Radio telescope

A radio telescope is a form of directional radio antenna used in radio astronomy and in tracking and collecting data from satellites and space probes. In their astronomical role they differ from optical telescopes in that they operate in the radio frequency portion of the electromagnetic spectrum where they can detect and collect data on radio sources. Radio telescopes are typically large parabolic ("dish") antenna used singularly or in an array. Radio observatories are located far from major centers of population in order to avoid electromagnetic interference (EMI) from radio, TV, radar, and other EMI emitting devices. This is similar to the locating of optical telescopes to avoid light pollution, with the difference being that radio observatories will be placed in valleys to further shield them from EMI as opposed to clear air mountain tops for optical observatories.

Early radio telescopes

The first radio antenna used to identify an astronomical radio source was one built by Karl Guthe Jansky, an engineer with Bell Telephone Laboratories, in 1931. Jansky was assigned the job of identifying sources of static that might interfere with radio telephone service. Jansky's antenna was designed to receive short wave radio signals at a frequency of 20.5 MHz (wavelength about 14.6 m). It was mounted on a turntable that allowed it to rotate in any direction, earning it the name "Jansky's merry-go-round". It had a diameter of approximately 100 ft (30 m). and stood 20 ft (6 m). tall. By rotating the antenna on a set of four Ford Model-T tires, the direction of the received interfering radio source (static) could be pinpointed. A small shed to the side of the antenna housed an analog pen-and-paper recording system. After recording signals from all directions for several months, Jansky eventually categorized them into three types of static: nearby thunderstorms, distant thunderstorms, and a faint steady hiss of unknown origin. Jansky finally determined that the "faint hiss" repeated on a cycle of 23 hours and 56 minutes. This four-minute lag is a typical an astronomical sidereal day, the time it takes any "fixed" object located on the celestial sphere to pass overhead twice. By comparing his observations with optical astronomical maps, Jansky concluded that the radiation was coming from the Milky Way and was strongest in the direction of the center of the galaxy, in the constellation of Sagittarius.

Grote Reber was one of the pioneers of what became known as radio astronomy when he built the first parabolic "dish" radio telescope (9 m in diameter) in 1937. He was instrumental in repeating Karl Guthe Jansky's pioneering but somewhat simple work, and went on to conduct the first sky survey in the radio frequencies. After World War II, substantial improvements in radio astronomy technology were made by astronomers in Europe, Australia and the United States, and the field of radio astronomy began to blossom.

Radio telescope types

The range of frequencies in the electromagnetic spectrum that makes up the radio spectrum is very large. This means the variety and types of antennas that are used as radio telescopes vary in design, size, and configuration. At wavelengths of 30 meters to 3 meters (10 MHz - 100 MHz), they are generally directional antenna arrays similar to "TV antennas" or large stationary reflectors with moveable focal points. Since the wave length being observed with these types of antennas are so long, the "reflector" surfaces can be constructed from coarse wire mesh. At shorter wavelengths “dish” style radio telescopes predominate. The angular resolution of a dish style antenna is a function of the diameter of the dish in proportion to the wavelength of the electromagnetic radiation being observed. This dictates the size of the dish a radio telescope needs to have a useful resolution. Radio telescopes operating at wavelengths of 3 meters to 30 cm (100 MHz to 1 GHz) are usually well over 100 meters in diameter. Telescopes working at wavelengths above 30 cm (1 GHz) range in size from 3 to 90 meters in diameter.

Big dishes

File:Jodrell Bank Observatory.Lovell telescope.jpg

The 76.0 m Lovell radio telescope at Jodrell Bank Observatory which, at the time of its construction, was the largest steerable dish radio telescope in the world.

