TASS image compression

["Robert J. Bradbury" <bradbury@aeiveos.com>]

I have been looking at the problem of image compression
of the TASS data as Tom has indicated to me that there
has been some discussion of this in the past.

However what I find in the mail archives seems to be
five or more years old.

So I've decided to document and experiment a little bit
with the data and programs which may be available.

For documentation, see: [1].

Worth noting is that the FITSPRESS and compFITS program
sources seem to be unavailable [if anyone has these or
knows who might please let me know].

So, working with the HCOMPRESS program from STSCI it
looks like TASS images can be compressed...

With scale factors of 1, 2, 4, 8, 16, 32, 64 & 128,
the hcompress approach (actually fcompress script) reduces
the normal TASS FITS image from 8,415,350 bytes (~8 MB)
to a fractional size as follows:
 1: 0.48, 2: 0.46, 4: 0.41, 8: 0.35, 16: 0.29, 32: 0.23,
 64: 0.16, 128: 0.10

So, using a scale compression factor of 128 one reduces
the size of the images by about an order of magnitude.
The question then becomes where, if one uses a potentially
lossy compression strategy, a reduction in the image information
begins to significantly impact the accuracy of the data.
(As the discussions I've seen would tend to suggest that we
don't have a good feel for the sources of and/or long term values
and trends of errors in the raw data.)

But there has to be some level at which semi-lossy compression
is not going to negatively impact the data coming out of the
reduction pipeline.  Of course, when one is trying to deal
with difficult cases (identification of binaries, etc.) one
may want to return to the original data -- but for most of
what one wants to do the question is can the data be dealt
with in a compressed (noise-reduced?) format?

Robert

1. http://www.aeiveos.com/~bradbury/Astronomy/AIC.html
[If anyone has any good references to be added to this page
please send them to me.]