Atmospheric turbulence in phase-referenced and wide-field interferometric images

Application to the Square Kilometre Array

¹ Max-Planck-Institut für Radiastronomie, Auf dem Hügel 69, 53121 Bonn, Germany e-mail: imartiv@mpifr.de
² Dpt. Astronomia i Astrofísica, Universitat de València, Dr. Moliner 50, 46100 Burjassot, Spain

Received: 3 December 2009
Accepted: 2 April 2010

Abstract

Phase referencing is a standard calibration procedure in radio interferometry. It allows us to detect weak sources by using quasi-simultaneous observations of closeby sources acting as calibrators. However, atmospheric turbulence may introduce strong differences in the optical paths of the signals of the target and calibrator and affect, or even waste, phase referencing in cases of relatively large calibrator-to-target separations and/or bad weather. The situation is similar in wide-field interferometric observations. We present the results of a Monte Carlo study of the astrometric precision and sensitivity of an interferometric array (a realization of the Square Kilometre Array, SKA) in phase-referenced and wide-field observations. These simulations can be extrapolated to other arrays by applying the corresponding corrections. We consider several effects from the turbulent atmosphere (i.e., ionosphere and wet component of the troposphere) and also from the antenna receivers. We study the changes in dynamic range and astrometric precision as a function of observing frequency, source separation, and strength of the turbulence. We find that, for frequencies between 1 and 10 GHz, it is possible to obtain images with high fidelity, although the atmosphere strongly limits the sensitivity of the instrument compared to the case with no atmosphere. Outside this frequency window, the dynamic range of the images and the accuracy of the source positions decrease. We also find that, even if a good model of the atmospheric turbulence (with an accuracy of 99%) is used in the imaging, residual effects from the turbulence can still limit the dynamic ranges of deep, high-contrast (10⁵–10⁶), images.