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<H2><A NAME="SECTION000514000000000000000">
The Hipparcos Catalogue and the ICRS</A>
</H2>
With effect from the beginning of 1998, the IAU adopted a new
reference system to replace FK5 J2000. The new system, called the
International Celestial Reference System (ICRS), differs profoundly
from all predecessors in that the link with solar-system dynamics
was broken; the ICRS axes are defined in terms of the directions
of a set of extragalactic sources, not in terms of the mean equator and
equinox at a given reference epoch. Although the ICRS and FK5 coordinates
of any given object are almost the same, the orientation of the new frame
was essentially arbitrary, and the close match to FK5 J2000 was contrived
purely for reasons of continuity and convenience.
<P>
A distinction is made between the reference <I>system</I> (the ICRS)
and <I>frame</I> (ICRF). The ICRS is the set of prescriptions and
conventions together with the modelling required to define, at any
time, a triad of axes. The ICRF is a practical realization, and
currently consists of a catalogue of equatorial coordinates for 608
extragalactic radio sources observed by VLBI.
<P>
The best optical realization of the ICRF currently available is the
Hipparcos catalogue. The extragalactic sources were not directly
observable by the Hipparcos satellite and so the link from Hipparcos
to ICRF was established through a variety of indirect techniques: VLBI and
conventional interferometry of radio stars, photographic astrometry
and so on. The Hipparcos frame is aligned to the ICRF to within about
0.5 mas and 0.5 mas/year (at epoch 1991.25).
<P>
The Hipparcos catalogue includes all of the FK5 stars, which has enabled
the orientation and spin of the latter to be studied. At epoch J2000,
the misalignment of the FK5 frame with respect to Hipparcos
(and hence ICRS) are about 32 mas and 1 mas/year respectively.
Consequently, for many practical purposes, including pointing
telescopes, the IAU 1976-1982 conventions on reference frames and
Earth orientation remain adequate and there is no need to change to
Hipparcos coordinates, new precession/nutation models and so on.
However, for the most exacting astrometric applications, SLALIB
provides some support for Hipparcos coordinates in the form of
four new routines:
sla_FK52H and
sla_H2FK5,
which transform FK5 positions and proper motions to the Hipparcos frame
and <I>vice versa,</I> and
sla_FK5HZ and
sla_HFK5Z,
where the transformations are for stars whose Hipparcos proper motion is
zero.
<P>
Further information on the ICRS can be found in the paper by M.Feissel
and F.Mignard, Astron.Astrophys. 331, L33-L36 (1988).
<P>
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<ADDRESS>
<I>SLALIB --- Positional Astronomy Library<BR>Starlink User Note 67<BR>P. T. Wallace<BR>12 October 1999<BR>E-mail:ptw@star.rl.ac.uk</I>
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