Constraining Direction-Dependent Instrumental Polarisation: A New Technique for Polarisation Angle Calibration

Abstract

Direction-dependent instrumental polarisation introduces wide-field polarimetric aberrations and limits the dynamic range of low-frequency interferometric images. We therefore provide a detailed two-dimensional analysis of the Giant Metrewave Radio Telescope (GMRT) primary beam in full-Stokes at 325 MHz and 610 MHz. We find that the directional dependence is essentially independent of the feed and is dominated by the curvature of the dishes reflecting mesh. The developed beam models are used to reduce wide-field instrumental polarisation in 610 MHz observations by subtracting the expected response from the $uv$-data itself. Furthermore, a new technique for polarisation angle calibration is presented that allows for calibration using an unpolarised source and therefore can be implemented at arbitrarily low observational frequencies. This technique has the advantage that it calibrates the polarisation angle independently of ionospheric Faraday rotation and source variability. It also removes the need for known polarised sources on the sky - which are scarce at low frequencies. We use the technique to retrieve the Rotation Measure of pulsar B1937+21 at 325 MHz, finding it to be consistent with previous independent measurements. An extended version of this method may be useful for verifying the calibration of other interferometers intended for polarimetric surveys.