Issues and restrictions specific to VLA-EVLA baselines

A number of known problems is specific to crossed baselines, i.e. baselines formed by a traditional VLA antenna and an upgraded EVLA antenna. This class of problem does not occur on VLA - VLA or on EVLA - EVLA baselines, and will automatically disappear once there are no more VLA antennas left in the array. In other words, this is a transition issue.

One way to avoid these problems is to flag all VLA - EVLA baselines, but given the ever increasing number of EVLA antennas this means throwing out a lot of useful data. Obviously these problems do not affect observations which use only EVLA antennas only, such as when observing outside the classical VLA bands.

Known problems of this type are:

  • Whenever the (VLA) fluke synthesizers are reset there is the potential for phase jumps on VLA - EVLA baselines. Such resets occur a) on any frequency change, however small, and b) any change in bandwidth. In either case, the observer should bracket the source scan by calibrator scans at exactly the same frequency/fluke and bandwidth settings

    For this same reason, we do not recommend using Doppler tracking, as the fluke changes resulting from Doppler tracking have the potential to cause phase jumps. Further discussion of the use of fixed-frequency observing, including the consequences for dynamically scheduled projects, can be found in this document

  • The EVLA bandpass is enough different from that of the VLA that about a 6% closure error is present on EVLA - VLA baselines in continuum modes using 50 MHz bandwidths. This closure error will be larger in continuum modes at narrower bandwidths. The closure error can be mitigated by observing a strong source with known structure and using the AIPS task BLCAL to determine baseline-based closure corrections. For observers who are interested in continuum observations for bandwidths of 25 MHz or narrower we recommend the use of spectral line mode (i.e. pseudo-continuum), allowing the effect to be dealt with with standard bandpass calibration

    When observing in continuum mode, make sure to observe a baseline calibrator at least once every hour in order to be able to run BLCAL in the post-processing stage. This baseline calibrator should be a source with known structure, and can be anywhere in the sky. By far the best is a strong (> 1 Jy) unconfused point-source, such as a 'P' calibrator with no UV restrictions for the frequency and configuration. A slightly resolved source is acceptable, but not recommended, as an accurate model of the structure is necessary to prevent erroneous solutions. For details about determining and applying baseline solutions see the closure errors - continuum data paragraph in the post-processing section.

    When observing in spectral line mode, the standard 'channel 0' created by FILLM will be affected by these closure errors. We therefore suggest to create a new 'channel 0' from the line data. This is described in further detail in the closure errors - spectral line data paragraph in the post-processing section.

Modified on Tuesday, 29-Jan-2013 13:57:49 MST by Gustaaf van Moorsel