Selecting a Phased-Array Mode

The strength and structure of your VLBI target source and the VLA configuration available determine the appropriate type of phased-array observations. Strength guidelines were given in Section 9.2. Structure guidelines are based on the fact that the VLA cannot be phased up (VLA mode VA) on resolved sources. To determine whether structure will be a problem, you must know what configuration the VLA will be in during your VLBI observing. Use the fact that at 5.0 GHz in the A configuration the FWHM of the (naturally weighted) instantaneous synthesized beam is about $0.5''$, and the scaling factor between adjacent configurations is 3.285. VLA mode VA will work on a strong enough source if the VLA configuration is small enough that the entire VLBI target source is unresolved, or if the VLA configuration is large enough that any structure adjacent to the VLBI target source is resolved out; in the latter case, structure larger than about ten times the instantaneous synthesized beam will be resolved out.

Use VLA mode VX if the source is too weak to phase up on and/or is strongly resolved. In this case occasional scans, in mode VA, of a VLA phase calibrator must be inserted into the VLA observing file, so the array can be phased up. In the A and B configurations, in particular, the VLA should be phased at intervals of only 10-30 m and when moving more than about $15^{\circ}$. Use the shorter phasing interval at the shorter wavelengths. Phasing up from scratch, after changing frequencies or after observing in normal interferometer mode, requires a dwell time of about 3 m on the phase calibrator; VLA slew times plus a maximum of 30 s of VLA setup time should be added to this 3 m. Touching up the phasing after a mode VX scan usually requires about 1 or 2 m on the phase calibrator; again, VLA slew and setup times should be added to this calibrator dwell time.

Phase calibrators should be selected from the ``VLA Calibrator Manual''. If your VLBI target source is strong enough to be in the Manual, then you can use the Manual's structural information to decide whether to observe it in mode VA or mode VX. For your configuration and wavelength, mode VA will work fine on quality ``P'' or ``S'' sources, provided the ANTSOL limits tabulated include all baselines in the configuration. If these conditions are not met then observe your VLBI target source in mode VX, with occasional mode VA scans of a suitable nearby phase calibrator.

At frequencies of 1.7 GHz and lower, sporadic RFI can corrupt the phasing during a mode VA scan. If these corrupted mode VA phases are held during the subsequent mode VX scan, then the array will be poorly phased while on your VLBI target source, meaning that the VLA's collecting area could be seriously degraded. If such interference problems are expected, then you should instruct the VLA operator to stop mode VA phasing scans as soon as the array is phased and to ``skip into'' following mode VX scans. Such action will avoid conducting mode VX scans with interference-corrupted mode VA phases. Visiting observers can provide such VLA operator instructions verbally, while absentee observers can relay such special instructions to the VLA operator via notes in the program cover information.