APERTIF: Phased array feeds for the westerbork synthesis radio telescope

Present day synthesis radio telescopes have limited survey capabilities because of field of view restrictions. A novel method to form multiple beams on the sky is to employ a phased array feed (PAF). In telescopes with small f/D ratios, it is the only way to form closely packed beams on the sky. An additional advantage of this technique is that a PAF allows optimizing the secondary beam in terms of sensitivity, sidelobes and polarization characteristics. APERTIF (“APERture Tile In Focus”) is a PAF system that is being developed for the Westerbork Synthesis Radio Telescope (WSRT) to increase its survey speed with a factor 20. This paper presents a system overview of APERTIF and measurement results that demonstrate the unique capabilities of PAFs in practice: Wide field of view (scan range), low system temperature, excellent illumination efficiency, synthesis imaging and a significant reduction of the reflector - feed interaction.