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Instrumental Polarization Calibration

For the continuum polarimetry data set, it is necessary to calibrate the instrumental polarization (D) before high-quality polarization images can be formed. Accurate D calibration depends on an appropriate polarization model for the calibrator. If the calibrator is unpolarized (Stokes Q = U $\sim$ 0), and point-like^1.2, the simple total intensity model (non-zero Stokes I only) used in the G calibration can be used as a polarization model. If this is the case, the D calibration can be obtained at the same time as G by issuing an additional calC.setsolve execution before the calC.solve is executed.

If the instrumental polarization calibrator is significantly polarized (as in the case of 3C 286) a non-zero model for Stokes Q and U must be determined. This model can be obtained by imaging the full polarization image prior to D calibration, as described below. In the near future, a function for simply summing the polarization visibilities will be available to provide an initial polarization model estimate.

For the time being, the polarization model of 3C 286 is obtained by imaging the corrected data after applying the G (and P) calibration. Since 3C 286 is virtually a point source at this frequency, the Stokes I, Q, and U values are obtained using the image.statistics function in the image tool.

The calibrater tool operations used in solving for D are similar to those used to solve for G. However, note that one additional parameter--preavg--is used explicitly in the calibrater.setsolve execution. Since only one D solution is desired for the entire observation, the solution interval is set very long, and the data for each baseline within the solution interval would be averaged before the solution is determined. However, the parallactic angle varies on timescales much shorter than the solution interval^1.3. Thus, the preavg parameter is set to prevent averaging of the data beyond an acceptable timescale for the parallactic angle.