Calibration artefacts in radio interferometry. III. Phase-only calibration and primary beam correction

TL;DR

This paper investigates calibration artefacts in radio interferometry, revealing that phase-only calibration produces anti-ghosts significantly brighter than those from combined calibration, especially after primary beam correction.

Contribution

It demonstrates that phase-only calibration results in brighter anti-ghosts, amplified further by primary beam correction, expanding understanding of calibration artefacts in radio interferometry.

Findings

Anti-ghosts from phase-only calibration are N-times brighter with N antennas.

Primary beam correction amplifies the brightness of anti-ghosts.

Spurious symmetrization effects are linked to calibration methods.

Abstract

This is the third installment in a series of papers in which we investigate calibration artefacts. Calibration artefacts (also known as ghosts or spurious sources) are created when we calibrate with an incomplete model. In the first two papers of this series we developed a mathematical framework which enabled us to study the ghosting mechanism itself. An interesting concomitant of the second paper was that ghosts appear in symmetrical pairs. This could possibly account for spurious symmetrization. Spurious symmetrization refers to the appearance of a spurious source (the anti-ghost) symmetrically opposite an unmodelled source around a modelled source. The analysis in the first two papers indicates that the anti-ghost is usually very faint, in particular when a large number of antennas are used. This suggests that spurious symmetrization will mainly occur at an almost undetectable flux level. In this paper, we show that phase-only calibration produces an anti-ghost that is $N$-times (where $N$ denotes the number of antennas in the array) as bright as the one produced by phase and amplitude calibration and that this already bright ghost can be further amplified by the primary beam correction.