Reliability of uGMRT Band-4 Polarimetry: Results from a Quadrature Hybrid Polarizer Bypass Experiment

TL;DR

This study identifies a systematic polarization calibration issue in uGMRT Band 4 caused by the Quadrature Hybrid polarizer and demonstrates that bypassing it significantly improves polarimetric accuracy, enabling reliable sub-GHz polarization measurements.

Contribution

The paper reveals the Quadrature Hybrid polarizer as the main source of calibration instability and shows that bypassing it enhances polarimetric performance in uGMRT Band 4.

Findings

Instrumental leakage reduced from 10-15% to 2-5%.

Residual leakage after calibration decreased to less than 0.2%.

Accurate polarization angle rotation measured after bypassing the QH.

Abstract

Polarimetric observations at sub-GHz frequencies offer unique access to the magnetized universe through Faraday rotation and depolarization studies, but achieving reliable polarization calibration at these frequencies remains challenging. We report the identification and resolution of a systematic polarization calibration instability in the upgraded Giant Metrewave Radio Telescope (uGMRT) Band 4 (550--750\,MHz). Through diagnostic observations of multiple calibrators, we discovered that the cross-hand phase response varies with the fractional polarization of the observed source, violating the fundamental assumption of calibration transferability in radio interferometry. Systematic engineering tests traced this behaviour to the Quadrature Hybrid (QH) polarizer in the frontend signal chain. We conducted a controlled experiment in which the QH was bypassed in seven antennas, converting them to linear polarization feeds. The bypassed system shows dramatically improved performance: instrumental leakage reduced from 10--15\% to 2--5\%, residual leakage after calibration reduced from $\sim$0.5\% to less than $0.2\%$, and stable cross-hand phases independent of source polarization. For the polarized source DA\,240 (RM\,$=$\,3.3\,rad\,m$^{-2}$), the QH-bypassed system accurately recovers the expected $25^\circ$ polarization angle rotation across the band, which the with QH system fails to reproduce. These results establish that the QH polarizer is the dominant source of polarimetric instability in uGMRT Band\,4 and demonstrate that its removal enables reliable sub-GHz polarimetry. We recommend the linear feed configuration for science cases requiring accurate polarization angle and rotation measure measurements.