Phased Array Feeds

The two open-ended frontiers in radio astronomy instrumentation are collecting area and field of view. Greater collecting area translates directly into greater sensitivity while increased field of view increases observing efficiency and makes feasible new areas of science, such as searches for transient radio sources, rare types of pulsars, and surveys of large portions of our Galaxy and of the universe. Steady progress will be made in more affordable collecting area, but the main limitation will continue to be cost. Greater field of view depends much more on technology breakthroughs that can happen in the next decade with adequate support for research and development in antennas and receiver systems. There are three basic methods for obtaining greater field of view: aperture arrays of small elements, each of which sees most of the sky and beams are formed by combining their signals in electronics, smaller reflector antennas that have broader beams, and arrays of reflector antenna feeds that form the equivalent of a radio camera. This white paper outlines a technology development program for a specific form of radio camera called a phased array feed. This type of feed retains the efficiency of the best waveguide feed while fully sampling the telescope focal plane and making complete use of the available information at the focal plane. Phased array feeds must be realized with system temperatures equal to or approaching those of the best single-beam receivers in use today. Otherwise, focal plane area and signal processing power will be squandered on restoring lost sensitivity instead of increasing observing efficiency. Cryogenic technology that can cool the key components of a large array is one area requiring R&D. Other areas include the design of array elements and low-noise amplifiers that are well matched in the presence of strong mutual coupling between array elements; the reduction of size, weight, power consumption and cost of receiver systems to make feasible the use of tens or hundreds of receivers at the focal plane; and greater data transport and signal processing bandwidth required by multi-element arrays. Given the current status of feed research and development, an array feed with preliminary science capability could be available within three to five years and mature operational instruments deployed soon thereafter.