Efficient wide-field radio interferometry response

Abstract

Radio interferometers do not measure the sky brightness distribution directly, but measure a modified Fourier transform of it. Imaging algorithms therefore need a computational representation of the linear measurement operator and its adjoint, regardless of the specific chosen imaging algorithm. In this paper, we present a C++ implementation of the radio interferometric measurement operator for wide-field measurements that is based on so-called improved w-stacking. It can provide high accuracy (down to ≈10−12), is based on a new gridding kernel that allows smaller kernel support for given accuracy, dynamically chooses kernel, kernel support, and oversampling factor for maximum performance, uses piece-wise polynomial approximation for cheap evaluations of the gridding kernel, treats the visibilities in cache-friendly order, uses explicit vectorisation if available, and comes with a parallelisation scheme that scales well also in the adjoint direction (which is a problem for many previous implementations). The implementation has a small memory footprint in the sense that temporary internal data structures are much smaller than the respective input and output data, allowing in-memory processing of data sets that needed to be read from disk or distributed across several compute nodes before.

Ultra-Steep Spectrum Radio `Jellyfish' Uncovered in Abell 2877

“…The primary calibrator for each observation window was PKS 1934-638 and a secondary calibrator PKS 0048-427 was observed periodically throughout to ensure phase calibration. We imaged the observation using WSClean (Offringa et al 2014) using the new WGridder backend (Arras et al 2021). The band was separated into four equally sized output channels using its multifrequency synthesis algorithm (Offringa & Smirnov 2017), baselines were weighted using the Briggs scheme with robustness parameter 0, and Cleaning was performed down to the image noise inside a mask set at a factor of 3 times the noise.…”

A practical preconditioner for wide-field continuum imaging of radio interferometric data

“…This map can be implemented efficiently as a 2D non-uniform fast Fourier transform per imaging band with a correction that accounts for wide field effects (see eg. Arras et al (2020b)). We use the dask wrappers of the wgridder Arras et al (2020b) in codex-africanus (https://github.com/ska-sa/codex-africanus) to implement the measurement operator throughout and use dask-ms (https://github.com/ska-sa/dask-ms) as the data access layer (see Perkins et al (2021)).…”

“…Arras et al (2020b)). We use the dask wrappers of the wgridder Arras et al (2020b) in codex-africanus (https://github.com/ska-sa/codex-africanus) to implement the measurement operator throughout and use dask-ms (https://github.com/ska-sa/dask-ms) as the data access layer (see Perkins et al (2021)).…”

The merging galaxy cluster Abell 3266 at low radio frequencies

“…Namely, individual 2-min snapshots are calibrated and imaged independently prior to linear co-addition/mosaicking. Calibration takes place in-field with a local sky model generated from the GaLactic and Extragalactic All-sky MWA (GLEAM) survey (Wayth et al 2015;Hurley-Walker et al 2017) and SUMSS (see Duchesne et al 2020, for details) and multiscale, multi-frequency deconvolution is performed using (Offringa et al 2014;Offringa & Smirnov 2017) with the algorithm (Arras et al 2021;Ye et al 2021). General astrometry and brightness scaling prior to co-addition is also as detailed in Duchesne et al (2020) and makes use of the aforementioned local sky model.…”