Thursday, August 6, 2009

Amplitude Self-Calibration and Flux Levels

If you want to get really high dynamic range, self-calibration is a necessary. (See this post for the basics of self-calibrating in AIPS.) However, getting the fluxes right (especially for weak extended emission) is tough. See below for more details.

Usually you start off with a phase calibration boxing more and more sources as you improve the model. I usually start boxing fairly conservatively and then box more freely in later phase self-calibration interations. The amplitude self-calibration, however, is slightly different in that you need to include all the emission that's real and none that's not, i.e., you need to make the best possible image of your source. If you don't include all the emission (say the diffuse stuff), then you are essentially saying that there's zero flux in that part of the image and the fluxes in the resulting image will be forced downward closer to that value.

There are a couple things to check to see how well you're doing with the flux estimates. One is to plot the visibility amplitudes vs. uv distance. They should be more or less the same. If you have amplitudes in the amplitude self-calibration data that are significantly below that in the unself-calibrated data you've got problems. Another thing to check is how much total flux you're recovering. The total clean components you use (given in the output of IMAGR) should be close to the total flux in the image and the fluxes of sources shouldn't go down after amplitude self-calibration.

Thanks to Crystal Brogan and Bill Cotton for their help with this issue.

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