Correcting the gains for polarization of the calibrators

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Next: Establishing the flux density scale Up: Basic Calibration Fundamentals Previous: Solving for complex gain, bandpass and leakage

Correcting the gains for polarization of the calibrators

The gain solutions were generated with an unpolarized unit point source model for all calibrators unrecognized by Imager.setjy. In practice calibrators often have noticable linear polarization of up to 10% or so. This is not a problem for telescopes with circular feeds, but for telescopes with linear feeds, like the ATCA, this causes a small time variation in the gains proportional to the percentage polarization of the calibrator. The calibrator polarization can be determined from the variation of the X and Y gain solutions with changing feed position angle. After determining the polarization we can correct the gains for this effect. Note that this only works correctly if the calibrator has been observed over a sufficient range in position angle. Generally 5 or more scans with a span of at least 90 degrees in position angle is sufficient. The quality of the polarization solution can be checked by looking at the fit error, it should generally be smaller than 0.5% in Q and U. The correction is performed using the Calibrater.linpolcor function.

cal.linpolcor(tablein='cal.G', fields="0405-385 2254-367");

This corrects the calibration table entries for sources 0405-385 and 2254-367 and updates the table. If you add a tableout argument, it will write the updated table to a new file. (Note: using double quotes like in the example above produces a vector of values, split at each space, another way to specify the fields argument would be: fields=['0405-385','2254-367']).

Next: Establishing the flux density scale Up: Basic Calibration Fundamentals Previous: Solving for complex gain, bandpass and leakage Contents