74001525 2197472 radio telescopes working

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Radio Telescopes Working.

Radio telescopes are used to study naturally occurring radio emission from stars, galaxies,

quasars, and other astronomical objects between wavelengths of about 10 meters (30megahertz [MHz]) and 1 millimeter (300 gigahertz [GHz]). At wavelengths longer than about

20 centimeters (1.5 GHz), irregularities in the ionosphere distort the incoming signals. Thiscauses a phenomenon known as scintillation, which is analogous to the twinkling of stars seen

at optical wavelengths. The absorption of cosmic radio waves by the ionosphere becomes

more important as wavelength increases.

At wavelengths longer than about 10 meters, Radio observations of the cosmic sources atthese wavelengths are difficult from ground-based radio telescopes, so that the effective

angular resolution and image quality is limited only by the size of the instrument.

Principles of Operation

Radio telescopes vary widely, but they all have two basic components:

(1) Large radio antenna and(2) Sensitive radiometer or radio receiver.


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The sensitivity of a radio telescope--i.e., the ability to measure weak sources of radio

emission--depends on the area and efficiency of the antenna and the sensitivity of theradio receiver used to amplify and detect the signals. For broadband continuum emission

the sensitivity also depends on the bandwidth of the receiver. Because cosmic radio

sources are extremely weak, radio telescopes are usually very large and only the most

sensitive radio receivers are used. Moreover, weak cosmic signals can be easily maskedby terrestrial radio interference, and great effort is taken to protect radio telescopes from

man-made interference.

The most familiar type of radio telescope is the radio reflector consisting of a parabolicantenna--the so-called dish or filled-aperture telescope--which operates in the same

manner as a television-satellite receiving antenna to focus the incoming radiation onto a

small antenna referred to as the feed, a term that originated with antennas used for radar

transmissions. In a radio telescope the feed is typically a waveguide horn and transfers theincoming signal to the sensitive radio receiver. Cryogenically cooled solid-state amplifiers

with very low internal noise are used to obtain the best possible sensitivity.

In some radio telescopes the parabolic surface is equatorially mounted, with one axisparallel to the rotation axis of the Earth. Equatorial mounts are attractive because they

allow the telescope to follow a position in the sky as the Earth rotates by moving the

antenna about a single axis parallel to the Earth's axis of rotation. But equatoriallymounted radio telescopes are difficult and expensive to build. In most modern radio

telescopes a digital computer is used to drive the telescope about the azimuth and

elevation axes to follow the motion of a radio source across the sky.

Observing times up to many hours are expended and sophisticated signal-processingtechniques are used to detect astronomical radio signals that are as much as one milliontimes weaker than the noise generated in the receiver. Signal-processing and analysis are

usually done in a digital computer.

The performance of a radio telescope is limited by

various factors:


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The accuracy of a reflecting surface that may depart from the ideal shape because of

manufacturing irregularities;The effect of wind load; thermal deformations that cause differential expansion and

contraction; and deflections due to changes in gravitational forces as the antenna is

pointed to different parts of the sky. Departures from a perfect parabolic surface become

important when they are a few percent or more of the wavelength of operation. Sincesmall structures can be built with greater precision than larger ones, radio telescopes

designed for operation at millimeter wavelength are typically only a few tens of meters

across, whereas those designed for operation at centimeter wavelengths range up to 100meters in diameter.

Radio telescopes are used to measure broad-bandwidth continuum radiation as well as

spectroscopic features due to atomic and molecular lines found in the radio spectrum ofastronomical objects. In early radio telescopes, spectroscopic observations were made by

tuning a receiver across a sufficiently large frequency range to cover the various

frequencies of interest. This procedure, however, was extremely time-consuming and

greatly restricted observations. Modern radio telescopes observe simultaneously at a largenumber of frequencies by dividing the signals up into as many as several thousand

separate frequency channels that may range over a total bandwidth of tens to hundreds of

megahertz.

The angular resolution, or ability of a radio telescope to distinguish fine detail in the sky,

depends on the wavelength of observations divided by the size of the instrument. Yet,

even the largest antennas, when used at their shortest operating wavelength, have anangular resolution only a little better than one arc minute, which is comparable to that of

the unaided human eye at optical wavelengths. Because radio telescopes operate at much

longer wavelengths than do optical telescopes, radio telescopes must be much larger than

optical telescopes to achieve the same angular resolution.At radio wavelengths, the distortions introduced by the atmosphere are less important than

at optical wavelengths, and so the theoretical angular resolution of a radio telescope can in

practice be achieved even for the largest dimensions. Also, because radio signals are easyto distribute over large distances without distortion, it is possible to build radio telescopes

of essentially unlimited dimensions. In fact, the history of radio astronomy has been one

of solving engineering problems to construct radio telescopes of continually increasingangular resolution.

The high angular resolution of radio telescopes is achieved by using the principles ofinterferometry to synthesize a very large effective aperture from a number of small

elements. In a simple two-element radio interferometer, the signals from an unresolved, or

"point," source alternately arrive in phase and out of phase as the Earth rotates and causes

a change in the difference in path from the radio source to the two elements of theinterferometer. This produces interference fringes in a manner similar to that in an optical

interferometer. If the radio source has finite angular size, then the difference in path length

to the elements of the interferometer varies across the source. The measured interference


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fringes from each interferometer pair thus depend on the detailed nature of the radio

"brightness" distribution in the sky.The rotation of the Earth can sample a sufficient number of Fourier components with

which to synthesize the effect of a large aperture and thereby reconstruct high-resolution

images of the radio sky. The laborious computational task of doing Fourier transforms to

obtain images from the interferometer data is accomplished with high-speed computersand the fast Fourier transform (FFT).

Application..

Radio Communication_

Sound and radio waves are different phenomena. Sound consists of pressure variations inmatter, such as air or water. Sound will not travel through a vacuum. Radio waves, like

visible light, infrared, ultraviolet, X-rays and gamma rays, are electromagnetic waves that

do travel through a vacuum. When you turn on a radio you hear sounds because the

transmitter at the radio station has converted the sound waves into electromagnetic waves,which are then encoded onto an electromagnetic wave in the radio frequency range

(generally in the range of 500-1600 kHz for AM stations, or 86-107 MHz for FMstations). Radio electromagnetic waves are used because they can travel very large

distances through the atmosphere without being greatly attenuated due to scattering or

absorption. Yourradio receives the radio waves, decodes this information, and uses a

speaker to change it back into a sound wave. An animated gif of this process is given

below.

74001525 2197472 radio telescopes working

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