Radio interferometer

The invention: An astronomical instrument that combines multiple

radio telescopes into a single system that makes possible the

exploration of distant space.

The people behind the invention:

Sir Martin Ryle (1918-1984), an English astronomer

Karl Jansky (1905-1950), an American radio engineer

Hendrik Christoffel van de Hulst (1918- ), a Dutch radio

astronomer

Harold Irving Ewan (1922- ), an American astrophysicist

Edward Mills Purcell (1912-1997), an American physicist

Seeing with Radio

Since the early 1600’s, astronomers have relied on optical telescopes

for viewing stellar objects. Optical telescopes detect the

visible light from stars, galaxies, quasars, and other astronomical

objects. Throughout the late twentieth century, astronomers developed

more powerful optical telescopes for peering deeper into the

cosmos and viewing objects located hundreds of millions of lightyears

away from the earth.





In 1933, Karl Jansky, an American radio engineer with Bell Telephone

Laboratories, constructed a radio antenna receiver for locating

sources of telephone interference. Jansky discovered a daily radio

burst that he was able to trace to the center of the Milky Way

galaxy. In 1935, Grote Reber, another American radio engineer, followed

up Jansky’s work with the construction of the first dishshaped

“radio” telescope. Reber used his 9-meter-diameter radio

telescope to repeat Jansky’s experiments and to locate other radio

sources in space. He was able to map precisely the locations of various

radio sources in space, some of which later were identified as

galaxies and quasars.

Following World War II (that is, after 1945), radio astronomy

blossomed with the help of surplus radar equipment. Radio astronomy

tries to locate objects in space by picking up the radio waves that they emit. In 1944, the Dutch astronomer Hendrik Christoffel

van de Hulst had proposed that hydrogen atoms emit radio waves

with a 21-centimeter wavelength. Because hydrogen is the most

abundant element in the universe, van de Hulst’s discovery had explained

the nature of extraterrestrial radio waves. His theory later

was confirmed by the American radio astronomers Harold Irving

Ewen and Edward Mills Purcell of Harvard University.

By coupling the newly invented computer technology with radio

telescopes, astronomers were able to generate a radio image of a star

almost identical to the star’s optical image. Amajor advantage of radio

telescopes over optical telescopes is the ability of radio telescopes

to detect extraterrestrial radio emissions day or night, as well as their

ability to bypass the cosmic dust that dims or blocks visible light.

More with Less

After 1945, major research groups were formed in England, Australia,

and The Netherlands. Sir Martin Ryle was head of the Mullard

Radio Astronomy Observatory of the Cavendish Laboratory,

University of Cambridge. He had worked with radar for the Telecommunications

Research Establishment during World War II.

The radio telescopes developed by Ryle and other astronomers

operate on the same basic principle as satellite television receivers.

A constant stream of radio waves strikes the parabolic-shaped reflector

dish, which aims all the radio waves at a focusing point

above the dish. The focusing point directs the concentrated radio

beam to the center of the dish, where it is sent to a radio receiver,

then an amplifier, and finally to a chart recorder or computer.

With large-diameter radio telescopes, astronomers can locate

stars and galaxies that cannot be seen with optical telescopes. This

ability to detect more distant objects is called “resolution.” Like

optical telescopes, large-diameter radio telescopes have better resolution

than smaller ones. Very large radio telescopes were constructed

in the late 1950’s and early 1960’s (Jodrell Bank, England;

Green Bank, West Virginia; Arecibo, Puerto Rico). Instead of just

building larger radio telescopes to achieve greater resolution, however,

Ryle developed a method called “interferometry.” In Ryle’s

method, a computer is used to combine the incoming radio waves of two or more movable radio telescopes pointed at the same stellar

object.

Suppose that one had a 30-meter-diameter radio telescope. Its radio

wave-collecting area would be limited to its diameter. If a second

identical 30-meter-diameter radio telescope was linked with

the first, then one would have an interferometer. The two radio telescopes

would point exactly at the same stellar object, and the radio

emissions from this object captured by the two telescopes would be

combined by computer to produce a higher-resolution image. If the

two radio telescopes were located 1.6 kilometers apart, then their

combined resolution would be equivalent to that of a single radio

telescope dish 1.6 kilometers in diameter.

Ryle constructed the first true radio telescope interferometer at

the Mullard Radio Astronomy Observatory in 1955. He used combinations

of radio telescopes to produce interferometers containing

about twelve radio receivers. Ryle’s interferometer greatly improved

radio telescope resolution for detecting stellar radio sources, mapping

the locations of stars and galaxies, assisting in the discovery of  “quasars” (quasi-stellar radio sources), measuring the earth’s rotation

around the Sun, and measuring the motion of the solar system

through space.

Consequences

Following Ryle’s discovery, interferometers were constructed at

radio astronomy observatories throughout the world. The United

States established the National Radio Astronomy Observatory (NRAO)

in rural Green Bank, West Virginia. The NRAO is operated by nine

eastern universities and is funded by the National Science Foundation.

At Green Bank, a three-telescope interferometer was constructed,

with each radio telescope having a 26-meter-diameter

dish. During the late 1970’s, theNRAOconstructed the largest radio

interferometer in the world, the Very Large Array (VLA). The VLA,

located approximately 80 kilometers west of Socorro, New Mexico,

consists of twenty-seven 25-meter-diameter radio telescopes linked

by a supercomputer. The VLA has a resolution equivalent to that of

a single radio telescope 32 kilometers in diameter.

Even larger radio telescope interferometers can be created with

a technique known as “very long baseline interferometry” (VLBI).

VLBI has been used to construct a radio telescope having an effective

diameter of several thousand kilometers. Such an arrangement

involves the precise synchronization of radio telescopes located

in several different parts of the world. Supernova 1987A in

the Large Magellanic Cloud was studied using a VLBI arrangement

between observatories located in Australia, South America,

and South Africa.

Launching radio telescopes into orbit and linking them with

ground-based radio telescopes could produce a radio telescope

whose effective diameter would be larger than that of the earth.

Such instruments will enable astronomers to map the distribution

of galaxies, quasars, and other cosmic objects, to understand the

origin and evolution of the universe, and possibly to detect meaningful

radio signals from extraterrestrial civilizations.