(Adapted and expanded from Sky & Telescope;
last updated October 2007.)

The next generation of big radio telescopes won't look anything like today's massive dishes. Instead of giant steel constructions towering into the sky, the future will belong to more economical arrays of many small antennas hugging the ground. And, in a historic role reversal, searchers for extraterrestrial intelligence are blazing a trail for conventional radio astronomy to follow.

That is the vision behind the Allen Telescope Array (ATA), formerly named the One Hectare Telescope — the world's first large radio observatory designed for SETI from the get-go. The SETI Institute and the University of California at Berkeley's Radio Astronomy Lab are building an array that, with sufficient funding, should eventually include 350 dishes, having a total collecting area of about 10,000 square meters (one hectare, or 2.47 acres). The Allen Telescope Array is currently under construction and beginning operations at Berkeley's Hat Creek radio observatory site in northern California. The instrument will be used for ordinary radio astronomy at the same time as it performs its SETI work.

In October 2007 the SETI Institute announced that the ATA's first 42 dishes, known as "ATA-42," have begun science observations.

"The ATA is being constructed in four stages," says the SETI Institute; "the ATA-42, ATA-98, ATA-206, and ATA-350, each number representing the number of dishes in the array at a given time." Construction beyond ATA-42 will require additional private funding. The institute says it hopes that the full, 350-dish version will be funded and finished around 2010.

The strategy is to substitute smart electronics and truly massive data processing for big, expensive mechanics. As computing power grows ever cheaper, radio engineers can use interferometry to combine signals spanning a very broad range of frequencies from many low-cost antenna elements. This will result in a single, unified instrument that should be able to perform tricks never before feasible. In particular, the ATA will be capable of "multibeaming," using software to observe several separate targets simultaneously in the same patch of sky.

The individual dishes are 20 feet (6.1 meters) in diameter, with unobstructed apertures and offset signal feeds. Each dish is equipped with a specially designed receiver that can listen to every frequency in nearly the entire "microwave window" that comes through Earth's atmosphere clearly, from about 0.5 to 11.2 gigahertz. Given enough computing power, the receivers' outputs could be divided into billions of narrow radio channels, so that each channel can be examined for signs of an artificial narrowband signal in deep space.

Much of the money and effort have gone into the instrument's "back end," the computers and other parts that integrate and analyze the signals — tasks that have posed severe technical hurdles. In fact, today's computing technology is inadequate to do the full job envisioned. The hope is that computing power will continue to improve, allowing the finished instrument to be upgraded as time goes on.

The primary strategy announced for the ATA will be to do a targeted search of nearby Sun-like stars, one by one. Eventually the ATA should examine 100,000 or more target stars at frequencies across the microwave window — a vast undertaking compared to the 800 or so stars that were targeted at 1.2 to 3.0 gigahertz by the SETI Institute's Project Phoenix (see SETI Searches Today).

More recently, some SETI astronomers have been coming around to a different view: that the best chance of success is not in targeting nearby stars one by one, but in choosing star-rich swaths of the Milky Way for deep, protracted scrutiny — thereby examining a much larger number of stars, even though most of the stars would be very far away and thus would require the aliens to be transmitting with truly unearthly power (see Smarter SETI Strategy). The ATA will give this strategy too a try. ATA-42 will sweep many millions of stars in a swath along the Milky Way's plane near the galactic center, in a project set to last six months.

Meanwhile, Berkeley radio astronomers plan to use some of the ATA's time for such projects as timing pulsars, mapping the hydrogen in the Milky Way and other galaxies, measuring primordial deuterium, and examining the hearts of star-forming regions. Says Leo Blitz, director of the Berkeley Radio Astronomy Lab, "Our goal is nothing short of standing the way radio astronomy has been done up to now on its head."

The full complement of 350 dishes are to be spaced across an area about 0.7 kilometer wide. This design is a compromise. It will yield fairly high resolution for radio astronomy with fairly narrow beams, which, however, are less than optimum for SETI. Wide beams are desirable for SETI searches, because they encompass the most stars at once.

The ATA is being funded by private donations through the SETI Institute. The major donor is Microsoft cofounder Paul G. Allen. The SETI Institute is also funded by many smaller donors; $50 gets you a one-year membership. A donation of $100,000 gets your name on a dish. The SETI Institute said in October 2007 that the project has cost $50 million so far.

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