Direction-dependent calibration with image-domain gridding

TL;DR

This paper introduces a novel calibration method for radio interferometry that uses image-domain gridding to model direction-dependent effects continuously, improving wide-field image quality over traditional discrete approaches.

Contribution

The paper presents a new calibration technique employing image-domain gridding to replace discrete directional models, eliminating discontinuities and enhancing image quality in radio interferometry.

Findings

Improved image quality with the new calibration method.

Better physical plausibility of systematic error modeling.

Comparison shows advantages over traditional facet-based calibration.

Abstract

Wide-field images made by radio interferometers are invariably affected by direction-dependent systematic effects such as the ionosphere or the beam pattern. Calibration along a set of discrete directions in the sky is the default technique to estimate and correct these systematic errors. However, additional processing such as smoothing and mosaicing are required to reconcile the step wise variation of the estimated systematic errors at the edges of the discrete directions (facets). We overcome the discrete nature of direction-dependent calibration by using image-domain gridding as the model for the calibration. Instead of discrete directions in the sky, calibration is performed using a basis that represents a set of sub-grids in the Fourier space. This automatically removes the need for extra operations to recover the wide-field systematics error model without any discontinuity. We provide results based on LOFAR data where we compare the traditional facet-based (discrete directional gains) calibration with the proposed approach. The comparison shows improved image quality, mainly because of the physical plausibility of the proposed approach as opposed to using a piecewise constant model for direction-dependent systematic errors.