Full Characterisation of the Polarimetric Primary Beams of the GRAO 32-m Telescope

Abstract

Direction-dependent instrumental polarisation is a major systematic limitation in high-fidelity single-dish radio polarimetry, yet a unified characterisation of beam leakage and the conditioning of polarisation recovery is lacking for the Ghana Radio Astronomy Observatory (GRAO) 32-m telescope. We aim to establish a quantitative, direction-dependent polarimetric beam model at 5.0 and 6.7 GHz. High-resolution GRASP electromagnetic simulations of the nominal telescope configuration are used to derive the full complex Jones response and corresponding Mueller matrices, from which beam shape, beam squint, instrumental Stokes leakage, and intrinsic cross-polarisation ratio (IXR) are evaluated across the primary beam. The Stokes I beams have half-power beam widths of 396.4 and 295.3 arcsec at 5.0 and 6.7 GHz, respectively, with main-beam efficiencies of 56.4 and 55.6 per cent. At 5.0 GHz, the circular-polarisation beam squint is 9.96 arcsec (2.5 per cent of the HPBW), whereas no statistically significant squint is detected at 6.7 GHz. Leakage from Stokes I into linear polarisation remains below 0.1 per cent within the half-power beam but increases substantially towards the sidelobes. IXR reaches approximately 80 dB on axis and decreases with angular offset. These results establish the intrinsic electromagnetic polarimetric response of the GRAO 32-m telescope and provide a quantitative baseline for direction-dependent calibration and subsequent observational validation.