Radio telescope

Radio telescopes are typically large parabolic ("dish") antennas similar to those employed in tracking and communicating with satellites and space probes.

Jansky's antenna was an array of dipoles and reflectors designed to receive short wave radio signals at a frequency of 20.5 MHz (wavelength about 14.6 meters).

Thus Jansky suspected that the hiss originated outside of the Solar System, and by comparing his observations with optical astronomical maps, Jansky concluded that the radiation was coming from the Milky Way Galaxy and was strongest in the direction of the center of the galaxy, in the constellation of Sagittarius.

He built the first parabolic "dish" radio telescope, 9 metres (30 ft) in diameter, in his back yard in Wheaton, Illinois in 1937.

As a consequence, the types of antennas that are used as radio telescopes vary widely in design, size, and configuration.

Telescopes working at wavelengths shorter than 30 cm (above 1 GHz) range in size from 3 to 90 meters in diameter.

[citation needed] The increasing use of radio frequencies for communication makes astronomical observations more and more difficult (see Open spectrum).

By changing the shape of the dish and moving the feed cabin on its cables, the telescope can be steered to point to any region of the sky up to 40° from the zenith.

The above stationary dishes are not fully "steerable"; they can only be aimed at points in an area of the sky near the zenith, and cannot receive from sources near the horizon.

The fourth-largest fully steerable radio telescopes are six 70-meter dishes: three Russian RT-70, and three in the NASA Deep Space Network.

All of the telescopes in the array are widely separated and are usually connected using coaxial cable, waveguide, optical fiber, or other type of transmission line.

For example, the Very Large Array (VLA) near Socorro, New Mexico has 27 telescopes with 351 independent baselines at once, which achieves a resolution of 0.2 arc seconds at 3 cm wavelengths.

[12] Martin Ryle's group in Cambridge obtained a Nobel Prize for interferometry and aperture synthesis.

[13] The Lloyd's mirror interferometer was also developed independently in 1946 by Joseph Pawsey's group at the University of Sydney.

The world's largest physically connected telescope, the Square Kilometre Array (SKA), is planned to start operations in 2027.

[15] Many astronomical objects are not only observable in visible light but also emit radiation at radio wavelengths.