Subject: a FITS puzzle, suggestions welcome

From: sla at helios.ucsc.edu (Steve Allen)
Date: 19 Sep 91 04:26:32 GMT

Here at Lick the engineering shops have begun construction of a dewar
which will eventually hold a 2x2 mosaic of Ford 2048**2 chips.
The adjacent edges of the CCDs will be cut off so that they can be butted
together with very little gap.

When the entirety of all 4 chips is readout, we expect that the best
method of storing these images is as a 3-d FITS array.  This is exactly
akin to the method used in the HST's WF/PC images, which has a very
similar layout (conceptually).

The question we have not yet answered is this:
What happens when subregions of the CCDs are readout as part of a single
exposure?  More precisely, if the subregions of the 4 CCDs do not have
the same dimensions, how can these images be stored as a FITS image?

Obviously we could simply pick the largest dimensions in each direction,
and pad the edges of the images with Null values for pixels which were not
read.  However, when dealing with up to 4096x4096 pixels, these
Null values begin to take up A LOT of space on disk, tape, whatever;
it would really be preferable not to store them at all.

Again, we could store such images as 4 separate FITS files, but that is
also unappealing.  We would like to keep data from the same exposure in
one file.

Any suggestions are welcome.  Let's see how busy this newsgroup might be.

Steve Allen
sla at helios.ucsc.edu
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