3.2. Coordinate Systems at the VLA

      The VLA supports the J2000 coordinate system.  Details on
this implementation are given by Barry Clark in VLA Computer
Memorandum 167 of May 11, 1983.  The complete description of the J2000
coordinate system can be found in the USNO circular 163 edited by
G. H. Kaplan of the Naval Research Laboratory.  See also Johnston et
al 1995 (AJ 110, 880).  In summary, positions given in the J2000
system will be precessed in accordance with the recommendations of
USNO 163.  Positions given in the B1950 coordinate system will employ
adjustments, so that they are effectively processed by the
recommendations of the Explanatory Supplement of 1960.  Positions of
any other epoch will, currently, be precessed by the recommendations
of the Explanatory Supplement of 1960.  The most serious consequence
of this is that planetary coordinates given in apparent coordinates of
observing date are assumed to be in the system of the FK4.  It seems
likely that as soon as the system of FK5 comes into greater use for
the production of planetary ephemerides, we shall reverse this
decision, and use the FK5 and the formulae of USNO 163 for epochs
other than 1950.

   The B1950 system used at the VLA is a bit of an historical oddity.
The VLA needed calibrators with accurate positions before the various
astrometric (VLBI) lists existed, so they were measured using a
reference frame based on a handful of positions (of order 10) from
observations with the Green Bank three-element interferometer in
1979.9; those sources were therefore all referred to the B1950 system,
epoch 1979.9. When the astrometric lists started coming out from JPL
and Goddard, NRAO both added new sources and replaced the old
positions with the new, more accurate measurements; these are in the
J2000 system, precessed to epoch 2000.0.  So the VLA has two
fundamental systems: the B1950, epoch 1979.9 frame most natural for
the old calibrators, and the J2000, epoch 2000.0 frame most natural
for the new ones.  In transforming from one frame to the other,
e.g. to get a B1950 position for a source originally measured in
J2000, both the VLA on-line computers and the OBSERVE program which
creates the VLA schedule are "hard-wired" to precess between (B1950,
epoch 1979.9) and (J2000, epoch 2000.0).  All positions measured at
the VLA are in one of these two frames.  Just recently positions
from the reference frame of Eubanks 1995-1 have been adopted 
for 475 sources in the calibrator manual, replacing many less
accurate positions, and bringing the VLA and VLBA onto the same
reference frame.  

      It now seems profitable for anyone interested in the highest
positional accuracy to use J2000 coordinates for all future
observations, unless compatibility with previous observations is
critical and the whole series does not span enough time to be unduly
confused by the known error in Newcomb's precession constant.  (It is
probably less work to put previous observations in J2000 coordinates
than it is to calculate the corrections to the 1950 coordinates of
various dates, if the observations span more than a couple of years).
For the general VLA user, the pressure to change systems is not so
strong--he/she must consider whether it is more important that his
observation remain compatible with previous observations of the
object, or whether it should be compatible with future accurate
astrometry, either radio or optical.  We do not recommend changing to
J2000 coordinates for an object which you have observed here before,
and might conceivably wish to combine the old (u,v) data--there is no
point in just asking for trouble.  However, we encourage the use of
J2000 coordinates for new observations.  It seems inevitable that
J2000 coordinates are going to come into general use, and the sooner
we can get through the painful transition period, the better off we
all shall be.

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