Design of a Radio Telescope

(Foundations of Astronomy, 6^th Edition)

Radio telescopes are not always constructed in the same exact way. There can be variations of the antennas and receivers. The following describes a typical radio telescope.

A radio telescope can be divided into 4 functional parts. The four parts are: the reflector dish, the antenna, the amplifier and the receiver/recorder. The large dish that most people associate with a radio telescope is used to focus the radio waves. Since radio waves are much longer than visible light waves, the dish of the radio telescope does not need to be perfectly smooth, like the ground mirrors of a reflecting telescope. In fact, some radio telescopes use a wire mesh as the reflecting dish. It will still be able to deflect the long waves to a detection device.

The radio telescope reflector pictured above is able to be moved at the base to point to an area of interest in the sky. It has an alt-azimuth drive, a motorized device that allows movement up and down and side to side. Some radio telescopes such as Arecibo in Puerto Rico have their reflectors constructed in the ground. This is done to support a large dish; the dish at Arecibo is 300 meters in diameter. With the dish in the ground, the telescope can’t be pointed and so the operators must rely on the rotation of the earth to point at areas of interest.

The antenna of the radio telescope is at the center of the dish, and perched above it. The waves that are reflected by the dish are directed at the antenna. The antenna absorbs radio energy and the signal is carried by a cable to an amplifying device, to strengthen the signal. After preamplification, the signal is carried to a building which houses the receiver. The receiver further amplifies the signal and the signal is integrated and may be recorded on an analog recorder, usually pen on moving chart paper. The signal is then transmitted to a computer which will record the signal in digital form and the data can then be further processed if desired. Oftentimes, the data is converted into a contour map. The contour map shows areas of higher and lower densities of radio waves. This representation can be manipulated to show colors or shading which is of course not actually visible to the human eye. The contour map gives us a picture that we can understand easily of where matter is found in between stars.

Arecibo Observatory (NRAO/ AUI/ NSF)

Improving the Signal

Radio signals received by radio telescopes are rather weak. A large reflector dish such as the one constructed at Arecibo is one way to improve the detection of radio waves. There are many things on earth that can interfere with signal detection from space. In fact, the radio signals from space are thousands of times weaker than TV or radio signals found on earth. Radio telescopes are located far from cities in order to reduce unwanted noise.

Radio signals can be improved by using more than one radio telescope in a system. By adding the signals from radio telescopes together, the resolving power can be improved. When radio telescopes are linked in this way, it is called a radio interferometer. The Very Large Array (VLA) located in New Mexico and Arizona has the ability to employ up to 27 dishes. This arrangement simulates a radio telescope with a diameter of 40 kilometers. The Very Long Baseline Array (VLBA) is an interferometer that is spread from Hawaii to the Virgin Islands; its effective diameter is 8000 kilometers.

Very Large Array (NRAO/ AUI/ NSF)