Description

task 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.

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.