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.