(Adapted and expanded from Sky & Telescope;
last updated June 2008.)

The next generation of big radio telescopes won't look anything like the massive dishes of old. 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 have been 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).

As of June 2008, the first 42 dishes of the Allen Telescope Array are beginning their science 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.

"The ATA is being constructed in four stages," according to 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. But that will require another $40 million in grants and donations.

The ATA is being funded by private donations through the SETI Institute. The major donor is Microsoft cofounder Paul G. Allen, who has given 30 million. The SETI Institute is also funded by many smaller donors; $50 gets you a one-year membership and a slick magazine, and $100,000 gets your name on a dish. The SETI Institute said in October 2007 that the project has cost $50 million so far.

The strategy is to substitute smart electronics and 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 is 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.

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 should be able to listen to 100 million channels simultaneously. This band of channels can be marched up and down the microwave spectrum to cover wider frequency ranges, one block at a time. If the number of simultaneous channels can be increased, the ATA's efficiency for SETI work will increase proportionally.

Each of the ATA's simultaneous beams (aimings at separate targets) will require its own data-collection and analysis system. The plan is to start with three beams and increase their number to 16 as costs allow.

Thomas Pierson, chief executive officer for the SETI Institute, says an important goal is to provide long-term upgrade capability. "The Allen Telescope Array can be improved constantly, at relatively low cost. For instance, the telescope can be made more powerful by improving the software and incorporating new computing hardware, which continues to get better and less expensive. It can also be made more sensitive by adding more dishes."

Bigger projects are also in the works. The ATA should serve as a test bed for the much more ambitious Square Kilometer Array, an instrument that radio astronomers worldwide hope to build starting around 2011 and to finish by 2020. It would be designed on the same idea of "aperture synthesis" as the ATA but would have 100 times the collecting area of ATA-350 (and would have impressive SETI potential of its own).

Another aperture-synthesis radio telescope is LOFAR, now being completed in the Netherlands, which will use 15,000 small antenna elements to make high-resolution images of the sky at low frequencies from 10 to 250 MHz. LOFAR has already begun observations.

The SETI Institute funded an effort at Ohio State University to push its own antenna-array efforts one step further. Robert Dixon and Steve Ellington built an early version of an "Omnidirectional Search System" (OSS), a fully "steerable" radio telescope that has no moving parts at all. Instead of dishes, each of its elements is a simple small, fixed antenna sensitive to the entire sky. Aiming was done entirely in software by combining signals from all the antennas through interferometry. Dixon and Ellington dubbed their prototype the Argus omnidirectional radio telescope.

An 8-antenna prototype used 21 computers linked together to process its signals into a dynamic radio map of the sky (for a narrow range of frequencies). Its was successfully programmed to blank out artificial interference from satellites crossing the sky; "black spots" in its sensitivity can be set to track problem satellites, hiding them from view. The SETI Institute is itself working to explore omnidirectional-array technology.

The limiting factor for this ambitious idea is lack of sufficient computing power. To see the whole sky, each antenna has to be small, hardly larger than the wavelength itself; no dishes allowed. So the total collecting area is tiny. And if even a small, 64-antenna array were to scan and fully analyze all available microwave frequencies, it would require the power of about 100,000 desktop computers.

And as for a full-up version, one capable of watching all the sky well at all good microwave frequencies all the time?

"The present estimate," says the SETI Institute's Frank Drake, "is that a computer system which can carry out about ten to the twentieth power calculations per second is required. This is presently beyond both our technical and financial capability. However, if the increases in computer power and decreases in cost follow their historical trends — that is, following Moore's Law [which suggests that computing power per dollar doubles every 18 months] — the required capability at an affordable cost should be available in perhaps a decade."

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Alan M. MacRobert is a senior editor of Sky & Telescope.

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