Amazing Facts

Radio Telescope

The largest single-aperture radio telescope fills a valley in Puerto
Rico. Called the Arecibo radio telescope, it's a giant dish antenna
305 meters (1000 feet) in diameter.

Most radio telescopes look like large satellite or radar dishes. The
dish is a metal reflector that concentrates radio waves just like an
optical telescope's mirror concentrates light waves. Instead of
looking at the output with a lens or a camera, a radio telescope
uses a radio receiver to pick up the signals.

Since many astronomical objects create radio noise, the radio
telescope can often detect the faint signals of a distant pulsar,
quasar, interstellar gas cloud or galaxy. The Sun and Jupiter also
broadcast strong radio signals.

Most radio telescopes are able to point anywhere in the sky, like a
radar dish. Arecibo, however, has a unique design. Because of its
huge size, the dish is rigidly fixed, held by towers much like a
suspension bridge.

Since the big dish can't move, to aim the radio telescope, small
movable radio antennas are suspended high above the center of the
dish. The incoming signals bounce off the dish and are focused onto
the small antennas. By aiming the small antennas in different
directions, astronomers pick up signals that have bounced off the
dish from different parts of the sky.

There is also a powerful transmitter at the antenna that allows
Arecibo to be used for radar. The antenna was originally built to
study the upper atmosphere region called the ionosphere, which radio
signals bounce off. The radio signals would start from the
transmitter, bounce off the big dish, then bounce off the ionosphere
(which has electrons knocked off atoms by the Sun's ultraviolet

Some of the reflected radio waves return to the antenna and are
detected. Measuring how long the signals take to come back tells how
far the layer is. The strength of the signal tells how dense the
ionosphere is at that level.

But the transmitter can be tuned to higher frequencies that pass
straight through the ionosphere and into space. The signals can be
aimed at the Moon, Venus, Mercury, and other solar-system objects.

Even though the reflections are extremely weak, supersensitive
receivers can detect them. Sometimes another radiotelescope is used
together with Arecibo. NASA's Deep Space Network (DSN) in Southern
California is often used, with one antenna serving as the
transmitter and the other as the receiver.

One of the most exciting uses of the Arecibo dish is in the search
for extraterrestrial intelligence (SETI). NASA's High-Resolution
Microwave Survey (HRMS) project sometimes will use it to search for
such signals.

Related Tags: Telescope  Antenna  Radio  
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