Chapter 3
Radio Telescopes: Discovering the Invisible Universe

In 1932, while working for Bell Labs in New Jersey, physicist Karl Jansky made a rather unsettling discovery. Jansky's work dealt with the problems of radio and telephone communications, and he had been assigned the task of tracking down the crackling static noises that plagued early overseas telephone reception. Stymied by the source of the interference, Jansky constructed a large but crude antenna system that he could rotate. He then recorded in magnetic tape two well-known kinds of atmospheric static: sharp crashes from local thunderstorms and noise from distant thunderstorms. After listening to his recordings, however, he identified something he had never noticed before: a weak third sort of static that sounded more like a hiss.

Keen on tracing the origin of the hiss, Jansky constructed a better rotating antenna that looked similar to a merry-go-round—a circle of both vertical and horizontal metal rods with a rotation that allowed it to track the hiss. Jansky rotated his antenna and carefully measured the timing of the faint hiss. After looking at his findings, Jansky determined that the noise was not generated by an earthbound meteorological event. Instead, he deduced that the faint hiss had to be coming from somewhere in the center of the Milky Way.

At the age of twenty-six, Jansky had made a historic discovery. As a physicist, Jansky understood that all forms of energy waves passing through the universe, including light, were electromagnetic waves of varying lengths. All are generated when atoms and molecules collide with each other, forming waves, much as a stone tossed into a pool of water forms waves. One of the characteristics of long radio waves that differentiate them from visible light waves is that they can be heard but not seen. Therefore, Jansky concluded that bodies deep in space could and did emit invisible light rays as well as visible ones.

Light waves that are characterized by a particular range of electromagnetic wavelengths between seven hundred and four hundred nanometers (a nanometers is one-billionth of a meter) happen to be visible to the human eye, yet they account for only a small fraction of the electromagnetic spectrum. All other light waves are either too long or too short to be seen. Those that are much longer, typically ranging from slightly less than one inch to hundreds of feet, fall into the spectrum called radio waves. Nonetheless, they are forms of electromagnetic light waves.

Jansky's fortuitous discovery gave birth to a new branch of astronomy called radio astronomy. He determined that all cosmic objects emit radio waves (as well as other energy waves) that can reveal just as much as, and often more than, visible light. Following Jansky's discovery, astronomers developed more sophisticated means of collecting radio waves than his rudimentary merry-go-round. These innovative devices allowed astronomers to gather radio waves, analyze them, and even convert them into spectacular photographs.

The First Radio Telescope

In 1937, Grote Reber, an American radio engineer, read about Jansky's work and set out to construct the first professional radio telescope. Reber understood that radio waves were no different from visible light waves except for their longer lengths and lower energy levels. With this information, he realized the best design for an efficient radio telescope would be to copy designs of existing optical telescopes.

Since radio waves can be very long, very large surfaces would be needed to capture and focus them. Mirrors made of lightweight and inexpensive wire mesh would be more practical than glass mirrors yet just as efficient for capturing the much longer radio wavelengths. Although the wire mesh mirrors have holes in them that visible light passes through, longer radio waves reflect off them just as visible light waves reflect off glass mirrors. Reber also understood that radio wave mirrors would require a focal point, but instead of it being an eyepiece, it would be a receiver capable of capturing and then amplifying the waves for recording and converting into photographs. The reason radio telescopes are so similar to visible light telescopes is because the physics of all electromagnetic energy is so similar; both visible light and radio waves move through space at the speed of light and both reflect off a variety of surfaces in the same way.

Reber's first radio telescope was a thirty-foot reflector mirror set up in his backyard in Illinois. Reber spent long hours every night scanning the skies with his telescope. He worked at night because in 1937 automobile engines emitted sparks out of their tailpipes that created too much interference during the daytime. At night, fewer people drove their cars and Reber could get better readings.

Success was a matter of trial and error. Reber designed the first telescope to detect wavelengths of about six inches, but it failed to detect any signals. The second one, designed for two-foot-long waves, also failed. Finally, he adjusted for six-foot waves and was successful in detecting signals from the Milky Way, confirming Jansky's earlier discovery. Reber continued his

investigations of radio sources and confirmed that they arose throughout the Milky Way. In 1944, he published the first maps of the galaxy based solely on radio waves.

