11 March 2010

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.

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