In Situ Antenna Performance Evaluation of the LOFAR Phased Array Radio Telescope

The low frequency array (LOFAR) is a phased array radio telescope that is currently being built in The Netherlands with extensions throughout Europe. It was officially opened on June 12, 2010 and is an important pathfinder for the square kilometre array. The Dutch LOFAR system will consist of 36 stations covering the 10-250 MHz frequency range. In this paper we discuss the sky noise limited design of the antenna system and present a novel technique to obtain the ratio of effective area and system temperature directly from the calibration results, despite the presence of multiple sources within the 2π sr field of view of the antennas. This ratio is the key sensitivity parameter for radio telescopes. The presented technique allows in situ performance evaluation using astronomical calibration sources, i.e., without the use of reference sources, a controlled environment or lab equipment. We use this technique to evaluate the performance of some of the already available LOFAR hardware and demonstrate that LOFAR has the desired sky noise dominated performance.