On Thu 2004-03-11T12:20:12 -0500, Thomas McGlynn hath writ:
My feeling is that if the two elements of data are so tightly
coupled that they would normally be put in the same FITS file,
then one would consider them to be the same dataset. At the HEASARC
datasets generally comprise more than one FITS file. I can't think
off the top of my head of an example where a given FITS file would
naturally split up into multiple IDs.

We are currently experimenting with a new observing mode for a
mosaic of CCDs on an echelle spectrograph. The notion is that
if the red end of the spectrum has a very different flux than the
blue end, it might be good to run multiple exposures of one CCD
while running one exposure of another CCD.

For example, if the blue flux is low enough to be limited by readout
noise then that CCD wants one long exposure. But if the red flux is
notably higher then it might be nice to run two or three sub-exposures
there so as to be able to remove cosmic rays.

Although our ideal plan would be to write all of the readouts into
separate IMAGE extensions of one FITS file, engineering constraints
may cause us initially to write the different sub-exposures into
separate FITS files which we would want to recombine later.

So I am not convinced that the rules about splitting files and
combining HDUs are simple enough not to require the IDENT in
each of the HDUs.

--
Steve Allen UCO/Lick Observatory Santa Cruz, CA 95064
Voice: +1 831 459 3046 http://www.ucolick.org/~sla
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