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Tuned to the Universe PUBLIC ACCESS

Precision on a Grand Scale Brings in Clearer Signals from Distant Time and Space.

[+] Author Notes

Tom Gibson. based in Lewisburg. PA is a freelance writer specializing in technology and engineering.

Mechanical Engineering 126(12), 36-38 (Dec 01, 2004) (3 pages) doi:10.1115/1.2004-DEC-3

The Robert C. Byrd Green Bank Telescope has advanced the knowledge of astronomy with some key discoveries. It assisted in observing the first known double-pulsar binary systems, improving astronomers’ understanding of pulsars and allowing the most precise test yet of general relativity. The telescope would have an offset feed arm, meaning the sub-reflector at the focal point would lie outside the path of incident radio waves, increasing the viewing area by as much as 10 percent. Measuring 100 by 110 meters, the telescope’s oval reflector dish has two acres of surface area. The surface would consist of 2004 panels that could be adjusted in and out to fine-tune the shape of the surface. The basic structure of the telescope consists of the rotating lower section, known as the alidade, and the tipping structure, which tilts within it. The telescope operates over a greater band of wavelengths than any telescope before it and points with greater accuracy. It can zero in on targets within an arc-second, the angle subtended by a dime from a mile away.

The Robert C. Byrd Green Bank Telescope looks a bit out of place in the rugged West Virginia mountains. At 485 feet tall, the mammoth 16-million-pound structure stands higher than the Statue of Liberty and ranks as one of the world's largest radio telescopes.

But the astronomers who work at the observatory in Green Bank, W. Va., aren't so much impressed by the telescope's size as by its accuracy, the result of careful and creative engineering. They've waited for it since the project began in 1989, and they're just now starting to use it on a regular basis.

In its short life, the telescope has already advanced the knowledge of astronomy with some key discoveries. It assisted in observing the first known double-pulsar binary systems, improving astronomers' understanding of pulsars—radio-wave-emitting stars with strong magnetic fields-and allowing the most precise test yet of general relativity. It also detected high-velocity neutral hydrogen clouds surrounding the Andromeda galaxy; astronomers think the clouds might be long-sought-after galactic building blocks.

Green Bank is one of several sites operated around the Western Hemisphere that constitute the National Radio Astronomy Observatory. The facility is operated for the National Science Foundation by Associated Universities Inc., a nonprofit corporation formed by nine northeastern universities. Other observatories are in Socorro, N.M.; Tucson, Ariz., and Santiago, Chile.

Many objects in the universe emit radio waves, which can be detected from distances beyond the range of optical telescopes. Radio telescopes allow scientists to determine the makeup and age of celestial matter. Radio astronomy has resulted in many spinoff technologies used in areas ranging from the study of earthquakes to the early detection of breast cancer.

Like a Satellite TV Dish

Because of its size and accuracy, the new telescope will help researchers peer deeper into the heavens than ever and further advance theories on the origins of the universe. In particular, the device can detect gases at higher frequencies than was previously possible, meaning astronomers can explore carbon monoxide, which has the highest frequency in the gas spectrum. The substance abounds in the Milky Way, often in regions where new stars are made.

A radio telescope is a parabolic surface reflector antenna that works much like a TV satellite dish. Incident radio-frequency waves bounce off the dish to a sub-reflector placed at the focal point of the parabola. The sub-reflector directs waves to a feed horn at the center of the dish, where they're converted to electric current. The signal enters an amplifier and then a computer, which records it on magnetic tape or displays the information as an image on a screen. Waves of different intensities are given different colors to create a photograph-like depiction, so astronomers can plot images of stars, galaxies, pulsars, quasars, and supernovas, and piece together the evolution of the universe.

Planning for the Green Bank Telescope began in 1988 after the much-used 300-foot radio telescope at the observatory collapsed from fatigue. "The overall design was in the back of people's minds at the time, and once that happened, it provided the impetus to bring the new idea on line," said Mark McKinnon, an assistant scientist at Green Bank throughout most of the project. The observatory received funding from Congress for the $75 million project in 1989.

Lee King, a now-retired engineer who worked out of the observatory's headquarters in Charlottesville, Va., had designed numerous telescopes for the observatory's various sites, and served as chief engineer for the Green Bank telescope. Scientists and engineers at Charlottesville did preliminary design work with help from the Jet Propulsion Laboratory in Pasadena, Calif.

According to McKinnon, "When radio astronomers build a new instrument, they don't settle for the status quo." The Green Bank telescope would sport a host of new design features. For starters, the structure's steel members would have welded joints rather than the bolted connections used in the past.

Dave Seaman, who was a mechanical engineer on the project, said that welding stiffens the structure "because you have infinitesimal amounts of slippage at bolt joints, and these can add up."

The telescope would have an offset feed arm, meaning the subreflector at the focal point would lie outside the path of incident radio waves, increasing the viewing area by as much as 10 percent. This is accomplished by having the dish consist of a shallow section of a parabola to one side of its centerline rather than symmetrical about it.

