Polarization Calibration (SJ GEORGE, P ALEXANDER, D FORD)

Dr. Sam George was appointed as a replacement for Joern Geisbuesch and started work on 1 June 2011.

Accurate measurements of instrumental polarization and correction of polarization cross-talk between the components of the Stokes vector is critical to the science of polarimetry. Measuring the Stokes vector resulting form the instrument, we can retrieve the Muller matrix using a non-linear-least-squared fit based on the known fractional form of the matrix of the calibration optics assuming a static instrument Muller matrix. Good polarization calibration must deliver both the instrumental polarization and the position angle calibration. An unpolarized source allows one to determining the instrumental polarization and does not require parallactic angle coverage. A polarized source can only give the position angle calibration and the polarization needs to be known a priori. Sources with unknown polarization need sufficient parallactic angle coverage.

Generally the instrumental polarization is solved using a linear approximation where the cross-terms in more than a single product of the instrumental or source polarization are ignored in the measurement equation. This is not a sufficient approach for data where there is high (of order a few percent) instrumental polarization leakage, for this we need a generalised non-linearised solution. This is of course, important for high dynamic range wide field total intensity imaging.

The current implementation in Common Astronomy Software Applications package (CASA) uses the small amplitude approximation. Though this works fairly well on observations with small leakage terms the residual leakage is still seen in the maps produced. We are implementing a non-linear method that removes this restriction. In python (using CASA to access the measurement set) we have implemented a task which extracts the useful data from the measurement set efficiently. The observations of a known unpolarized source is then used as a model of the leakage and this signal is removed on a channel by channel basis whilst making the assumption that this is non-time varying. This has been applied to both test data and 610~MHz Giant Metrewave Radio Telescope (GMRT) data. The calibration returned gives a similar scatter to that obtained via CASA's `polcal' task. An improved method of solving the full instrumental Muller matrix is being implemented. Currently, the routine does a fit to the data and returns the Muller matrix per channel, though at the moment when the inverse is applied to the data the results are disappointing.

A new set of plotting tools have also been developed inside of CASA using matplotlib - these allow for quick visualisation of the polarization calibration data. For example, these animated plotting of Stokes Q,U as a function of time for all channels, plotting Q/I vs U/I for all times as a function of channel and antenna, showing a given number of antennas.

Deliverable #1: Improved CASA polarization calibration routine and associated plot tools Completion date: October 2011

Having completed this first deliverable we will develop the work to concentrate on the wide-field aspects Man months in this period: 3 month (SG) 0.1 month (PA) 0.2 month (DF)