# Description

The **polcal** task supports solving for systematic calibration relating to the linear and circular polarization sensitivity of synthesis observations, namely, the instrumental polarization and cross-hand phase.

The heuristics of polarization calibration are described in more detail here.

See "Solving for Calibration" for more information on the task parameters **polcal** shares with all solving tasks, including data selection, general solving properties and arranging prior calibration. Below we describe parameters unique to **polcal**, and those common parameters with unique properties. Since parallactic angle is usually an important part of instrumental polarization calibration, the **polcal** task will implicitly turn *parang=True*, and it is not a user-setable parameter.

# Polarization Calibration type: *poltype*

The *poltype* parameter supports a range of modes to accommodate a variety of situations. Except where noted, these options are not basis-specific.

*'Df' -*Solve for instrumental polarization (leakage D-terms), using the transform of a specified IQU model; requires no parallactic angle coverage, but if the source polarization is non-zero, the gain calibration must have the correct R-L phase registration. (Note: this is unlikely, so just use*'D+X'*to let the cross-hand phase registration float). This will produce a calibration table of type D.*'Df+X'*- For circular basis only, solve for instrumental polarization D-terms and the cross-hand phase correction, using the transform of an IQU model; this mode requires at least 2 distinct parallactic angles to separate the net instrumental polarization and the cross-hand phase (same as position angle for circulars). This will produce a calibration table of type D. Please note: no table of type X will be produced, so you must follow this by a run of**polcal**with*poltype='Xf'*(see below)*'Df+QU'*- For circular basis only, solve for instrumental polarization and source Q+iU; requires at least 3 distinct parallactic angles to separate the net instrumental polarization from the*apparent*source Q and U. This will produce a calibration table of type D.- 'Xf' - Solve only for the cross-hand phase (same as position angle for the circular basis); best to use this after getting the D-terms from one of the above modes. Requires the observation of a calibrator with known Q+iU (or at least known U/Q). This will produce a calibration table of type X.
- '
*Dflls'*- A specialized mode which explicitly solves a linearized form of the cross-hand data for the D-terms. - '
*Xfparang+QU*' - For observations of a distinctly linearly polarized calibrator over a significant parallactic angle range, solve for the apparent cross-hand phase spectrum (channelized) and integrated (per-spw) fractional linear polarization, Q and U. This mode is intended for use prior to solving for the instrumental polarization (e.g., Df) and works for either the linear or circular basis, as long as there is adequate parallactic angle to isolate the polarized calibrator signal and cross-hand phase in the visibility cross-hands from any instrumental polarization contribution. A significantly polarized (> few %) calibrator should be used, and a non-zero polarized model prior should be specified in the smodel parameter. For the linear basis, the prior model (if reasonably accurate) will be used to correctly resolve the 180 degree ambiguity in the cross-hand phase and Q,U sign. For the circular basis, an accurate prior polarized model will achieve a good nominal position angle calibration; if the polarization model is specified without accurate prior position angle knowledge, the resulting Q,U estimate will be relative, and a traditional position angle calibration will be necessary (after solving for the instrumental polarization). *'PosAng'*- Solve directly for the absolute position angle offset spectrum, for either the linear or circular basis. The resulting calibration table stores the position angle offset directly, as an angle in radians. Note that for the circular basis (only!), this is interchangeable with the position angle calibration traditionally provided by general cross-hand phase calibration (e.g., poltype='Xf', for which position angle is 0.5 times the cross-hand phase). For the linear basis, this is the only way to directly calibrate the absolute position angle.

'Xfparang+QU' and 'PosAng' are considered "experimental" in CASA 5.5. As more experience is gained in their use, additional advice will be added here.

In each of these options, the *'f'* causes channelized solutions to be obtained, which is usually desirable for modern instruments. Omitting the *'f'* will generate strictly unchannelized solutions.