Systematic Polarization Errors from Parallactic-Angle Dependent Leakage in Pseudo-Circular Feeds

Abstract

Wideband radio interferometers increasingly rely on analog quadrature hybrids to synthesize circular polarization from linear feeds. These systems are typically calibrated under the assumption that instrumental polarization leakage can be represented as a static complex offset, independent of parallactic angle. In this work, we demonstrate that this assumption breaks down in the presence of realistic hybrid imperfections. We show that amplitude and phase errors in the hybrid $\mathbf{H}(ν)$ introduce a non-commutative interaction with parallactic rotation $\mathbf{R}(χ)$, such that [$\mathbf{H}(ν)\mathbf{R}(χ) \neq \mathbf{R}(χ)\mathbf{H}(ν)$] leading to a time-dependent effective leakage term that rotates in the Stokes $(Q,U)$ plane. This effect causes systematic distortions in polarization angle and introduces frequency-dependent biases that can mimic or corrupt Faraday rotation measurements. We derive a first-order analytic model for this leakage and demonstrate that it manifests as a deterministic, geometrically modulated error proportional to total intensity. To mitigate this effect, we introduce a Static Offset Pre-correction (SOP) method that operates in the antenna frame, inverting the hybrid response prior to parallactic de-rotation. Unlike conventional calibration approaches, SOP removes the non-commutative error in the Jones domain, preventing its projection into the sky frame. Our results show that hybrid-induced leakage is not merely a calibration artifact but a fundamental systematic error that must be addressed to achieve high-fidelity wideband polarimetry.