Positional Accuracy & Astrometry
In preparation for observing, the a-priori position 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 and atmospheric phases as determined on the reference source. The accuracy of the calibration determines the accuracy of the positions in the image. (Note that phase self-calibration imposes the assumed position of the model, i.e., makes the position indeterminate. Therefore, an absolute position cannot be determined after self-calibration, but relative positions between features within a self-calibrated image are valid.)
It may help to think of astrometry in 2 steps, narrow and wide-field.
In narrow-field astrometry, 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. Even under more typical conditions, 10% of the beam is readily achieved.
Astrometric calibrators are marked 'J2000 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.
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.
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. Dynamic scheduling uses the API data to run a project under suitable conditions, specified by the user. Note that VLBI projects using the phased VLA will typically be fixed date, not dynamically scheduled.
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. 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. These effects are handled somewhat differently in the post-processing packages, the current situation is described below. Improvements are in progress, check with EVLA staff.
The dominant widefield effects are differential aberration and EOP errors. Data exported from the EVLA archive as a CASA measurement set will have reasonably accurate EOP attached by the CASA FILLER, based on BIPM values pertaining to the observation date, but determined after the fact, on the date of export. These EOP differ slightly from those used at correlation (observation) time, when only predictions are available. The difference is usually negligible. However, CASA (program FIXVIS) does not, as of June 2013, correct for the differential aberration. On the other hand, AIPS corrects differential aberration (program UVFIX), but, because there is no EOP data in the archived ASDM, the UVFITS file imported by BDF2AIPS has zeros in the antenna table where the EOP should be. One can edit in the EOP and proceed with UVFIX, but it is better that the ASDM stores EOP in the first place. This feature is expected by late 2013. The capabilities of both CASA and AIPS should be equivalent in this regard by 2014.
