I've been wondering lately-- when you run an Amplitude & Phase Self-Cal run, often times this can make certain parts of your data look anomalously high or low. Usually it's pretty mild, but a couple of times, it has been rather dramatic. A bit distressing, as I don't really understand why CALIB chose to make my amplitudes worse...
But anyway, does anyone out there have opinions on whether you should flag your data again after an A&P self cal? If you do flag, do you then copy the flag table over to the un-A&P-self-calibrated data set, and the re-self-calibrate using this new flag table? And iterate so on till no more "bad" data pops up on the A&P self cal?
Sunday, June 8, 2008
Flagging After A&P Self Calibration?
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Are you doing a phase-only selfcal before the A&P? What frequency is the data you're looking at? In general (at least the kind of data I work with), if this is happening, and especially if it's dramatic (and the data didn't look bad beforehand) it's a sign that something has gone wrong with the self-cal and playing with the parameters is probably in order...
Yes, I am doing at least one iteration of Phase-only self cal before the A&P. I've noticed this problem mostly with 20cm data I think, but maybe also with C and X band, I'm not sure.
Hmmm...but one thing I wonder is this. You can know if your calibrator has bad phases because you know its phases should all be around zero. You can flag on phase for the calibrator. However, you can't do this for your source, because the phases are all crazy and wildly range from -180 to +180. I really do believe that sometimes data is bad in phase without being obviously bad in amplitude. Maybe self cal is telling you something, if after you make all the phases behave with a 'P' self cal, and then run an 'A&P' self cal, and some new bad data pops up? Maybe it is sort of translating bad phase data (which you could not identify) into bad amplitude data (which you can identify)?
I think I've decided that if a LOT of newly-'bad' data pops up after A&P self cal, you should probably be skeptical of the self cal and do some more digging. But if just a few anomalous locations become more obviously anomalous, i would flag those.
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