[fitswcs] RADECSYS keyword values

[Patrick Wallace]

On Mon, 9 Nov 1998, Mark Calabretta wrote:

> Does anyone want to champion "ICRS" over "ICRF"?

I recommend "ICRF" because it's the reference frame rather than the
system.  "ICRS" means all the apparatus for defining and updating the
reference system.  All we're worried about in FITS is the reference
frame, hence ICRF is, I think, the best label to use.

n.b. The ICRF is (or is defined by) a set of extragalactic sources which
have been extensively observed by VLBI but which are faint in the
optical.  As a result of a multi-faceted campaign to link the Hipparcos
frame with the ICRF, the Hipparcos catalogue is currently the best
available implementation of the ICRF in the optical, and you could loosely
say the Hipparcos frame is the ICRF.  However, future work (especially
a second-epoch mission) will inevitably produce yet another optical
implementation of the ICRF, which is why I would use "Hipparcos"
rather than "ICRF" when dealing with optical images.

For all practical purposes at the 50mas level, J2000 FK5 and ICRF are
interchangeable.  Circa 1991, Hipparcos and ICRF were aligned to a
fraction of a mas (I'm out of town right now and can't look up the
exact figures) but the alignment is probably degrading at the rate
of a few mas per decade because of the limited proper-motion accuracy.

Patrick Wallace
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Starlink Project Manager                        Internet:  ptw at star.rl.ac.uk
Rutherford Appleton Laboratory                       Tel:    +44-1235-445372
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