Allen Telescope Array, Radio telescope array in Hat Creek, California, US.

The Allen Telescope Array is a radio interferometer of 42 dish antennas, each 6.1 m across, set on a dry plain at the Hat Creek Radio Observatory in Shasta County, California. Each white dish faces upward on a short base, and a segmented metal feed cone sits at the focus inside the reflector. Signals from all dishes are combined into one observation.

Construction of the array began in 2001 with funding from a private foundation, and first observations began in 2007. This marked the start of a new era in radio astronomy at the facility.

The array's name honours Microsoft co-founder Paul Allen, whose donations of $13.5 million and later more than $30 million in total funded the first two construction phases. Metal plaques on some of the six dishes carry the names of the donors who paid for them. Lead astronomer Seth Shostak has said he expects a signal to be found eventually, though he rules out meeting any civilisation soon, since the nearest likely lie several light years away.

The array stands at the Hat Creek Radio Observatory in Shasta County, about 470 km northeast of San Francisco. It is an operating research facility, not a museum, so there are no regular public tours and the antennas cannot be approached. SETI Institute observations run between 6 pm and 6 am daily, so daytime visits see the dishes idle.

Some of the six dishes carry small metal plaques naming the donors who paid for them, a detail easy to miss among the identical white reflectors. The array was first called the One Hectare Telescope before being renamed for Microsoft co-founder Paul Allen, who gave over $30 million.