IMPORTANT: Issues with the GMRT central square baselines (23 November 2020).

This is to inform all users of a problem that we have recently discovered that may affect your data. We are currently trying to fix the issue; however, this notice is to let you know of the problem and our current advice on how to handle it. We will be in touch with more details.

In essence, we have found that for the upgraded GMRT system, the GMRT Wideband Backend (GWB) visibilities show an offset which affects the baselines between central square antennas (i.e. baselines of the form CXX - CXX). Baselines between central square antennas and arm antennas, or between arm antennas, appear to be affected much less, or not at all. The offset is most severe in Band-5, and less in Band-4 and Band-3; the typical offset in Band-5 is ~15% on the central square baselines, while that in Band-3 and Band-4 is ~5-10%. The issue appears to have affected the GWB data since October 2018. Data from the GMRT Software Backend (GSB) of the legacy GMRT system are not affected by this problem.

As of now, we recommend the following:

  • 1. Spectral line observers at all bands, but especially in Band-5, should make sure to ask for the GSB data to be recorded, in parallel with the GWB data. We expect that, in most cases, the GSB data would be sufficient for meeting the spectral line science goals. In case of questions on how to set up the GSB for your observations, please write to Nissim Kanekar at the address .
  • 2. If your science goals are based on compact sources, then we advise that you simply flag out the 91 baselines between central square antennas (CXX - CXX) throughout; this would affect your RMS noise by only ~10%, and so should not seriously hinder your science goals.
  • 3. Continuum observers interested in extended emission are advised to exclude the UV range corresponding to antenna separations <~ 1 km, while calibrating their data, so that the offset does not affect the calibration. The extent of the offset would then be visible on plotting the calibrated visibilities on the flux or phase calibrators versus UV distance (e.g. using UVPLT in AIPS). If you do see an offset in the central square baselines on doing the above, please write to C. H. Ishwara-Chandra at , and we will try to help you with the analysis.

We will update you as soon as we have resolved the issue, at which time normal procedures for using the GWB data can be followed.