Subject: HST data in FITS format

From: hanisch at stsci.edu (Bob Hanisch)
Date: 2 Apr 92 15:56:39 GMT

I just want to try to clarify a few points about HST data and FITS format.
HST data is intended to fully conform to the FITS standard, and we have taken
some pains to make the data fit into the simpler FITS structures.  We use a
variant of the FITS `random groups' format for our internal data structures,
but map our data to standard FITS images for export.  We do utilize the IEEE
floating point format for the sake of efficiency and accuracy.  Scaling the
data to 8 bits can be dangerous, as data drop outs and other bad pixels (such
as strong cosmic ray hits) can affect the scaling profoundly, putting all of
the interesting data values into just a few scaled values.

Concerning the WF/PC data formats:  the data consists of four independent 
arrays of 800x800 pixels, not a single 1600x1600 array.  In fact, the four
subarrays cannot be simply patched together to form a contiguous 1600x1600
pixel image.  There is some overlap of the images, and the four subimages are
not perfectly aligned.  To construct a single image you would have to resample
3 of the 4 frames.  Observers may wish to do this for display purposes, but
we do not advocate this or force it upon observers by providing a combined
image.

The four 800x800 images are provided to observers in one of two ways:  (1) as
a standard FITS image with three data dimensions, 800x800x4, or (2) as four
independent FITS images, each simply 800x800.  This is an option that users
specify to us when they request data.  Option (1) is preferred:  you should
bear in mind that a WFPC observation actually consists of a number of images
(the actual data, both raw and calibrated, data quality files, etc.) and if all
of these are broken up into four independent pieces it becomes very difficult
to track what is what.  However, the choice is up to the observer.  The STSDAS
FITS reading software will combine the data back together into our standard
internal format.

If anyone finds problems with HST FITS format data, I would appreciate hearing
about it.
-- 
Robert J. Hanisch                       Tel. (410)338-4910   Fax. (410)338-5090
Space Telescope Science Institute       NSI: hanisch at stsci.edu [130.167.1.2]
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