A direction-dependent framework for visibility plane mosaicing and primary beam correction

TL;DR

This paper addresses the challenge of imaging extended, low-surface-brightness radio emission over wide fields in the presence of direction-dependent effects. It introduces a fully direction-dependent visibility-plane mosaicing framework that coherently combines multiple pointings on a single tangent plane while applying primary-beam corrections during joint deconvolution, implemented with DDFacet and killMS. Through simulations and a real MeerKAT survey of the Shapley Supercluster, the authors show that 3GC-enabled visibility-plane mosaicing yields higher flux fidelity, more accurate spectral indices, and greater completeness than traditional image-plane mosaicing, especially beyond the half-power beam width. The approach yields deeper, higher dynamic-range images with consistent flux scales, suggesting substantial benefits for deep wide-field surveys and SKA-scale pipelines, including potential reductions in observing density and improvements for low-surface-brightness source studies. Future work will extend to polarization and broader bandwidths, further optimization of compute resources, and deployment in next-generation calibration frameworks.

Abstract

With the increasing sensitivity of modern radio interferometers, it has become important to image objects larger than the field of view while optimising sensitivity and image fidelity. We present a coherent visibility plane direction-dependent imaging, calibration and mosaicing framework. Our simulations and application to real MeerKAT data show that this joint deconvolution and primary beam correction approach, coupled with direction-dependent calibration, allows for deeper mosaics with greater fidelity and increased accuracy of recovered flux densities and spectral indices, especially beyond the half-power beam width. Our best-case mosaic produces precise flux values within a 6% uncertainty and spectral indices within 20\% throughout the imaged area, and is fully complete out to twice the radii and half the flux density than the image plane equivalent. The application to archival wideband MeerKAT 1283 MHz data produces the deepest high-resolution image of the Shapley Supercluster Core, with a sensitivity of 3.6 $μ$Jy/beam within the primary beam at a 7$^{\prime\prime}$ resolution, constituting a $\sim$ 50% increase in dynamic range over the image plane counterpart, and a fluxscale that is consistent within 10% across the entire field of view. The compute time for the direction-dependent visibility plane mosaic was comparable to the sum of the times needed to perform direction-dependent calibration on the individual pointings. Our results suggest that visibility plane mosaicing with its capability for deeper deconvolution could improve the efficiency of deep and wide surveys, particularly for on-the-fly mapping and studies of low surface brightness sources, and could form the basis of future calibration pipelines for SKA-scale instruments.

Figures (11)

• Figure 1: Source intensity cumulative density function (CDF) for the suite of simulations.
• Figure 2: Cutouts of the simulated images. Abbreviations: Im = image, vis = visibility, PB = primary beam, DDE = direction-dependent effects. Root mean squared (rms) noises: 17.2, 17.0, 5.1, 10.1, 9.7, 1.9 $\mu$Jy/beam
• Figure 3: Recovered point source peak flux densities for the various simulations. The vertical dashed lines indicate the edge of the PB main lobe at a power of 0.1. The horizontal dashed lines show the unity level. Each simulation is labelled (a)-(f) according to the text.
• Figure 4: Average percentage difference (top) and completeness fractions (bottom) as a function of radial distance (left) and source intensity (right). Each simulation is colour-coded.
• Figure 5: Recovered point source spectral index for the various simulations. The vertical dashed lines indicate the edge of the PB main lobe at a power of 0.1. The horizontal dashed lines show the zero level. The inset zooms into the inner 1$^{\circ}$ to emphasise the spread.