A radio telescope is a telescope designed to pick up radio waves from space. It is made up of several main parts.
The dish is an antenna which focuses the transmissions from the satellite onto the receiver in much the same way as your TV dish focuses signals onto the TV receiver. The bigger the dish, the better the signal so radio telescopes can be huge. In fact the biggest telescopes in the world are radio telescopes.
The design of the antenna will depend on the frequency of radio waves you are listening for. When considering the properties of an radio astronomy antenna its often useful to think in terms of it transmitting (rather than receiving).
– Reciprocity theorem: Performance of an antenna when collecting radiation form a point source at infinity may be studied by considering its properties as a transmitter:
– Far-field pattern (the antenna’s “beam”) is the Fourier Transform of aperture plane electric field distribution:
A receiver is an electronic device that receives radio waves and converts the information carried by them to a usable form. It is used with the antenna. The antenna intercepts radio waves and converts them to tiny alternating currents which are applied to the receiver, and the receiver extracts the desired information. The receiver uses filters to separate the chosen frequency signal from all the other signals picked up by the antenna.
An amplifier is used to boost weak signals. Amplifiers are extremely sensitive and are normally cooled to very low temperatures to minimise interference due to the noise generated by the movement of the atoms in the metal, called thermal noise
A recorder is used to save the received data. Most radio telescopes nowadays record directly to some form of computer memory disk as astronomers use sophisticated software to process and analyse the data.
Radio telescopes can be used singly or linked together electronically in an array. Unlike optical telescopes, radio telescopes can be used in the daytime as well as at night as they are detecting radio waves and not light waves which are obscured by the sun during daylight hours. One problem with radio telescopes is electromagnetic interference, so radio telescopes are generally located in remote area’s far from population centres.
The world’s largest filled-aperture (full dish) radio telescope is the Five hundred meter Aperture Spherical Telescope (FAST) in China. The 500-meter-diameter (1,600 ft) dish with an area as large as 30 football fields is built into a natural Karst depression in the landscape in Guizhou province. This telescope cannot move; the antenna is in a cabin suspended above the dish on cables. The active dish is composed of 4450 moveable panels controlled by a computer. 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. Although the dish is 500 meters in diameter, only a 300-meter circular area on the dish is illuminated by the feed antenna at any given time, so the actual effective aperture is 300 meters. The dish can only be aimed at points in an area of the sky near the zenith and cannot receive from sources near the horizon.
In my next blog I will look at what radio waves are and where they come from.