Calibration errors in interferometric radio polarimetry

Abstract

Residual calibration errors are difficult to predict in interferometric radio polarimetry because they depend on the employed observational calibration strategy, encompassing the Stokes vector of the calibrator and parallactic angle coverage. This work presents analytic derivations and simulations that enable examination of residual on-axis instrumental leakage and position angle errors for a suite of calibration strategies. The focus is on arrays comprising alt-az antennas with common feeds over which parallactic angle is approximately uniform. The results indicate that calibration schemes requiring parallactic angle coverage in the linear feed basis (e.g. ALMA) need only observe over 30 deg, beyond which no significant improvements in calibration accuracy are obtained. In the circular feed basis (e.g. VLA above 1 GHz), 30 deg is also appropriate when the Stokes vector of the leakage calibrator is known a priori, but this rises to 90 deg when the Stokes vector is unknown. These findings illustrate and quantify concepts that were previously obscure rules of thumb.