Phase jump on CA03

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Phase jump on CA03

Postby abutler » Fri Feb 06, 2015 8:28 pm

Hi all,

I've encountered a problem in the calibration for the secondary (2333-528) on our 21cm continuum data taken on 2014-12-28. After I calibrated the data, I checked the calibration solutions by typing the following command:

gpplt vis=2014-12-28/2333-528.2100 yaxis=phase options=xygains device=/xs

I see the plots as shown in the image attached (2014-12-28_phase_2333-528.tiff). The large, sudden jump in the XY phase for CA03 worries me. When the jump occurred (UT ~4:00), there weren't any problems going on with the array. Is that normal? And given that we are only interested in stokes I, will it matter? I didn't see this phenomenon on any of our 20 other observation days. There has been a phase drift problem on CA03 ever since sometime in November/December 2014 (see the attached vis screenshots). Could this be part of the problem? Is the data for CA03 going to be recoverable if its phases were drifting, as seen in the screenshots?

You can also see less severe phase offsets for CA01, CA05 and CA06 after UT ~11:00 (there was a problem with the array at around UT ~10:00-11:00, but another pcal wasn't necessary after the problem occurred). Is this normal? These phase offsets may be due to bad data that hasn't been flagged yet, so I will look into that...

Phases for XY gains for 2333-528 on 2014-12-28
2014-12-28_phase_2333-528.jpg (138.49 KiB) Viewed 7549 times
Phase drift for CA03 on 2015-01-19
2015-01-19.jpg (86.34 KiB) Viewed 7549 times
Phase drift for CA03 on 2014-12-24
24-12-2014.jpg (201.65 KiB) Viewed 7549 times
Posts: 11
Joined: Fri Feb 06, 2015 1:28 pm

Re: Phase jump on CA03

Postby ste616 » Wed Mar 04, 2015 1:57 pm

Hi Andrew,

This is the CA03 phase jump problem, and it has been around for quite some time, although it got much worse in the latter half of 2014. It was quite resistant to our efforts to fix it, although we think we finally found the problem mid-Feb 2015 and replaced the faulty parts.

Anyway, the problem is not that the phase jumped. A phase jump can always be accounted for via calibration, but only if you're always measuring the phase. The problem is that it is most likely that the phase jumped in between phase calibrator observations. Therefore, between those two phase calibrator observations, some/most/all of the target data will have the wrong phase assigned to it via interpolation between those two scans.

For 16cm data, this may not cause much of a problem. And certainly, if you can make an image of your target and then self-calibrate with the sky model derived from that image, you should be able to correct the data. If you can't self-calibrate, my advice would be to flag CA03 between the two calibrator scans that occur either side of the the phase jump.

The other phase drifts you see late in your run on the other antennas are perfectly normal, and are probably due to atmospheric and elevation effects. Those effects are the primary reason why you observe phase calibrators.
Jamie Stevens
ATCA Senior System Scientist
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