Re: Compression

[hjohnson@pluto.njcc.com (Herbert R Johnson)]

On Fri, 23 Oct 1998 14:26:42 -0700, Chris Albertson <chris@topdog.pas1.logicon.com> wrote:
*>
*>No, FITS is a plain n dimensional array.  In the FITS user guide
*>is a reference to a _proposed_ compression standard.(Warnock et al)..

As Arne says, there is no FITS standard.

*>Didn't someone here on this list look into compression and it's
*>effect of photometry?  I'm sure I saw it written up complete
*>with some plots.

It was well discussed.

*>Delta encoding works OK for images (After whacking off 
*>the noisy low order bits) but it does not take advantage of the
*>2 (or more) dimensional-ness of the data.

That is certainly true. Compression schemes that "match" the data to
be compressed do better. Schemes that produce "acceptable" data loss
do better still.

*>I don't think this is a big deal.  Currently you can keep _all_ the
*>raw MkIV data at a pretty low cost on tape or CD. 

As previously discussed, a night's viewing on Mark III will fit on ONE CD-ROM:
but Mark IV's proposed night's data may not. There was no consensus on tape
in our discussion; the consensus was that CD-ROM was a very convenient
medium that did not suffer from incompatibilities as did tape. I don't
care to reopen that discussion as I have nothing to add to it: but
I did want to introduce the notion to (eventually) establish the
dynamic range (i.e. reliable number of bits) from the Mark IV.

Herb Johnson

Herbert R. Johnson              http://pluto.njcc.com/~hjohnson
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