[fitswcs] RADECSYS keyword values

[Arnold Rots]

My interpretation was that a specific collection of sources defined
ICRF, just like the Hipparcos catalog defined the Hipparcos frame.

>From these frames, a reference system, ICRS, was derived that is meant
to be used as the successor of FK5, to express the coordinates of any
object in an inertial frame.  Though one might be justified in
labeling the coordinates of sources that were used in the definition
of the ICRF with RADECSYS='ICRF', it would seem to me that 'ICRS' is
the proper designation for everything else.

Hipparcos is really a galactic frame.  The accuracy of the tie was
0.6 mas in orientation, 0.25 mas/yr in rotation, at epoch 1991.25.

  - Arnold Rots

Pat Wallace wrote:

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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Arnold H. Rots                                         AXAF Science Center
Smithsonian Astrophysical Observatory                tel:  +1 617 496 7701
60 Garden Street, MS 81                              fax:  +1 617 495 7356
Cambridge, MA 02138                             arots at head-cfa.harvard.edu
USA                                     http://hea-www.harvard.edu/~arots/
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