Capabilities of the VLA+PT Link

The VLA is not equipped at 13 and 50 cm, so those frequencies are not available to be used with the link. All the VLA frequency bands (400, 90, 20, 6, 3.6, 2, 1.3 and 0.7 cm) are available to be used with the VLA+PT link. In particular the 400 cm band is a new addition for the January 2002 configuration, although users are cautioned that the use of this band will be highly experimental and good results are not guaranteed.

The use of 400 and 90 cm simultaneously (4P band in VLA parlance) is now supported on the VLA+PT link. However, Jobserve does not at this time recognize this capability. To use this mode, contact Mark Claussen at mclausse@nrao.edu or at (505) 835-7284.

The usual caveats as to radio-frequency interference apply at the lower frequency bands. (See the VLA documentation on Special Observing Modes and the VLA Observational Status Summary for further VLA information.)

The image sensitivity of the VLA+PT link for a given frequency band is that of the VLA, with uniform weighting of the visibilities. Using natural weighting will weight the baselines to PT down so far that the full resolution will not be realized.  In the imaging step, Briggs' robust weighting scheme should likely be applied to obtain the best match between sensitivity and resolution desired.

The technique of fast switching, used extensively with the VLA at 1.3 and 0.7 cm in order to reduce phase variations arising in the troposphere at high frequencies, requires that the two computer systems controlling the VLA and PT be synchronous. This is not necessarily the case, as at the moment there is no direct communication between the two, and the scheduling is done in LST in the VLA case, and UT in the VLBA case.  We have implemented and tested a method whereby fast switching is possible but it may not work efficiently at cycle times less than about 2 minutes. We strongly recommend, that, immediately previous to beginning a fast switching scan, that a short (~30 seconds), normal scan on the primary source be inserted, to guarantee that both the VLA and PT are on the source before fast switching begins.

The PT VLBA antenna does not have referenced pointing enabled. Thus when this technique is being used on the VLA, the PT antenna will not actually do any pointing. Since the pointing on the VLBA antennas is better than most VLA antennas, we don't think that this will adversely affect high-freqeuncy observing programs.

In our testing to date, we have found that phase drifts due to the Earth's ionosphere are a significant contributor to the phase change over time on scales of several to tens of minutes. Thus, we strongly suggest that daytime observations at low frequencies (20 cm and longer) not be attempted. This warning is echoed in Barry Clark's newsletter articles. This time of maximum solar activity only exacerbates the problem.