Calibration of radio interferometers – the state of the art in radio astronomy

by Dr Josef Borg

An aerial impression of the spiral arrangement of stations for SKA-Low in Western Australia. Credits: Jodrell Bank Centre for Astrophysics

This article appeared on the Times of Malta on July 4, 2021. It is being reproduced here with the consent of the author.

The sheer size of radio dishes, moving slowly as they track particular cosmic targets, has become synonymous with radio astronomy over the decades that this relatively novel branch of astronomy has been pursued. Due to the nature of the long wavelength of radiowaves, obtaining high resolution images necessitates very large aperture telescopes. For comparison, an optical (visible light) telescope observing at 500nm (green light) can achieve a resolution of 0.5 arcseconds with 0.25m of aperture – a radio telescope observing at a frequency of 1GHz (30cm wavelength) needs to be 150km across to achieve the same resolution! For this reason, radio interferometry has replaced large single radio dishes as the instrument of choice in radio astronomy.

Radio interferometers make use of several smaller radio receivers which work in unison to observe a single target. The resolution of the entire array of radio antennas is defined by the maximum distance between antennas in the entire array – one can achieve the resolution of very large dishes by placing antennas the distance of the dishes’ diameter apart. The disadvantage this incurs in total aperture can be mitigated by using several of the cheaper, significantly easier to construct smaller antennas, thus constructing the radio interferometer – an array of receiving antennas.

Malta is one of several countries working on the construction of the Square Kilometre Array (SKA), which shall be the largest radio interferometer in the world upon its completion. Researchers within the Institute of Space Sciences and Astronomy (ISSA) at the University of Malta are working on the Low Frequency Aperture Array (LFAA), a component of SKA-Low – the low frequency component of the entire SKA. My doctoral dissertation, under the supervision of Kristian Zarb Adami and Alessio Magro at ISSA, indeed focused in part on the calibration of prototype verification stations for the LFAA. Calibration is a necessary step in the entire data processing pipeline for radio interferometry, since incoming radio waves are perturbed by a number of different effects before they are recorded by the individual antennas.

Therefore, calibration involves measuring the true telescope response and comparing it against a model telescope response, taking into account the sky being observed (and therefore, a sky model) as well as the actual telescope being used (a telescope model). My study focused on developing such a novel calibration routine for AAVS-1, in particular using per-element patterns to model the instrumental response and thus obtain far more accurate response models for the telescope. Apart from developing such calibration routines for the SKA, I also developed calibration pipelines for the BEST-2 array in Medicina, Bologna, which is primarily used for the observation of space debris, as well as for the MEXART array in Mexico, involved in interplanetary scintillation observations.

The work carried out in this doctoral dissertation was partially funded by the Endeavour Scholarship Scheme, which scholarships are part-financed by the European Union – European Social Fund (ESF) – Operational Programme II – Cohesion Policy 2014-2020.

Josef Borg completed a PhD in Astronomy at the Institute of Space Sciences and Astronomy, University of Malta, and is currently a researcher at the Faculty of Health Sciences at the University of Malta.