The Arecibo Radio Telescope

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Those who see the Arecibo radio telescope for the first time are astounded by the enormous size of the reflecting surface The huge spherical reflector is 1,000 feet in diameter and 167 feet deep, and covers an area of about twenty acres. The dish surface is made of almost forty thousand perforated aluminum panels, each measuring 3 feet by 6 feet, supported by a network of steel cables strung across the underlying dish to position them. Suspended 450 feet above the reflector is a nine-hundred-ton platform. Similar in design to a bridge, it hangs in midair on eighteen cables, which are strung from three reinforced concrete towers around the perimeter. Each tower is anchored to the ground with seven 3.25-inch-diameter steel bridge cables. Another system of three pairs of cables runs from each corner of the platform to large concrete blocks under the reflector. They are attached to giant jacks that allow adjustment of the height of each corner of the dish with millimeter precision.

Just below the triangular frame of the upper platform is a circular track on which the azimuth arm turns. Since the dish is embedded in the earth and cannot be rotated, the azimuth arm can be adjusted to point to particular positions in the sky. The azimuth arm is a bow-shaped structure 328 feet long that allows for positions anywhere up to twenty degrees from the vertical.

Hanging below the azimuth arm are various antennae, each tuned to a narrow band of frequencies. The antennae point downward and are designed specially for the Arecibo spherical reflector. Aiming a feed antenna at a certain point on the reflector allows radio emissions originating from a very small area of the sky in line with the feed antenna to be focused.

The massive radio telescope at Arecibo in Puerto Rico can detect the source of extremely distant radio waves.

The Arecibo telescope detects the source of radio waves more distant than any other radio telescope. It has scoured the cosmos from within the nearby solar system to within 5 percent of the edge of the universe, 12 billion light-years away. Arecibo studies the properties of planets, stars, comets, and asteroids within the Milky Way, as well as more exotic cosmic entities from the farthest reaches of the universe, such as supernovas and even black holes. Many of the radio waves emitted from deep space billions of light-years away arrive so weak that only the Big Ear can detect them. One of Arecibo's unique capabilities is its ability to analyze surface properties of distant objects by transmitting radar waves to them and then capturing the echo that bounces back. To perform such an operation, Arecibo possesses a one-megawatt planetary radar transmitter located in a special room. Following a short transmission period, the dish awaits the echo's return. Analyzing the returning echoes provides information about surface properties, the size of the targeted object, and its distance from Earth.

Yet, as the great astronomer and author Isaac Asimov noted about Arecibo and other large radio telescopes, "Even the largest radio telescopes are not very good at resolution if they are regarded as single structures in themselves. They can't be capturing the size of the wavelengths they deal with."10 What Asimov meant by his comment was that radio wavelengths that exceeded the diameters of large telescopes were only partially captured, therefore part of the cosmic information is lost.

Fortunately for Asimov and all radio astronomers, Martin Ryle at Cambridge University in England had already begun work on a solution to that problem. In the late 1950s, he described a new science called in-terferometry that could link multiple telescopes, located many miles apart, to form a network of radio telescopes working as one to piece together any information from partially captured radio waves.

Long-Baseline Interferometry

Ryle understood that massive radio telescopes, many miles in diameter, were desirable but impossible to build. As an alternative, he proposed the ingenious solution that one could be synthesized by linking many smaller ones. Working in unison, their signals could be combined to produce cosmic maps and photographs far superior to those produced by Arecibo alone.

In 1964, Caltech initiated interferometry with twin dishes at the Owens Valley Radio Observatory in California. Each was ninety feet in diameter and mounted on railroad tracks so they could be moved varying distances from each other, with a maximum separation of sixteen hundred feet. Both were cabled together to a central data-gathering laboratory where the captured radio waves could be amplified and then combined to form a single stronger signal that could later be transformed into a photograph.

Astronomers were astonished when they realized that just thirty years following the initial discoveries of Jansky and Reber, radio telescopes had developed the best method for observing the universe in sharp detail. They mulled over their success and wondered what quality imagery they might achieve by using in-terferometry to link multiple telescopes together over greater distances. Such a notion, called long-baseline interferometry (LBI), was on the horizon.

By the mid-1970s, radio astronomers were eager to experiment with LBI to generate better resolution of distant objects. At the heart of LBI is a large array of multiple telescopes interconnected by interferomet-ric equipment. Many new radio telescopes were constructed and are still in use. The largest is the Very Large Array (VLA) on the plains of San Agustin fifty miles west of Socorro, New Mexico.

The VLA

One of the world's premier astronomical radio observatories, the VLA consists of twenty-seven radio dishes

The position of the twenty-seven radio telescopes of New Mexico's Very Large Array can be adjusted to measure wavelengths from distant objects in multiple configurations.

in a Y-shaped configuration. Each dish is eighty-two feet in diameter, weighs 230 tons, and is mounted on rails to provide movement. When data are electronically combined from the array, the resulting resolution is equivalent to a single antenna twenty-two miles in diameter.

