCROTn        TROTAn       RROTAi 
Rotation Matrix   CDiiijjj   ijCDn         TCDnnmm      RCDiijj 
Transform Matrix  CDi_j       N/A           N/A          N/A 
Axis Units        CUNITi     iCUNIn        TCUNIn       RCUNIi 
Coord. Epoch      EQUINOX    EQUINOX or    EQUINOX or   EQUINOX 
                             EQUINn        EQUINn 
Date of Obs.      DATE-OBS   DATE-OBS or   DATE-OBS or  DATE-OBS 
                             DATOBn        DATOBn 
MJD of Obs.       MJD-OBS    MJD-OBS or    MJD-OBS or   MJD-OBS  
                             MJDOBn        MJDOBn 
Reference Frame   RADECSYS   RADECSYS or   RADECSYS or  RADECSYS 
                             RADECn        RADECn  
Coord. Rotation   LONGPOLE   LONGPOLE or   LONGPOLE or  LONGPOLE 
                             LONGPn        LONGPn 
Proj. Param. 1    PROJP1     PROJP1 or     PROJP1 or    PROJP1 
                             P1PARn        P1PARn 
Proj. Param. 2    PROJP2     PROJP2 or     PROJP2 or    PROJP2 
                             P2PARn        P2PARn  
--------------------------------------------------------------------
Notes:
  1)  i, j, ii, jj, iii, and jjj represent an image axis number.
  2)  n, nn, and mm represent a column number in a table.

-Bill Pence
 HEASARC/GSFC/NASA