GMRT Technical memo: constraining direction depedent instrumental polarization

摘要: Direction-dependent instrumental polarisation introduces wide-field polarimetric aberrations and limits the dynamic range of low-frequency interferometric images. This restricts the region of the primary-beam usable for scientific measurement. The wide-field beam at the GMRT has been constrained by observations in full-Stokes at 325 MHz and 610 MHz, and found to be essentially frequency-independent across an observing band. The directional dependence is largely independent of the feed and is dominated by the curvature of the dishes reflecting mesh, causing the 325 MHz and 610 MHz polarisation beams to scale with the Stokes I FWHM. This direction-dependent polarisation averages down considerably for observations with a large range in parallactic angle. I use the developed beam model to reduce direction-dependent effects in 610 MHz uv-data. An analysis of the Stokes I beam at 610 MHz suggests the beam has a ‘squircle’shaped geometry, potentially affecting the dynamic range obtainable at the GMRT. Furthermore, I also develop a new technique for polarisation angle calibration using the wide-field response of the GMRT. This technique has the advantage that it calibrates the polarisation angle independently of ionospheric variation and source variability. It also removes the need for known polarised sources on the sky–which are scarce at low frequencies.