Measuring 100 by 110 meters, the telescope's oval reflector dish has two acres of surface area. The surface would consist of 2,004 panels that could be adjusted in and out to fine-tune the shape of the surface. "The idea is to keep the telescope in focus all the time," McKinnon said. "That becomes very important at high-frequency observations, where the telescope will have its biggest impact." The active surface minimizes distortions caused by structural deflection from temperature change, irregular loading such as snow, and the variation in gravitational forces at different tilt angles. Mechanisms for manipulating the surface shape actually come from previous radio telescopes, but at Green Bank they are used on a larger scale and in more precise ways with extensive electronic control.

The observatory awarded a contract to Radiation Systems Inc., an antenna specialist in Sterling, Va., for the design, construction, and testing of the telescope. Radiation Systems chose Loral, a company in San Jose, Calif., to perform engineering work, and Electro Space Industries, a division of Radiation Systems, to design the control systems.

The observatory at Green Bank hired Seaman, plus a project manager and a structural engineer, for the telescope project. Working with engineers at Loral, they had a year and a half to come up with a detailed design before construction began in July 1991. The engineers hired by the observatory stayed through most of the project; the structural engineer retired in 2003, the project manager left after the project was finished, and Seaman left in 1998.

Much of the observatory engineers' effort focused on stress analysis to ensure structural integrity of Loral's design. Seaman recalled, "We actually ran more than 95 computer models—different design configurations. Each one of those models had about 80 different load cases for various loads of wind, ice, and snow." They also tested models in a wind tunnel to determine the structure's aerodynamic characteristics.

Construction began with excavation 20 feet deep to a hard layer of shale and the pouring of 4,000 cubic yards of concrete for a foundation. With this in place, workers installed the azimuth track and the pintle bearing, a 12-foot-diameter ball bearing around which the entire structure pivots. Sixteen 52-inch-diameter wheels rotate the structure as they travel around a circular steel track that's 210 feet in diameter. To ensure precise rotation, the track was grouted in place by hand and leveled to within 0.003 inch with a water level that uses motor-driven micrometers.

The basic structure of the telescope consists of the rotating lower section, known as the alidade, and the tipping structure, which tilts within it. The latter contains the dish and reflector arm, and pivots about a shaft made from a 36-inch pipe turning in one-meter roller bearings. Over two million pounds of counterweight on the tipping structure balance the load, making it easier to tilt.

Radiation Systems fabricated the alidade at its plant in Mexia, Texas, and then measured it in the summer heat. The company disassembled the alidade for shipment and later reassembled it in the cold of a West Virginia winter. The distance between the two towers of the alidade that support the tipping structure decreased by an inch and a half from initial measurements because of thermal contraction. But the engineers predicted this through calculations and ensured the correct final dimension by preloading the towers, actually pulling them toward each other.

Direct current electric motors drive both the rotation and tilting functions in conjunction with an elaborate servo control system. This allows operators to accelerate and decelerate the antenna smoothly, and to make fine adjustments. Eight azimuth wheels each have two 30-horsepower motors, while the elevation drive uses eight 40-hp motors. Each motor has a gear reduction box, a tachometer, and a fail-safe electric brake on its tailshaft. Each azimuth drive wheel has two pinions driving its bull gear to apply a counter torque; one rotates in the reverse direction, removing backlash from the gears. Receiving signals from a myriad of sensors, the computer-controlled servo system applies power and brakes as needed.

The aluminum surface panels of the dish's active surface measure 2×2½ meters each and adjust by means of custom-designed linear ball screw actuators placed at the intersections of the plates' corners. A computerized system of lasers bounces beams off mirrors and reflectors to constantly check the surface, determine its optimum position, and send signals to the actuators. Technicians can also input data from finite element modeling and thermal analysis to further optimize the shape. The dish surface has 2,209 electric actuators for individual plates, each powered by a dc motor and controlled by a dedicated programmable logic controller.

With construction complete, the telescope saw first light on Aug. 22, 2000, when a team led by Mark McKinnon used it to track a galaxy across the sky, and it began regular scientific observations on a limited basis in early 2001. Astronomers and engineers emphasize that its performance will evolve and improve with time as they break in different configurations of equipment on it.

Dennis Egan, head of the mechanical engineering division at the observatory in Green Bank, said, "We're doing a lot of commissioning, but we're also doing a lot of science now. We've got quite a few receivers that are up, and it is an operating telescope."

Richard Prestage, deputy site director at Green Bank, said, "The mechanical structure of the telescope itself is exceeding the expectations from the design phase. The pointing performance is better than we had hoped. The surface is better than the spec the contractors were required to deliver."

The telescope operates over a greater band of wave-lengths than any telescope before it and also points with greater accuracy. It can zero in on targets within an arcsecond, the angle subtended by a dime from a mile away.

Because of its size and complexities, many people wondered how the finished product would peform. According to Egan, "It's a wonderful structure, behaving very much as predicted. It's a real poster child for numerical analysis."

Copyright © 2004 by ASME
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