Through his trials, Reber learned that his telescope was capable of detecting a broad spectrum of radio wavelengths and that he could tune it to locate and record specific ones of interest. He was also the first to recognize that much of deep space—which appears to be a blur of dust and gas storms when viewed with optical telescopes—can look extraordinarily detailed when viewed with radio telescopes. The reason for the greater resolution is that the longer wavelengths pass through the dust and gas of deep space, unlike visible wavelengths that are blocked or refracted.

How Radio Telescopes Work

Radio telescopes work with three basic components. Each must have a large metallic mirror (also called a dish or antenna), a focus point called the feed, and a sensitive radiometer or radio receiver. The sensitivity of a radio telescope—its ability to capture and analyze weak sources of radio waves—depends on the area and efficiency of the dish and the sensitivity of the receiver used to amplify the signals. Cosmic radio sources can be extremely weak because they sometimes emanate billions of light-years away. Since radio waves can be weak, observing times up to many hours are sometimes needed to capture their signals.

Weak radio waves also demand careful aiming of the mirror. In some radio telescopes the dish surface is equatorially mounted; one axis is parallel to the rotation axis of the earth. This mounting technique allows the telescope to follow a position in the sky as the earth rotates by moving the antenna parallel to the earth's axis of rotation.

Once properly aimed, the dish operates in the same manner as a television satellite antenna to capture and focus incoming radiation onto the feed that is suspended a few feet above the center of the dish. Radio waves are then transferred from the feed to the receiver by way of a coaxial cable. In the earliest form of radio telescope, such as that designed by Reber, the receiver was directly coupled to the feed, but today, the detected signal is carried away from the feed to a nearby electronics laboratory where it can be amplified, recorded, analyzed, and converted into pictures.

The conversion of nonvisible waves into visible pictures is now performed by complex computer software programs. Their job is to take the amplified waves, determine their individual profiles based on several characteristics, assign a numeric value, and then convert their values into dots, called pixels, which are transferred onto photographic paper. Sometimes following hours of capturing distant radio waves, thousands of numeric values are combined by computers to form one dazzling color photograph of tens of thousands of pixels depicting an expansive galaxy swirling in a dense, gaseous region billions of light-years away.

The Big Ear

As the size of radio dishes grew to capture longer electromagnetic wavelengths from deeper space, a consortium of American radio astronomers stumbled across a naturally occurring concave depression in the earth's limestone surface in the mountainous jungle of Arecibo, Puerto Rico. In 1963, recognizing that this depression might be a perfect place to construct an enormous radio observatory, scientists began construction of a dish one thousand feet in diameter that was immediately dubbed "the Big Ear." Besides the natural depression, the surrounding jungle acted as a buffer to keep towns and highways at a safe distance, thereby minimizing terrestrial interference with incoming celestial signals. The giant size of the reflector is what makes the Arecibo Observatory unique. It is the largest telescope on the planet, which means it is also the world's most sensitive radio telescope. Other radio telescopes may require observing times of several hours to collect enough energy and data from a distant radio source, whereas at Arecibo such an observation may require just a few minutes.

Such a massive telescope embedded in the earth's crust has one major flaw: It cannot be aimed by rotating or tipping the dish. To resolve this problem, astrophysicists realized they would need to aim the telescope by moving the feed rather than the dish. The feed at Arecibo, which is suspended 450 feet above the dish, hangs in midair on eighteen cables. It is a bow-shaped structure 328 feet long that can be moved side to side and positioned anywhere up to twenty degrees from the vertical to focus on objects in deep space. Aiming the feed at a certain point above the dish enables radio emissions originating from a very small area of the sky in line with the feed to be accurately focused, thereby producing superb photographs.

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 interferometry 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 interferometry 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 interferometric 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

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

The development of long-baseline radio interferometry 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 interferometry tantalized the imaginations of astronomers. If telescopes 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 high-speed 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 long-wave 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.