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How SETI Works

Dishes for the Sky
Interferometers combine images from several radio telescopes to make one image that looks like it was taken from one large dish.
Interferometers combine images from several radio telescopes to make one image that looks like it was taken from one large dish.

If ET is communicating by radio, how can we detect such signals? Radio signals are waves of light, like visible light, infra-red light (heat) and X-rays. But radio signals have longer wavelengths than these other forms of light. To detect ET radio signals, you use a radio telescope. A radio telescope is a radio receiver similar to the radio that you have in your house or car. It has the following parts:

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Diagram of the parts of a radio telescope (Cassegrain design).

Hover over the labels for a call-out of each piece.

  • Dish - A parabolic reflector ("bucket") that collects the radio waves and brings them to a focus (like a mirror in a reflecting telescope). The telescope in the diagram is a Cassegrain design, which uses a sub-reflector (like the secondary mirror in a reflecting telescope) and feed horns to bring the radio waves to a focus behind the dish.
  • Antenna - Metal device (usually straight or coiled wire) located at the focus of the radio telescope. It converts the radio waves into an electric current when tuned to the correct frequency because the radio waves cause movements of electrons in the antenna. Noise The electronics in the radio telescope -- antenna, tuner, amplifier -- are often cooled with liquid nitrogen or liquid helium to reduce random electrical currents, or noise. The lower the noise, the easier it is to detect weak signals.
  • Tuner - Electrical device that separates a single radio signal from the thousands that come into the antenna. The tuner adjusts the frequency of the antenna to match a specific frequency among the incoming radio waves. SETI uses multichannel analyzers that allow them to tune multiple frequencies simultaneously.
  • Amplifier - Electrical device that increases the strength of a weak electrical current caused by an incoming radio signal.
  • Data recorders - Magnetic-tape or digital devices that store the signals from the amplifiers.
  • Auxiliary data instruments - Additional devices that encode information on the data tapes for interferometry (see below). These instruments include GPS receivers that record the position of the radio telescope and devices for precise time notations.
  • Computers - Computers are used to acquire and analyze data, as well as to control the telescope's movements.
  • Mechanical systems - Gears and motors on the horizontal and vertical axes are used to point and track the dish.

In general, large radio telescopes allow you to detect weak signals and resolve them -- so, the larger the dish, the greater the resolution of the signal. However, large dishes are difficult and expensive to construct and maintain. To get around this problem, radio astronomers use a technique called interferometry. Interferometry combines the signals from several small radio telescopes spread out over a large area to achieve the equivalent of one large dish over the same area (see the links on the next page for details on interferometry).

For more information about SETI and related topics, check out the links on the next page.