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Automatic Scheduled Online Calibration

To most effectively monitor the ATCA during observations, it is useful to do an online calibration, by setting the delays in the correlator. In this way, the amplitudes and phases calculated by the correlator and displayed by vis in real-time will be meaningful, and an observer may notice any problems if they occur.
In the CABB era, observers have had to perform online calibration manually. While the pre-CABB "cacal" command still exists (albeit in a different way), it is very rarely used and it is still not foolproof, nor applicable to all modes of observing.
With the advent of the rapid-response mode, there is a need to have a reliable way to calibrate the delays automatically. While having delays set to their default values will work for any CABB mode that provides 1 MHz continuum channels, any moderate delay error will cause uncorrectable decorrelation in any 64 MHz continuum channels. While 64 MHz modes are not used for a great deal of the time, it is still useful for rapid-response over-rides to be supported while the correlator is configured in this way, so we don't miss any interesting event simply due to unlucky timing.

Automatic Online Calibration

A method has been developed which uses the command fields of CABB schedules to automatically perform the online calibration routine. With careful planning of the correct sequence of commands, and knowledge of the RFI environment, this method can be quite reliable. The method is described here, and is implemented in the rapid-response service.

64 MHz calibration

Theory
To properly calibrate a 64 MHz zooms configuration so that the continuum channels are not decorrelating is straightforward. The following steps are completely general for all bands, and if executed properly on a source which is fluxy enough, the calibration will be successful.
  1. Focus the antennas.
  2. Set the reference antenna to one which is near the centre.
  3. Enable tvmedian in the correlator.
  4. Set the attenuation servo target.
  5. Turn the attenuation servo on, and wait several cycles for the servo to work before turning it off.
  6. Tell the correlator to calculate amplitude, phase and delay error from the first zoom in each IF.
  7. Reset the delays.
  8. Increase the delay averaging to a small value to compensate for the smaller bandwidth available from the zoom bands.
  9. Set the tvchannel range to the default centre part of the zoom band.
  10. Wait for at least 3 cycles of good data on source, then give the dcal command.
  11. After the delays have been corrected, tell the correlator to calculate amplitude, phase and delay error from the continuum bands.
  12. Set the tvchannel range to a part of the band free of RFI.
  13. Decrease the delay averaging, preferably to 1.
  14. Wait for at least 3 cycles of good data on source, then give the dcal command.
  15. After the delays have been corrected, wait for at least 3 cycles of good data on source and give the acal command.
  16. After the amplitudes have been corrected, wait for at least 3 cycles of good data on source and give the pcal command.
Practice
Executing these steps properly, reliably, and as efficiently as possible, is not easy. With some testing though, the following scan setup has been shown to work.
  • Scan 1:
    • Move towards setup calibrator source at desired frequency. Since the following commands do not need visibilities to work, this scan can be set as "Normal", and only needs to be 4 cycles long.
    • Set focus with "foc def". We keep the minimum required text because the "Cmd" field has a length limit, and for efficiency's sake we want to keep the number of scans to a minimum.
    • Set the reference antenna to CA03 with "set ref ca03".
    • Ensure tvmedian is set on with "cor tvmed on on".
    • Ensure attenuator servo target is 20 with "cor tatts 20".
    • In many cases, issuing two correlator commands in one cycle (both commands go at the start of the cycle) is unreliable, in the way that anything after the first command may only get executed after another command gets sent. But since the next command we will give (in the next cycle) will be to turn the attenuator servo on, whether the target gets set now or immediately before the the servo gets turned on is irrelevant.
    • Wait for the next cycle, with "wait 2".
    • Turn the attuenator servo on with "cor atts on".
  • Scan 2:
    • Once again, the commands in this scan do not need the telescope to be on source. But we do need the telescope on source for the next scan, and thus we must mark this scan as "Dwell", so it will move on only after the telescope gets to source. We need 5 cycles for this scan; we must therefore spend 5 cycles on source, even if we have 5 cycles before getting on source.
    • Tell the correlator to use the zooms for the visibility information with "cor calband z z".
    • Reset the delays with "cor reset delays".
    • Wait for the next cycle with "wait 1".
    • Increase the delay averaging with "cor delavg 8".
    • We are still running the attenuator servo, and we should give it a few cycles, with "wait 4".
    • Turn the attenuator servo off with "cor atts off".
  • Scan 3:
    • We should now be on source, and this scan will be to do a dcal in the zoom. We thus need it to be 12 cycles long. Since we will not include this scan during data reduction, we name it something obvious to it can be flagged.
    • Reset the tvchannels to default values with "cor tvch def".
    • Wait for at least 3 cycles, with "wait 7".
    • Do the delay calibration with "cor dcal".
    • Wait for the delay calibration to take effect, with "wait 12".
    • Tell the correlator to use the continuum bands for the visibility information with "cor calband f f".
  • Scan 4:
    • Reset the tvchannels to default for the continuum bands with "cor tvch def".
    • Wait for the next cycle with "wait 1".
    • Reset the delay averaging to a single channel with "cor delavg 1".
    • Wait for at least 3 cycles with "wait 7".
    • Do the delay calibration with "cor dcal".
    • Wait for the delay calibration to take effect, and a further 3 cycles minimum with "wait 13".
    • Set the amplitude scaling with "cor acal".
    • Wait for the amplitude calibration take effect with "wait 16".
    • Reset the phases with "cor pcal".

