Element gain drifts as an imaging dynamic range limitation in PAF-based interferometers

APERTIF (APERture Tile In Focus) [1] is a novel technology front end system that is currently being developed by ASTRON with the aim of replacing the conventional single-pixel horn feeds of the Westerbork Synthesis Radio Telescope (WSRT) by Phased Array Feeds (PAFs). These PAFs are comprised of 100+ small antenna elements and can form multiple beams on the sky for each antenna pointing direction, and hence significantly improve the survey speed of the current instrument. The design calls for 37 closely overlapping beams per each antenna (Fig. 1a), with an effective field of view of 8 square degrees. At present, a prototype called DIGESTIF, consisting of a PAF with 121 Vivaldi antennas connected to low noise amplifiers (LNAs) and a digital beamforming network, has been installed at one of the WSRT's 14 reflector antennas, and is being tested. This system operates at frequencies ranging from 1 to 1.75 GHz, with an instantaneous bandwidth of 300 MHz. The beamforming is performed off-line based on the measured correlations between the receiver channels [2]. Recently, DIGESTIF was equipped with an auxiliary calibrator system that can be used to track electronic gain variations of the PAF receiver during the observations [3]. Random drifts in these gains cause temporal instability of the compound beam patterns, which in turn affects interferometric imaging performance. In this paper, we describe a simulation framework for exploring this issue, and exercise it to determine whether these drifts can constitute an imaging dynamic range limitation for APERTIF.