Forty-two small dishes working as one: the Allen Telescope Array at Hat Creek

Background

The Allen Telescope Array is a radio telescope at Hat Creek in northern California, inside the Hat Creek Radio Observatory, roughly 300 miles north of San Francisco and a little north of Lassen Peak. In its present form it is made of 42 antennas, each six metres across, and the SETI Institute uses it to look for radio technosignatures while the same dishes also serve ordinary radio astronomy. The dishes do not work separately. Their signals are combined by the technique of interferometry, which lets the array make radio pictures of the sky, called imaging, or concentrate on one patch, called beamforming. This is the design its builders call a large number of small dishes: when the computers in the signal processing room put the 42 together, they give as many pixels on the sky as one dish a thousand feet across. The other strength is the feed, the part at the focus that takes in the radio waves. The original log-periodic Welch Feeds were streamlined into the present Antonio Feeds, which capture a very wide range of frequencies at once, and a new design called the Quad-Ridge Feed Horn is being worked on. The array is not the largest in the world; what sets it apart is a wide field of view and observing time dedicated to the search.