SETI@Arecibo: Arecibo is Better Than Ever

March 16, 2001

by Seth Shostak - Senior Astronomer

Project Phoenix is back at Arecibo, checking out nearby stars for signs of intelligent life. Astronomer Seth Shostak is reporting from the observatory once again, and will be home to his Arecibo Diaries. This is the fourth installment.

"This telescope’s no spring chicken," remarks astronomer Karen O’Neil wryly, "even if it is March."

Indeed, the Arecibo dish is hardly the new ‘scope on the block. It was conceived in 1959, and constructed in the early 1960s. There are a few employees at the observatory who’ve been here from the beginning, but not many. For most of the 140 folks who work here, Arecibo was always a facility, and not a dream.

Telescopes usually outlast their builders. They’re expensive, after all, which discourages thoughts of early disposal. California’s famed Palomar telescope, with its insouciant art deco dome, is more than a half-century old. The venerable Yerkes 40-inch (1-meter), 70 miles (113 kilometers) northwest of Chicago, dates from the Victorian era.

"Long in the tooth, yes. But Arecibo’s still on the cutting edge," notes O’Neil incisively. That’s because, like computer programs and frequent fliers, telescopes are routinely upgraded. The most recent major improvement to this economy-sized instrument was made between 1992 and 1997 when the Gregorian feed, housed in a 3-story high geodesic dome, was added to the Arecibo ‘scope.

Most telescopes – whether optical or radio – sport a parabolic reflector or mirror. A parabolic shape brings incoming rays to a single point, a fact you learned in seventh grade and probably forgot by eighth. But Arecibo’s 1,000-foot (305-meter) aluminum dish is teased into a spherical shape – like the inside of a ball that, if it were a complete sphere, would be voluminous enough to hold half the population of the U.S. or, should you prefer, 10 billion pounds of mixed nuts.

Thanks to this peculiar geometry, it’s possible to move the receiver assembly, which is suspended above the reflector, up to 140 feet (43 meters) away from center, thereby allowing the telescope to point anywhere in a circular cap of sky 40 degrees across. During the course of a day, Earth’s rotation slews an impressive one-third of the universe through that 40-degree patch.

That’s the good news. The bad news is that the spherical reflector doesn’t bring all the incoming radio waves to a single point of focus. Consequently, in those dim days before the Gregorian, the feed structures for Arecibo – the secondary antennas that funnel signals into the low-noise amplifiers – had to be unwieldy, complicated assemblies looking like 90-foot (27-meter) stacks of pie tins. When the winds were high, some poor devil would be lowered by winch to detach the lower part of the longest of these linear feeds to keep them from snapping off. Did I mention that they were also costly?

In addition, changing radio bands with this set-up was clumsy. You usually had to laboriously unbolt one of these spindly, metal proboscises, and affix another. It took all day.

But the Gregorian upgrade has fixed that. Two large metal mirrors within the dome bring all rays back to a single focus. (The geometry for this handy trick was first worked out by Scottish mathematician James Gregory in the 17th century, incidentally.) Simpler, small feeds now suffice, and these are mounted indoors on a rotating turret, much like lenses on a microscope. The stacked pie tins are passé, and observers can change bands at the proverbial touch of a button.

"Not only that," remarks O’Neil, "but the Gregorian has increased the ‘scope’s instantaneous bandwidth: its ability to eat large chunks of the radio dial."

As the most sensitive antenna on the planet, Arecibo – which is a facility of the National Science Foundation and operated under a cooperative agreement with Cornell University – is in high demand. Karen O’Neil is using it today to hunt for galaxies. Next week, Duncan Lorimer will be using it to investigate pulsars.

But for three weeks, the nights belong to us. Arecibo’s upgraded optics are turned to otherwise unremarkable stars as we look for the evidence of intelligent brethren. It may be old, but this doggone scope’s learned some new tricks.