Temporal beam pattern stability of a radio astronomy phased array feed

A novel method to form multiple beams on the sky with a reflector antenna is to employ a dense phased array feed (PAF). This technology is currently being developed to greatly increase the survey speed of existing and future radio telescopes. For high quality radio astronomical images the beam patterns of the telescope must be stable over the duration of the observation, which is typically 12 hours for an East-West array. This is more complicated for a PAF than for a horn feed because the beams of a PAF are formed by adding the responses of multiple elements. Electronic gain variations of the receiver channels will lead to beam pattern variations. In contrast, the beam pattern of a horn feed is determined by mechanics only. First, the beam stability requirements are derived for the APERTIF system, a PAF system which will be installed on the Westerbork Synthesis Radio Telescope (WSRT). From these requirements the stability of the individual receiver channels is derived. Measurements during ~3 hours demonstrate that the beam pattern stability of the existing horn feeds is according to the expectations. Good news is that the beam pattern of the PAF system is demonstrated to be equally stable during the same observation period under stable weather conditions.