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In contrast to an "ordinary" telescope, which produces visible light images, a radio telescope "sees" radio waves emitted by radio sources, typically by means of a large parabolic ("dish") antenna, or arrays of them.
The Parkes 64 metre radio telescope in New South Wales, Australia (the bigger of the two shown)
The best-known (and largest) radio telescope is the Arecibo radio telescope located in Arecibo, Puerto Rico. Another well-known radio
telescope is the Very Large Array (VLA), in Socorro, New Mexico. The largest radio telescope in Europe is the 100 metre diameter antenna in Effelsberg, Germany. A typical size of the single antenna of a radio telescope is 25 meter, dozens of radio telescopes with
comparable sizes are operated in radio observatories all over the world.
The sub-field of astronomy related to observations made through radio
telescopes is known as radio astronomy.
Many celestial objects, such as pulsars or active galaxies (like quasars), produce radio-frequency radiation
and so are best "visible" or even only visible in the radio region of electromagnetic spectrum. By examining the frequency, power and timing of radio emissions from
these objects, astronomers can improve our understanding of the Universe
Radio telescopes are also occasionally involved in SETI and tracking space probes (see Deep
Space Network).
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