Decoherence in LOFAR-VLBI beamforming

¹ Universita di Bologna, Via Zamboni 33, 40126 Bologna, Italy
e-mail: etienne.bonnassieux@unibo.it
² INAF, Instituto di Radioastronomia, Via Piero Gobetti 101, 40129 Bologna, Italy
³ Centre for Extragalactic Astronomy, Department of Physics, Durham University, Durham DH1 3LE, UK

Received: 13 January 2020
Accepted: 13 March 2020

Abstract

We show that the use of a superstation (a phased array created using multiple stations of an interferometric array) created in post-processing for LOFAR-VLBI observations introduces a direction-dependent loss of signal in the image. We show this effect using simulations and real data. Using the RIME formalism, we characterise it fully, and give limits under which this signal loss is negligible. Finally, we show that we are able to fully predict this effect. We close with guidelines for interferometric observers to avoid this effect in their observations, and a discussion of techniques which could limit this effect or do away with it entirely. The latter in particular will be relevant to the SKA should its long baselines be used to their fullest potential.