Characterizing the calibration systematics in radio interferometry due to partially modelled extended emission

Abstract

Calibration is the process by which we try to remove atmospheric and instrumental errors that corrupt the observations made by interferometers. Calibration is however an imperfect process and when we calibrate with an incomplete sky model systematics or artefacts manifest in our final images. One of the more striking artefacts that can appear in images are ghost sources. This paper explores these ghost artifacts in greater depth. We are particularly interested in the systematics or artefacts that are created if we partially model extended emission. A theoretical model which utilizes an east-west layout is developed (and empirically validated) that can give insight into the systematics so formed. The results from this study reveal that when we only partially model extended emission then the calibration systematics that form can manifest as extended emission in images. The aforementioned systematics alter the original extended source in various ways; the flux density of the extended source is modified and becomes comparable to the flux density of the source in the calibration sky model and its profile is slightly modified (it becomes more point like). More importantly, we found that the shorter baselines which are more sensitive to extended emission are in general more affected by the calibration systematics than the longer baselines are. In related works it was found that the number of antennas greatly influence the brightness of the created systematics. For the use case we investigate in the paper, however, we found this to not be the case.