Radio Telescopes

A radio telescope is capable of tuning in to the radio end of the electromagnetic spectrum. Stars emit all forms of radiation from their surfaces, but our eyes are only capable of seeing a very narrow band of visible light at 380-720 nm. A radio telescope can tune in to electromagnetic waves which are much, MUCH longer than a few hundred nanometers. The radio telescope listens to starlight at wavelengths of centimeters to kilometers, although the long wavelengths are seldom useful because of the sheer length between wavecrests. This may seem surprising, but recall that radio waves are really a form of "light," as electromagnetic radiation. These waves move at light speed of 300,000 km/sec, but their wavelengths are much longer and less energetic. Since radio waves easily pass through gas and dust, these telescopes can be very useful. First, a radio telescope can be operated 24 hours a day because sunlight and clouds do not block their energy. Second, these telescopes can see objects which might be obscured behind a dust cloud in interstellar or even intergalactic space. Stuff which the naked eye could not see with a reflecting or refracting telescope can be exposed by a radio telescope.

This is a diagram of an average radio telescope. Just like optical telescopes, energy from a star (galaxy in the image) can be focused to a narrow beam which is highly amplified ... sort of a reverse magnification of an image. The result is a highly enlarged signal from a distant source. The parts of a radio telescope include the reflecting dish, an antenna, an amplifier, and a recorder. These components work together to give astronomers a view into the radio emissions of deep space objects. Photographic film is not used to "see" the image, but computers can interpret the incoming signal and generate an intensity image of the radio wave source. The result is a computer-generated image whose colors differentiate the features of the radio source. Check out the NRAO site that will connect you directly to the National Radio Astronomy Observatory website and their page of images.



Most astronomy students have probably seen the movie "Contact." The first location where Ellie is working is the giant fixed radio telescope at Aricebo, Puerto Rico. This telescope is embedded in a natural mountain bowl and it used by astronomers to hunt for pulsars ... fast rotating neutron stars. Later, Ellie moved to New Mexico to the Very Large Array of radio telescopes. This unusual collection of telescopes can be moved by railway to various configurations and then wired together to act as one very large reception dish. This linkup of many radio telescopes is called interferometry and with this technique the VLA acts as one radio dish whose diameter is the distance between the farthest dishes. Thus the "size" of the VLA "dish" is 40 km. The Very Long Baseline Array (VLBA) consists of dishes spread from Hawaii to the Virgin Islands, and gives the effect of an 8000 km diameter dish. HALCA, the Highly Advanced Laboratory for Communications and Astronomy, consists of an 8 meter radio dish in orbit around the earth. Used with ground-based radio telescopes, this acts as a dish with a diameter of 32,000 km, thus greatly increasing the resolving power when looking at objects billions of light years distant. While the movie was fanciful in their extraterrestrial assumptions, to date no signal from intelligent life in space has been received.

I took this picture of a pair of radio dishes atop Mauna Kea. They are testing two different designs, one that is open to the air, and the other that has the "dish" covered. There are 36 mounting surfaces, one of which you can see in the foreground of the picture. The astronomers who will run this equipment will construct 8 radio telescopes of identical size and can move them in a great variety of patterns among the 36 mounting sites. More scopes could have been constructed, but the native Hawaiians have sacred ground near this particular location, and to minimize the affect of such construction interference, they number of radio scopes was kept to 8 mobile units.





So you have seen how we can listen to electromagnetic radiation with these giant telescopes, but remember that most of the wavelengths of the em spectrum are blocked by the earth's atmosphere. To really learn the details about stars, we need to send telescopes above the atmosphere aboard rocket-launched satellites. To learn about what these space telescopes have learned, click on Space Telescopes, or return to the Introduction to Light and Telescopes, the Syllabus, or the Home page.

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