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
a (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