[fitsbits] which WCS keywords?

[Phil Hodge]

I have a question about what world coordinate system (WCS) keywords to 
use for some image data that do not fit neatly into any of the three 
categories listed in "Representations of world coordinates in FITS," 
Greisen and Calabretta 2002, Astron & Astrophys, volume 395, pp 1061 - 
1075 (Paper I).
                                                                               
 
The three image formats for FITS files listed in Table 2 of Paper I 
are:  (a) a primary array or IMAGE extension (i.e. a conventional 
image), (b) a multidimensional array in a BINTABLE column, and (c) a 
tabulated list of pixels in a BINTABLE.  Different keywords are needed 
for these three cases because keywords for (b) include a column number 
and keywords for (c) include one or two column numbers.
                                                                               
 
The image format in question is for the Kepler spacecraft.  An array of 
CCD detectors will be used to take of images of a region of the sky.
See http://www.kepler.arc.nasa.gov/
Because of the large data volume, most of the data will be discarded; 
pixel values in a small region around each of the 170,000 or so targets 
will be returned from the spacecraft to Earth.  The (proposed) FITS data 
format is a BINTABLE extension for each chip, with two columns; one 
column has the raw pixel values and the other column has the calibrated 
pixel values.  It will not be possible to tell from the data file alone 
where a given table element is on the CCD; to avoid redundancy the pixel 
numbers are given in a separate reference table which is identified by a 
header keyword.  If the pixel numbers were in the data file, this would 
be case (c), a tabulated list of pixels.  The keywords for case (c) 
cannot be used for the Kepler data format,
however, because the keywords include the column numbers of the pixel 
coordinates, and those columns don't exist.  It seems to me that case 
(b) is reasonably close, even though the element at a given row and 
column is only a single value (1-D array of length 1).  The value in 
each row really is an image, albeit a very small one!  The keywords for 
case (b) include a column number, which in this case would be 1 for the 
raw data and 2 for the calibrated data.
                                                                               
 
My question is whether I would be stretching the limits of the FITS 
standard to use the WCS keywords for a "multidimensional array in a 
BINTABLE column" for Kepler data.  If so, is there a more reasonable 
alternative?  One point that I haven't mentioned but may be relevant is 
that the values of the WCS keywords will be nearly constant over a 
period of three months (the spacecraft will be rolled 90 degrees every 
three months).  There will be small, predictable shifts and scale 
changes due to velocity aberration, and such effects as pointing errors 
and temperature variations could introduce small, unpredictable shifts.
                                                                               
 
Phil