Positional Accuracy
In preparation for observing, the a-priori position of a target must be known to within the antenna primary beam, except perhaps for mosaicing observations. In the special case of using the phased VLA as a VLBI element, the a-priori position must be accurate to within the synthesized beam of the array.
In post-processing, target positions are typically determined from an image made after phase calibration, i.e. correcting the antenna phases as determined on the reference source. (Note that phase self-calibration imposes the assumed position of the model, i.e., makes the position indeterminate.)
It may help to think of astrometry in 2 steps, narrow and wide-field. In the former, the target is close to the phase tracking center and the antennas nod every few minutes between the target and a calibrator. Under good conditions of phase stability, accurate antenna positions, (so-called 'baselines'), a strong target, a close calibrator with accurately known position, and rapid switching, the accuracy can approach 1-2% of the synthesized beam, with a floor of ~2 mas. Under more typical conditions, 10% of the beam is readily achieved.
Astrometric calibrators are marked 'A' in the VLA calibrator list, and have an accuracy of ~2 mas. Other catalogs from the USNO and the VLBA are also useful, but offsets may exist between the VLA and VLBA centroids, arising from extended structure in the particular source, and the different resolutions of the arrays. Phase stability can be assessed in real time from the Atmospheric Phase Interferometer (API) at the VLA site, which uses observations of a geostationary satellite at ~12GHz.
For studies of proper motion and parallax, the absolute accuracy of a calibrator may be less important than its stability over time. Close or in-beam calibrators with poor a-priori positions can be used, and tied to the ICRF reference frame in the same or separate observations.
The widefield case is to determine the positions of targets within the primary beam, referenced to a calibrator within the beam or close by. In addition to the previous effects, there are distortions as a function of position in the field, from small errors in the Earth orientation parameters (EOP) used at correlation time, differential aberration, and phase gradients across the primary beam. These effects are handled somewhat differently in the various reduction packages. With no special effort, the errors build up to roughly ~1 synthesized beam at a separation of ~10^4 beams from the phase tracking center. Not all these errors are fully understood, and accurate recovery of positions over the full primary beam in the wideband, widefield case is a research area.
