The balcony of the Arecibo visitors center gives a panoramic view of the 1,000-foot-diameter dish and the antennas, the protective dome and the support structure above it, hung by massive cables from three concrete pillars. The panel in the foreground displays a schematic diagram of the system. Frank DiMeo/University Photography
ARECIBO, Puerto Rico Everyone knows that the Arecibo radio telescope is huge. But it's also remarkably accurate.
"It's a precision instrument that weighs 900 tons," said Michael Davis, project scientist for the world's largest radio telescope's recent upgrade, a job spanning some two decades from concept to conclusion.
The instrument is not the 1,000-foot dish that collects and focuses radio waves from space, but the massive structure that hangs above the dish to collect the radiation. Before the upgrade to this structure, suspended by thick cables from three concrete towers, it weighed about 600 tons. The upgrade consisted of adding a new, 90-ton radome containing two Gregorian reflectors that improve the sensitivity and resolution of the telescope. The rest of the extra weight comes from the reinforcement to the support structure needed to hold the new reflector and make the platform more stable.
In large optical telescopes, a concave mirror focuses incoming light rays onto an eyepiece or a photographic plate. A radio telescope works the same way, except that the reflector is made of metal to reflect radio waves, and the eyepiece is replaced by a radio receiver.
Most reflectors on both optical and radio telescopes are parabolic, with a curve that looks like part of a sphere but is a bit flatter. A parabolic reflector focuses incoming parallel rays to a sharp point, just like a convex lens, but the reflector must be pointed directly at the target.
But the Arecibo dish is far too large to aim; it's permanently fixed in its valley. So the designers adopted a compromise: They made the dish spherical and aim at various parts of the sky by moving the receiver above it.
Each part of the spherical dish reflects radio waves back in the same way, but any of those ways is a little less precise than from a parabolic reflector. Instead of reflecting incoming waves to a point at a fixed distance above the sphere, the Arecibo reflector sends them to a series of points at varying heights. Until the upgrade, the only way to collect the radiation was with a long, thin antenna known as a "line feed." Two line-feed antennae were mounted on cabinlike structures called carriage houses, both able to ride on a movable arm under the main supporting structure. By rotating the arm and moving the carriage houses along it, operators could aim the telescope anywhere in a 45-degree cone of the sky.
The new Gregorian reflector system overcomes the limitation of the spherical reflector and focuses incoming radio waves to a sharp point. The Gregorian system consists of two reflectors. Radio waves from the big dish focus first on the larger secondary reflector, 72 feet in diameter. This concentrates the beam into the smaller tertiary reflector, 26 feet in diameter, which in turn focuses the beam to a point. The device is called "Gregorian" because it resembles an eyepiece design by that name used in optical reflecting telescopes first used by James Gregory in the 17th century.
Each reflector has a complex mathematical shape, first calculated by German scientist Sebastian von Hoerner on a pocket calculator. Their function is similar to that of the corrective mirrors added to the Hubble Space Telescope; they compensate for the "spherical aberration" of the big dish.
"So now we have the shapes. But they have to be hung up in the sky, and they have to not fall down in a hurricane," Davis said.
Arecibo staff scientist Ben Hooghoudt conceived the idea of enclosing the reflectors in a dome to protect them from weather and especially to streamline them and protect them from wind. While most of the time the winds in Puerto Rico are less than 16 miles per hour, the island is directly in the path of many tropical hurricanes. "No rays pass through the dome," Davis said. "They enter through a 43-foot diameter hole in the bottom. The dome itself is made of aluminum, which has the added value that it keeps out radio-frequency interference."
The reflectors and dome were built on the floor of the dish then hoisted into place on the supporting structure above, replacing one of the two carriage houses on which line-feed antennae were mounted. On May 16, 1996, the day the dome was raised into place, a ground fog filled the dish.
"We literally saw the dome rising out of the mist," Davis recalled.
But there had been a lot of preparation for that day. "You don't take down a 35,000-pound carriage house and replace it with a 180,000-pound dome without first reinforcing the entire supporting structure," Davis pointed out.
"If it was a single L-beam, we'd weld or rivet another L-beam to it," Davis said. The structure had been supported by 12 cables extending from three concrete towers. Six new cables were added; instead of extending straight to the nearest corner of the structure, the new cables spread out from each supporting tower to widely separated points, helping to keep the structure from turning.
"We think of it as a telescope. It thinks of itself as a torsion pendulum," Davis explained, referring to the structure's tendency to rotate like a top hanging from a string.
More stability is provided by three pairs of cables that extend straight to the ground, through small holes in the dish, from each corner of the structure. These cables are attached to large jackscrews, capable of exerting up to 60 tons of pull, that are computer-controlled to keep the platform level and at the correct height, constantly adjusting for changes brought about by expansion and contraction of the metal structure with changes in temperature. They also act to counterbalance the weight of the dome as it's moved from one position to another.
"We need to maintain the focal point inside a one-eighth-inch circle, 450 feet in the sky," Davis explained. "The specification for the telescope is full accuracy in winds up to 16 mph and usable accuracy up to 30 mph."
When a hurricane comes, the arm is moved to a specially reinforced direction and all movable parts of the telescope are locked in place with inserted pins.
The same precision is adhered to in the shape and position of the reflectors. Aluminum panels in the big dish are positioned with a tolerance of only 3 centimeters, or a little more than 1 inch. Those in the secondary reflector are adjusted to within 12-thousandths of an inch, and those on the tertiary reflector to within eight-thousandths of an inch.
"The original system was built with 2-by-4 accuracy," Davis said. "Now we are going to machinist's accuracy."
The aluminum panels that make up the big dish were themselves the result of a previous upgrade in 1974, which greatly increased the frequency range of the telescope and made it useful to many new researchers. Davis expects the same thing to happen again.
"We can't say today what will come from the upgrades," he said. "But the upgrade increases the ease of use, flexibility and ease of adaptation to new uses. The most exciting things will come from people we don't know. Just as with the first upgrade, we will have a new surge of research. That's what makes it exciting."