Station Sensitivity Issue

Between 31st August 2015 and 7th June 2016 an issue existed where the sensitivity of some stations was reduced when observing using the HBA. The observed effect:


  • A number of stations may have had no sensitivity (i.e., not even RFI was visible in inspection plots) or obviously reduced sensitivity compared to others;

  • A number of stations may have suffered a sudden gain or loss in sensitivity during the observation;

  • The number of stations affected and the exact stations affected varied with observation and was not predictable. However:

    • It predominantly, but not exclusively, affected HBA1 core stations;

    • It was more apparent when larger numbers of stations were used: e.g., HBA_DUAL with the full array was strongly affected whereas it was only occasionally noticed in core-only observations.


When it occurred:


  • The effect appeared for any HBA observation following on from a previous HBA observation if the gap between observations was less than three minutes.

  • In a run of HBA observations, the third observation onwards would have a greater number of stations affected (possibly 10 or more).

     

The issue was caused by the HBA analogue tile beam being incorrectly set under some circumstances. There were two problems:


  1. The analogue beam was sometimes set incorrectly at the HBA tiles due to a communication failure. In the past the HBA analogue pointing was sent to the tiles at fixed intervals of 180s. An RSP Driver update issued on 31st August 2015 updated the analogue pointing only if had changed from that stored in cache. Communication failures could mean that the analogue beam was not updated when it should have been.

  2. The analogue tile beam is re-calculated every 180s (the so-called “analogue heartbeat”). If a new beam were set at a time which was not in sync with the heartbeat of a currently-set beam, setting of the new beam was delayed until the next heartbeat and the duration of the new beam was not changed to account for this difference in time. Since beam duration is currently equal to the observation duration, this could lead to the beam from an observation remaining set at the station for up to 180s after the formal end of the observation. When the gap between successive observations was one minute, this could lead to a situation where the beam of the previous observation remained set for up to two minutes at the start of the new observation. This has been the situation since 22nd March 2011, but only became noticeable when the gap between observations was reduced to one minute and only became an issue for the whole observation in combination with the other bug detailed above.


Both of these problems have now been resolved: The HBA analogue pointing is now updated every 10s, the duration of any beam will not exceed the duration of the observation and updates to the pointing have been made more robust against communication failures.

 

In the majority of cases, the effect on observations should be minimal:


Most interferometric observing runs are structured such that a long observation of the target is bookended by short observations of the calibrator of, typically, 10 minutes in length. In this case, the first calibrator will usually be unaffected and only the first few minutes of the target observation and the same for the second calibrator may be affected. The effect will be seen as low amplitudes for cross-correlation intensities on baselines where one or both stations suffer from reduced sensitivity.


The majority of beam-formed observations using the core only are unaffected. Unfortunately, it will not be possible to tell without studying the station dynamic spectra (which are only available for two months currently) or unless the beam intensity(s) are obviously reduced compared to expected.


We are very sorry, but there is no way in which data taken can be corrected for this issue.


Design: Kuenst.    Development: Dripl.    © 2020 ASTRON