The rails that provide movement for each antenna function the same as the zoom lens on a camera. By moving the antennae closer together or farther apart, astronomers can achieve either a wide-angle look into space or a tight telephoto view. Greatest detail is achieved when the array is at maximum disbursement. As the size of the array gradually decreases to the smallest spread, when the telescopes are all placed within four-tenths of a mile of the center, scientists achieve a wide-angle view of the overall structure of the object they are observing. By gathering wavelengths from the same distant object in multiple configurations, astronomers can capture a great deal more information.

The position of the twenty-seven radio telescopes of New Mexico's Very Large Array can be adjusted to measure wavelengths from distant objects in multiple configurations.

Today, configurations of the VLA are changed about every four months.

The development of long-baseline radio interfer-ometry showed images of the sky that were significantly finer than anything previously. VLA reveals detail as if the observer were 100,000 times closer to the object. Barry Clark, who currently directs the scheduling for the VLA, commented:

What [astronomers] want to do is to study everything from Jupiter to the most distant objects in the universe. Some of the most interesting results have come from regions where stars have recently formed, regions where stars have exploded, and regions of what might be supermassive black holes.11

During the early years of the twenty-first century, the long-baseline interferometer at the VLA has been used for a series of investigations into deep space, studying phenomena billions of light-years away. One recent project objective was to use the maximum capability of each of these telescopes to capture light from objects such as galaxies and quasars extremely far away and thus see them as they were when the universe was young. By comparing these ghost images from the early universe with the same type of objects at closer distances, and thus from a more recent past, astronomers can learn how these objects likely changed over billions of years.

A second function of the VLA is to make a detailed image of the supernova called 3C58. A supernova is the result of a cataclysmic explosion caused when a star exhausts its fuel and ends its life in a massive fiery fury. The new image of this debris from 3C58 will be compared with earlier images dating back to 1984 to learn how fast the material is moving outward from the explosion site and to monitor other changes in the supernova.

The success of the VLA using the latest interferome-try tantalized the imaginations of astronomers. If tele scopes spread over a twenty-two-mile baseline could improve the science of astronomy significantly, what might be the result of a baseline hundreds of times as long?

The Very Long Baseline Array

In the late 1980s, astronomers were ready to create a virtual radio telescope spanning thousands of miles. Using fundamentally the same interferometry and radio telescopes that were in use in Socorro, ten sites spanning the Pacific Ocean and the continental United States were selected as segments of the Very Long Baseline Array (VLBA).

In 1993, astronomers working for the National Radio Astronomy Observatory (NRAO) finished coordinating the network and began operating the VLBA, the world's largest telescope. Each of its ten sites is equipped with an eighty-foot dish antenna, and together they capture the same radio signals from deep space sources. The spread of the array, roughly five thousand miles, provides the VLBA with the highest resolution of any telescope. Astronomers working on the VLBA describe its resolution as being less than one milliarcsecond. This tiny angle corresponds to the width of a human hair as seen from ten miles. According to astronomers working on the VLBA, "This is equal to being able to read a newspaper in New York while standing in Los Angeles."12

The antennae, which operate unattended most of the time, are controlled by a single operator in Socorro. Astronomical data from the ten antennae are recorded on digital tape with the assistance of atomic clocks to capture precisely the same radio waves at each site. The atomic clocks are accurate to within one-billionth of a second per day, the equivalent of one second of deviation over 6 million years. The tapes are then shipped to Socorro where they are correlated by highspeed computers.

Since its inception, the VLBA has provided remarkably detailed photographs of the powerful cores of distant quasars, unusually bright remote objects that spew out tremendous amounts of energy. Before radio telescopes, quasars appeared to be simply bright distant stars, but with the VLBA, they are known to be millions of times brighter than stars. The VLBA has also provided precise measurements of the speed of debris from exploded supernovas at the cores of distant galaxies. Regarding the VLBA, astrophotographer Russ Dickman emphasizes,

Greater resolution is vital to astronomy because it shows more details, and details are clues to origins.

We have been looking at galaxy cores and quasars for a long time but we don't fully understand the processes. The key to what is happening is the core, near the central engine. That's because the "engine" —whether it's a black hole or some equally bizarre object—drives the entire galaxy.13

Physicists understood that if radio telescopes were effective at capturing long wavelengths, other types of telescopes might be capable of capturing very short wavelengths. Toward the end of the 1950s, while longwave radio and midlength visible light telescopes were probing the depths of space making new discoveries, astrophysicists were wondering what else they might discover by studying very short wavelengths of light. Very short wavelengths, much shorter than visible light, were known to exist, but the problem facing the astronomy community was how to capture them. Their very short wavelengths, often one-hundredth the length of visible light, are rarely able to penetrate the earth's insulating atmosphere. For this reason, earthbound telescopes would be of little value.

By the beginning of the 1960s, however, when America began rocketing satellites far above the earth's atmosphere, astronomers saw them as a solution for capturing very short wavelengths.

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