1 MHz calibration

Theory
It's not strictly necessary for 1 MHz mode to be calibrated online, since the delay will never have an error large enough for the phases to wrap entirely within a single 1 MHz channel, provided the system is working. Still, the benefits to monitoring are worth having for most observations. Most of the same steps that work in the 64 MHz mode work in 1 MHz mode, with the difference being that instead of using two different calibration bands (zooms and then continuum), we often need to use two different calibration windows (narrow range of channels, and then wide range).
  1. Focus the antennas.
  2. Set the reference antenna to one which is near the centre.
  3. Enable tvmedian in the correlator.
  4. Set the attenuation servo target.
  5. Turn the attenuation servo on, and wait several cycles for the servo to work before turning it off.
  6. Set a narrow, RFI-free tvchannel range.
  7. Reset the delays.
  8. Ensure that the delay averaging is set very small, preferably 1, to ensure we don't alias on phases that are potentially wrapping very quickly.
  9. Wait for at least 3 cycles of good data on source, then give the dcal command.
  10. After the delays have been corrected, set a much wider tvchannel range.
  11. Increase the delay averaging to a large value to ensure that RFI doesn't affect the calculation of the delays.
  12. Wait for at least 3 cycles of good data on source, then give the dcal command.
  13. After the delays have been corrected, wait for at least 3 cycles of good data on source and give the acal command.
  14. After the amplitudes have been corrected, wait for at least 3 cycles of good data on source and give the pcal command.
Practice
  • Scan 1:
    • Move towards setup calibrator source at desired frequency. Since the following commands do not need visibilities to work, this scan can be set as "Normal", but since we will be cycling the attenuator servo, it needs to run for about 9 cycles.
    • Set focus with "foc def".
    • Set the reference antenna to CA03 with "set ref ca03".
    • Ensure tvmedian is set on with "cor tvmed on on".
    • Ensure attenuator servo target is 20 with "cor tatts 20".
    • Wait for the next cycle with "wait 2".
    • Turn on the attenuator servo with "cor atts on". The rest of this scan is spent cycling the attenuator servo.
  • Scan 2:
    • Once again, the commands in this scan do not need the telescope to be on source. We need 4 cycles for this scan.
    • Turn the attenuator servo off with "cor atts off".
    • Wait for the next cycle with "wait 2".
    • Reset the delays with "cor reset delays".
    • Decrease the delay averaging with "cor delavg 1".
  • Scan 3:
    • Still don't need the telescope on source, but we do need the next scan to start on source, so we mark this scan as "Dwell". We need 4 cycles for this scan.
    • Reset flagging to default with "cor fflag f1 def;cor fflag f2 def".
    • Flag the birdies with "cor fflag f1 birdies;cor fflag f2 birdies".
  • Scan 4:
    • We should now be on source. We need this scan to be 21 cycles long, as we have a lot to do.
    • Wait for at least 3 cycles with "wait 7".
    • Do the delay calibration with "cor dcal".
    • Wait for the delay calibration to take effect with "wait 11".
    • Reset the tvchannels to default with "cor tvch def".
    • Wait for next cycle because we really need the next command to take effect immediately, with "wait 12".
    • Increase the delay averaging for RFI tolerance, with "cor delavg 64".
    • Wait for at least 3 cycles with "wait 17".
    • Do the delay calibration with "cor dcal".
  • Scan 5:
    • This last scan is for the amplitude and phase calibrations and needs to be only 10 cycles.
    • Wait for at least 3 cycles with "wait 7".
    • Do the amplitude calibration with "cor acal".
    • Wait for the next cycle with "wait 8".
    • Do the phase calibration with "cor pcal".

Original: Jamie Stevens (22-May-2019)