[fitswcs] Exhortation

[Don Wells]

Peter Bunclark writes:
 > ``.. the HST images were off in HFF by as much as 500_mas..''
 > Presumably that was because of the poor precision of GSC, not due to the
 > mechanism by which the coordinates were described?

I expect that the differences between the FK5 reference frame of
the GSC to which HST HFF data conform and the ICRS reference frame to
which the MERLIN and VLA imagery conform did contribute to the
discrepancies. The question I asked after the talk was whether this
was the only problem. The answer I got was that inadequate
compensation of distortions within the WFPC imagery was also
significant.

My understanding is that the (ICRS-FK5) reference frame problem is
well-understood by the pundits, and that we can expect that they
(e.g. Pat Wallace) will supply the rest of us (e.g. Bill Pence) with
code which we can incorporate in our applications to fix up CRVALi in
archival data which is referred to FK4/FK5/etc. I have heard that
there is already a new GSC which incorporates the (ICRS-FK5) change in
the star coordinates, and that Dave Monet is expected to deliver a new
version of his gigantic star catalog referred to the ICRS frame
'real-soon-now', so we can expect that future astronomy data
processing will be referred to ICRS automatically. Obviously we need
to agree on FITS conventions to indicate the reference frames for our
imagery; this is a FITS notation issue which the G&C paper addresses
in great detail (thanks, Pat!).

The description of distortion corrections is a FITS issue. The
relevant polynomical coefficients were probably present in FITS TABLE
extensions and/or special keywords in the FITS datasets which these
radio astronomers were analyzing.  These distortion notations are
proprietary to the HST pipeline processing and SDAS, and the radio
astronomers did their image comparisons in other software packages
(Classic-AIPS?)  which ignore the HST notations, so astronomy is
suffering because we (those of us on this mailing list) have not done
our job. We need to agree on a canonical interchange notation for
distortion corrections.

-Don