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Radio Telescopes

The dish shaped radio telescope uses a metal paraboloidal reflecting surface to focus incoming radiation to a point where the receiver is mounted.

Radio telescope diagram

Unlike optical telescopes, whose reflector needs to be extremely smooth and uniform, a radio dish need not have a perfect surface; it can even be made of separate panels covered in perforated mesh. This is because for a surface to be a good reflector its irregularities need to be no greater then one tenth of the observed wavelength. For example if observing at a wavelength of 21 cm the dish can have irregularities of up to 2.1 cm. When a detection is made the receiver produces an electrical current that is amplified, filtered for noise and then recorded.

Beam pattern and resolution of a radio telescope

The sensitivity of a telescope is a function of angle from the normal. The full width half max resolution angle is the minimum angle projected upon the sky that two sources can be resolved, i.e. if they were any closer together they would appear to be one object. The resolution of a telescope is dependant upon the diameter (D) of the dish and the wavelength at which measurements are being made. Radio waves have a large wavelength and so poor resolution compared to say optical wavelengths. To improve the resolution radio telescopes are typically made as large as possible, which also allows maximum photon collection, or used in the form of arrays. In an array two or more telescopes are used in conjunction and their signals combined. The separation distance can be very large, some arrays have telescopes situated on opposite ends of the earth, if this is the case D becomes the separation distance and the resolvable angle becomes very small.

Interference and protected bands

Transmitters on earth, TV, AM and FM, military channels etc, produce a myriad of radio signals covering nearly all the radio spectrum. A radio astronomer has the constant challenge of finding an area where transmissions are at a minimum, e.g. away from cities, or finding a frequency where there are no terrestrial signals. Having said this there are certain protected frequencies specifically put aside for radio astronomers, one such being 21 cm.