In the late 1950s and early 1960s saw the development of large single-dish radio telescopes. The largest individual radio telescope is the RATAN-600 (built in 1977 in the USSR, belongs to Russia since 1991) with 576 meter diameter of circular antenna (RATAN-600 description). Other two individual radio telescopes at Pushchino Radio Astronomy Observatory, Russia, designed specially for the low frequency observations, are between the largest in their class. LPA (LPA description (in Russian)) is 187 x 384 m size phased array meridional radio telescope, and DKR-1000 is 1000 x 1000 m cross radio telescope (DKR-1000 description (in Russian) ). The largest radio telescope in Europe is the 100 meter diameter antenna in Effelsberg, Germany, which also was the largest fully steerable telecope for 30 years until the Green Bank Telescope was opened in 2000. The largest radio telescope in the United States until 1998 was Ohio State University's The Big Ear.

File:Arecibo ObservatoryX.jpg

World's largest single-aperture radio telescope at Arecibo Observatory in Puerto Rico

Other well known disk radio telescopes include the Arecibo radio telescope located in Arecibo, Puerto Rico, which is steerable within about 20° of the zenith and is the largest single-aperture telescope (cf. multiple aperture telescope) ever to be constructed, and the fully steerable Lovell telescope at Jodrell Bank in the United Kingdom. A typical size of the single antenna of a radio telescope is 25 metre, dozens of radio telescopes with comparable sizes are operated in radio observatories all over the world.

Radio interferometry

The Very Large Array, an interferometric array formed from many smaller telescopes, like many larger radio telescopes.

One of the most notable developments came in 1946 with the introduction of the technique called astronomical interferometry. Astronomical radio interferometers usually consist either of arrays of parabolic dishes (e.g. the One-Mile Telescope), arrays of one-dimensional antennas (e.g. the Molonglo Observatory Synthesis Telescope) or two-dimensional arrays of omni-directional dipoles (e.g. Tony Hewish's Pulsar Array). All of the telescopes in the aray are widely separated and are connected together using coaxial cable, waveguide, optical fiber, or other type of transmission line. This not only increases the total signal collected, it can also be used in a process called Aperture synthesis to vastly increase resolution. This technique works by superposing (interfering) the signal waves from the different telescopes on the principle that waves that coincide with the same phase will add to each other while two waves that have opposite phases will cancel each other out. This creates a combed telescope that is the size of the antennas furthest apart in the array. In order to produce a high quality image, a large number of different separations between different telescopes are required (the projected separation between any two telescopes as seen from the radio source is called a baseline) - as many different baselines as possible are required in order to get a good quality image (For example the Very Large Array (VLA) in Socorro, New Mexico has 27 telescopes giving 351 independent baselines at once to achieve resolution of 0.2 arc seconds at 3 cm wavelengths^[1]). Martin Ryle's group in Cambridge obtained a Nobel Prize for interferometry and aperture synthesis^[2]. The Lloyd's mirror interferometer was also developed independently in 1946 by Joseph Pawsey's group at the University of Sydney^[3]. In the early 1950s the Cambridge Interferometer mapped the radio sky to produce the famous 2C and 3C surveys of radio sources. The largest existing radio telescope array is the Giant Metrewave Radio Telescope, located in Pune, India. A larger array, LOFAR (the 'LOw Frequency ARray') is currently being constructed in western Europe, consisting of 25 000 small antennas over an area several hundreds of kilometres in diameter.

Astronomical observations

Many astronomical objects are not only observable in visible light but also emit radiation at radio wavelegths. Besides observing energetic objects such as pulsars and quasars, radio telescopes are able to "image" most astronomical objects such as, galaxies, nebulae, and even radio emissions from planets.

Notes

^ gps.caltech.edu - Microwave Probing of the Invisible by Duane O. Muhleman
^ Nature 158 pp 339 1946
^ Nature 157 pp 158 1946

References

astronomytoday.com - "Radio Astronomy" by Sancar J Fredsti
Rohlfs, K., & Wilson, T. L. (2004). Tools of radio astronomy. Astronomy and astrophysics library. Berlin: Springer.
Asimov, I. (1979). Isaac Asimov's Book of facts; Sky Watchers. New York: Grosset & Dunlap. Page 390 - 399. ISBN 0803893477

[1] s.caltech.edu - Microwave Probing of the Invisible by Duane O. Muhleman

[2] Nature 158 pp 339 1946

[3] Nature 157 pp 158 1946

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