Heuristics and Strategies for Polarization Calibration

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4.4.5.1 Heuristics and Strategies for Polarization Calibration

Fundamentally, with good ordinary gain (and bandpass, if relevant) calibration already in hand, good polarization calibration must deliver both the instrumental polarization and position angle calibration. An unpolarized source can deliver only the first of these, but does not require parallactic angle coverage. A polarized source can only deliver the position angle calibration also if its polarization is known a priori. Sources that are polarized, but with unknown polarization, must always be observed with sufficient parallactic angle coverage, where ”sufficient” is determined by SNR and the details of the solving mode.

These principles are stated assuming the instrumental polarization solution is solved using the ”linear approximation” where cross-terms in more than a single product of the instrumental or source polarizations are ignored in the Measurement Equation (see § E). A more general non-linearized solution, with sufficient SNR, may enable some relaxation of the requirements indicated here, and modes supporting such an approach are currently under development.

For instrumental polarization calibration, there are 3 types of calibrator choice:

CASA Polarization Calibration Modes


Cal Polarization	Parallactic Angles	model	polmode	Result

unpolarized	any	set Q = U = 0	’D’ or ’Df’	D-terms only
known non-zero	2+ scans	set Q,U	’D+X’ or ’Df+X’	D-terms and PA
unknown	3+ scans	ignored	’D+QU’ or ’Df+QU’	D-terms and source

Note that the parallactic angle ranges spanned by the scans in the modes that require this should be large enough to give good separation between the components of the solution. In practice, 60^∘ is a good target.

Each of these solutions should be followed with a ’X’ solution on a source with known polarization position angle (and correct Q + iU in the model). ALERT: polmode=’D+X’ will soon be enhanced to deliver this automatically.

The polcal task will solve for the ’D’ or ’X’ terms using the model visibilities that are in the model attached to the MS. Calibration of the parallel hands must have already been carried out using gaincal and/or bandpass in order to align the phases over time and frequency. This calibration must be supplied through the gaintable parameters, but any cal-tables to be used in polcal must agree (e.g. have been derived from) the data in the DATA column and the FT of the model. Thus, for example, one would not use the cal-table produced by fluxscale as the rescaled amplitudes would no longer agree with the contents of the model.

Be careful when using resolved calibrators for polarization calibration. A particular problem is if the structure in Q and U is offset from that in I. Use of a point model, or a resolved model for I but point models for Q and U, can lead to errors in the ’X’ calibration. Use of a uvrange will help here. The use of a full-Stokes model with the correct polarization is the only way to ensure a correct calibration if these offsets are large.

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