Astronomers turn up the volume on cosmic signals
Allen Telescope Array will allow researchers to listen more closely to more of the universe

By Diane Ainsworth, Public Affairs

Sophisticated signal-processing circuitry and computer software allow astronomers to link any number of small radio antennas into an array, the functionality of which can equal or surpass that of huge single-dish antennas at a fraction of their cost. Eventually totaling 350 linked dishes, the Allen Telescope Array (seen here in an artist’s rendering), now under construction near Mt. Lassen, will enable researchers to listen for signals from an extraterrestrial civilization while studying other radio sources, such as pulsars, in distant galaxies. Image courtesy of the SETI Institute

11 September 2002 | The first of 350 low-cost antenna dishes, linked together with special electronics, are set to begin eavesdropping on the universe.

From the passenger seat of a Cessna 210 piloted by Berkeley astronomy professor Jack Welch, a visitor can just make out a handful of 20-foot-diameter antenna dishes, standing in serried salute to the sky, peeking out of a secluded clearing between Lassen Peak and Mount Shasta.

Shielded from radio interference by walls of volcanic rock, these electronic detectors are being primed by a team of Berkeley radio astronomers to listen for signals that originate more than 10 billion light-years out in space — so unimaginably far away that the astronomers expect the faint whisper of the Big Bang to be heard.

The first shipment of three dishes is fresh off a truck from Anderson Manufacturing in Idaho Falls, Idaho. Cookie-cutter clones will be produced over the next three years at a rate of one every other day. Once the array has grown to 350 dishes, the dishes will be interlinked, using a technique known as interferometry, to act as a single radio telescope. At roughly a third of the cost of the new 100-meter-diameter single-dish antenna at Green Bank, West Virginia, says Welch, the Allen Telescope Array (ATA) will provide 20 to 30 percent more collecting power, with a field of view large enough to observe 36 full moons in the sky at once.

Welch, whose specialty is radio astronomy, is the conceptual force behind the ATA project, which will monitor frequencies between 1,000 and 10,000 gigahertz in the microwave spectrum of the nearby universe, a “relatively quiet” area lacking much of the interference from naturally occurring cosmic static that complicates radio astronomers’ lives.

“This is where we would expect to detect artificial signals from intelligent life elsewhere in the galaxy,” says Welch. “It’s where SETI, the Search for Extraterrestrial Intelligence, takes place.”

The Allen array (named for computer billionaire Paul Allen, a major donor) will be making SETI observations constantly, a huge improvement over the limited time available for such investigations at other facilities.

This portion of the microwave spectrum is also a region where Welch and his colleagues will be able to observe a variety of cosmic sources — such as rapidly spinning pulsars, protoplanetary disks, and unusual molecular clouds called OH masers — at the same time as SETI monitoring is being done, thanks to new technology that will allow separate groups of astrono-mers to utilize the array’s data flow simultaneously.

In one such non-SETI endeavor, the Allen array will map, for the first time, all of the atomic hydrogen in the nearby universe. This is the material from which the stars and galaxies were (and still are) being made; mapping it — determining its distribution throughout the nearby universe — will tell scientists much about the structure of this region of the cosmos. That in turn will give them clues about how it evolved, says Leo Blitz, director of the Berkeley Radio Astronomy Laboratory.

Off-the-shelf hardware, cutting-edge technology

The first dish for the array is installed at the Hat Creek site. Photos courtesy of the SETI Institute

Turning up the volume on the universe at a non-astronomical price is the way of the world in radio astronomy today. But single, large-dish antennas are no longer the answer, says Blitz, who is coordinating the array project with the SETI Institute in Palo Alto. Interferometry, which enables the linking together of smaller dish antennas via electronic circuitry, has led to the placement of strings of radio telescopes around the world, all listening in on the strange crackles, hisses, and whirs of neighboring galaxies. Now webs of even smaller, more-inexpensive dishes can be constructed with off-the-shelf hardware and enhanced with state-of-the-art “split-beam” receiver technologies, at a fraction of the cost required to build a single large-dish antenna.

“We couldn’t have built this this five years ago,” Welch says of the Allen Telescope Array. “Now we’ve got the technology to make high-resolution observations simultaneously in 100 directions or more.”

Listening with a finely tuned ear is important. Cosmic radio signals are incredibly weak — one-trillionth of a watt or less — so they are easily drowned in a sea of more powerful local noise. The trick to achieving high spatial resolution is to combine the signals from many antenna dishes spread out over great distances. The collecting area of such an array increases with the number of antennas — like the Allen array, which is splayed across 2.5 acres of collecting area — and when the signals received by these antennas are synthesized, or combined, the acoustics of the cosmos are sharpened dramatically.

Speeding up the search
The Allen Telescope Array will allow astronomers to interweave signals received by individual dishes in sophisticated ways and observe multiple frequencies simultaneously, Welch says. With this new multi-beaming capability, Blitz adds, scientists will be able to search many more times the number of stars as they can right now, and so speed up their quest for signals from an extraterrestrial civilization.

At the same time, other radio astronomers will be able to carry out new studies of celestial phenomena previously beyond their view, given current observing capabilities. They may be listening for the whoosh of radio radiation from a coalescing massive black hole in another galaxy, or gathering radio data to study the onion-like shells shed by dying stars.

Now that the first three dishes of the array are in place near Berkeley’s Hat Creek Observatory, and while UC and the U.S. Forest Service hammer out final details of the site permit, Welch and a team of astronomers will begin testing the sophisticated signal-processing circuitry and software needed to combine and analyze hundreds of wideband signals at once.

“The hope as this array goes online,” says Donald Backer, a Berkeley research astronomer, “is to explore approaches toward building a next-generation synthesis-array telescope with an aggregate surface area of a square kilometer [0.4 square miles].”

The ability to observe much larger patches of the sky for much fainter signals — much as the Allen array is designed to do, but with even wider fields of view and even more precision — will aid astronomers in studying the beginnings of the Milky Way galaxy in the farthest reaches of the universe. The anticipated price tag for the Square Kilometer Array is about $1 billion, though, which demands that the project be international in scope. Astronomers from 12 countries have begun to collaborate, with the goal of bumping the project to the top of radio astronomy’s priority list.

If that dream becomes a reality, says Welch, “We would have a window on the earliest formative stages of the universe, when hydrogen first began to coalesce and form clouds of gas and dust. The capacity to observe that far back in time isn’t possible right now.”

The Square Kilometer Array, building on techniques and data derived from the Allen array, is expected to have a range of nearly 15 billion light years — which means it will detect signals from the very edge of the universe. Not a bad accomplishment for a project getting under way with three off-the-shelf antenna dishes in a remote California forest.