High Dynamic-Range Radio-Interferometric Images at 327 MHz

TL;DR

This paper discusses advanced calibration techniques for low-frequency radio interferometric imaging to improve image quality by correcting for ionospheric and instrumental effects that vary over time and across the field.

Contribution

It introduces methods for directionally-dependent calibration and ionospheric correction to enhance dynamic range in radio images at 327 MHz.

Findings

Higher dynamic range images achieved

Reduced spurious spectral signals

Effective correction of ionospheric effects

Abstract

Radio astronomical imaging using aperture synthesis telescopes requires deconvolution of the point spread function as well as calibration of the instrumental characteristics (primary beam) and foreground (ionospheric/atmospheric) effects. These effects vary in time and also across the field of view, resulting in directionally-dependent (DD), time-varying gains. The primary beam will deviate from the theoretical estimate in real cases at levels that will limit the dynamic range of images if left uncorrected. Ionospheric electron density variations cause time and position variable refraction of sources. At low frequencies and sufficiently high dynamic range this will also defocus the images producing error patterns that vary with position and also with frequency due to the chromatic aberration of synthesis telescopes. Superposition of such residual sidelobes can lead to spurious spectral signals. Field-based ionospheric calibration as well as "peeling" calibration of strong sources leads to images with higher dynamic range and lower spurious signals but will be limited by sensitivity on the necessary short-time scales. The results are improved images although some artifacts remain.