MMA Memo #208

Polarization Calibration of the MMA: Circular versus Linear Feeds

W. D. Cotton

May 13, 1998

Keywords:

This memo considers the effects of feed polarization type on the calibration of the MMA. Calibration of interferometers with circularly polarized feeds is only weakly affected by the polarization of typical calibrators whereas interferometers using linearly polarized feeds suffers from significant coupling of calibrator polarization and the determination of gains. This latter problem can be ignored if the calibrator is unresolved, both parallel handed correlations are always used and only total intensity is to be imaged. Uncorrected phase offsets between the two polarization channels of an interferometer with linear feeds will cause mutual corruptions of Stokes Q, U and V whereas this same calibration error in an interferometer with circularly polarized feeds rotates the apparent polarization angle of linear (Q+jU) polarization but does not otherwise corrupt the image.

Short term variations in polarization channel phase offsets mix total intensity and linear polarization in interferometers with linearly polarized feeds and total intensity and circular polarization with circularly polarized feeds. A system of monitoring this phase difference may be required.

Linearly polarized feeds require a wide range of parallactic angle for the determination of the calibrator linear polarization. This means that there can be no snapshot polarization projects with such a system. This is a fundamental limitation of the science that can be obtained with such an array. Calibration of the total intensity (Stokes' I) should be straight forward with either feed type but the details of the calibration of the polarization data differ.

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Last modified: 09 December, 1999