Recent Updates from the Observatory
On 10 KHz offset in the Local Oscillator in observations done with TGC : 24 Sep 2021
- 1. For continuum imaging observations: The LO offset of 10 KHz is unlikely to make any significant effect, and no corrective action may be needed except for the user to be aware of the slight shift in the RF band.
- 2. For spectral line observations: The LO offset of 10 KHz corresponds to a velocity offset of 2.1 km/s at ~ 1420 MHz and higher at lower frequencies by a factor of 1420/f, where f is the central frequency of the spectral line of your interest in MHz. To correct this offset, it is recommended to add +10 KHz to the frequency in the header before carrying out any data reduction.
- 3. For beam mode observations: The LO offset of 10 KHz introduces a systematic shift in frequencies interpreted by the analysis software. This can lead to an error in the DM estimates between 0.0005 to 0.001 pc-cm^-3 and a residual DM smear in the time-series due to dedispersion to incorrect frequencies. This can also introduce a small shift in the time-of-arrival of data. The magnitude of these errors depend on DM of the source being observed and will vary from one source to another. It also depends on observing frequency. The most significant effect is likely in Band 3 and 4, while Band 5 data is not affected except for very large DM sources. The header for all data taken with TGC system prior to 29 July 2021 needs to be corrected by adding +10 KHz to the frequency in the header before carrying out the data reduction.
Issues with the GMRT central square baselines : 23 November 2020
- 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 firstname.lastname@example.org .
- 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 email@example.com , and we will try to help you with the analysis.
This is to inform all potential users whose data might have been affected by a recently discovered issue about +10 KHz offset in the local oscillator settings in the observations conducted through the new control and monitor system of GMRT, the TGC, till 29th July 2021. This notice is to let you know of the problem, how to identify if your data suffered from this issue, and our current advice on how to handle it.
In essence, we have found that for the upgraded GMRT system, the local oscillator is set with an additional 10 KHz value for all observations carried out with the TGC (the new monitor and control system for the upgraded GMRT), from 18th October 2019 till 29th July 2021. It has to be noted that between 18th October 2019 and 5th January 2021, not all observations were done with TGC. Users can check the "ONLINE Machine" entry in the respective GTAC Logs to see if the observations were done using the TGC. Observations for which "ONLINE Machine" is not TGC (but is for e.g. "Shivneri/Lenyadri") are not affected by this problem.
The offset is always positive. For example, if the LO frequency is set to 500 MHz, the actual LO set is at 500.01 MHz.That is the offset is always positive. We have fixed the issues of 29th July 2021 and all observations carried out after this date don't have this problem. However, the implications of the offset vary from observations to observations, we describe below the possible implication of the above problem, for the 3 main modes of observations with the GMRT. In case your data needs correction, the best option is to add +10 KHz to the frequency in the header before carrying out any data reduction.
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:
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.