Understanding instrumental Stokes leakage in Murchison Widefield Array polarimetry
Abstract
This paper offers an electromagnetic, more specifically array theory, perspective on understanding strong instrumental polarization effects for planar lowfrequency "aperture arrays" with the Murchison Widefield Array (MWA) as an example. A longstanding issue that has been seen here is significant instrumental Stokes leakage after calibration, particularly in Stokes Q at high frequencies. A simple model that accounts for interelement mutual coupling is presented which explains the prominence of Q leakage seen when the array is scanned away from zenith in the principal planes. On these planes, the model predicts current imbalance in the X (EW) and Y (NS) dipoles and hence the Q leakage. Although helpful in concept, we find that this model is inadequate to explain the full details of the observation data. This finding motivates further experimentation with more rigorous models that account for both mutual coupling and embedded element patterns. Two more rigorous models are discussed: the "full" and "average" embedded element patterns. The viability of the full model is demonstrated by simulating current MWA practice of using a Hertzian dipole model as a Jones matrix estimate. We find that these results replicate the observed Q leakage to approximately 2 to 5%. Finally, we offer more direct indication for the level of improvement expected from upgrading the Jones matrix estimate with more rigorous models. Using the average embedded pattern as an estimate for the full model, we find that Q leakage of a few percent is achievable.
 Publication:

Radio Science
 Pub Date:
 January 2015
 DOI:
 10.1002/2014RS005517
 arXiv:
 arXiv:1412.4466
 Bibcode:
 2015RaSc...50...52S
 Keywords:

 polarimetry;
 antenna arrays;
 radio astronomy;
 antennas;
 Astrophysics  Instrumentation and Methods for Astrophysics
 EPrint:
 15 pages. Accepted for publication in Radio Science