[fitswcs] FK5-ICRS

[Andreas Wicenec]

Don Wells wrote:
> 
> 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.

The differences between FK5 J2000 mean equinox and equator and IRCS
should be less than 80 mas by definition of the ICRS (Arias et al., A&A,
303, 604, 1995). This number is just the uncertainity of the FK5 system.
BUT: The *realisation* (reference frame) of the FK5 system at J2000 by
the GSC is actually poor, not only because of the quality of the GSC
reduction but also because of the mean epoch of the POSS and SERC plates
used to construct the GSC.

> 
> 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!).


Even with those new reductions going on with GSC and USNO-Ax the
realisation of the ICRS by those catalogs will be not very good, because
of the epoch problem mentioned above. Even the Hipparcos frame will
degrade very soon because of the errors in proper motion determination.
The only way to maintain the accuracy is a follow-up astrometric
mission. The ICRS will remain stable in the radio regime, because the
IERS will continue to monitor some 600 radio sources to maintain this
system.

Andreas
-- 
Andreas Wicenec				awicenec at eso.org
ESO - European Southern Observatory
Tel.: (+49) (0)89 32006 470
Fax:  (+49) (0)89 32006 480