Radio astronomy has its roots back in the 1930's when Karl Jansky accidentally detected radio emission from the center of the Milky Way as part of his research on the interference on transatlantic phone lines. The British advanced radio antenna technology in their development of radar technology to fight the German warplanes in World War II. After the war, astronomers adapted the technology to detect radio waves coming from space.
The Robert C Byrd Green Bank Telescope (GBT) is the largest fully steerable radio telescope in the world; a much-improved replacement for the former 100-meter radio telescope. Although it has an unusual off-axis design to improve sensitivity and reduce distortions, the basic principles are the same as described in the cartoon at left. This image shows the underside lattice supports of the main reflector dish. Select the image to bring up an enlargement of the image. A zoomed-in image of the GBT is also available at this link (ask for a full-res version of the zoom image). A photo tour of Green Bank NRAO is available by selecting the tour link.
A radio telescope uses a large metal dish or wire mesh, usually parabolic-shaped, to reflect the radio waves to an antenna above the dish. An example of a mesh is shown at left. This was the mesh of the parabolic dish for the former 100-meter radio telescope at Green Bank, West Virginia (photo courtesy of National Radio Astronomy Observatory). Looking from underneath a radio telescope, a person can see the clouds in the sky overhead but to the much longer wavelength radio waves, the metal mesh is an excellent reflector. See also images from the Parkes Radio Telescope. Radio telescopes designed to also receive smaller wavelengths, such as the GBT pictured above, have solid metal dishes. The GBT's metal surface is made up of 2004 panels, each roughly the size of a queen-sized bed, mounted on actuators to fine-tune the shape as the telescope is tilted and wind speed and direction changes.
The signal from the antenna is sent to an amplifier to magnify the very faint signals. At the last step, the amplified signal is processed by a computer to turn the radio signals into an image that follows the shape of the radio emission. False colors are used to indicate the intensity of the radio emission at different locations. An example is shown below for Jupiter. Charged particles in its magnetic field produce a large amount of radio energy in donut-shaped regions around its center. A visible band image of Jupiter is shown below the radio image.
Radio telescopes are much larger than optical telescopes because radio wavelengths are much longer than optical wavelengths. The longer wavelengths means that the radio waves have lower energy than optical light waves. In order to collect enough radio photons to detect a signal, the radio dishes must be very large. Both optical and radio telescope reflectors use a parabolic shape to perfectly focus the light to a point. Increasing the size of the radio dish is also necessary in order to improve the clarity of the radio images. I will discuss the issue of image clarity further in the next two sections.
Radio telescopes detect the emission from cool clouds of hydrogen in the space between the stars. Hydrogen atoms are the most common type of atoms in the universe and much of the hydrogen gas is too far away from any star to produce emission in the optical wavelength band. In addition, there are cold clouds made of over a hundred different types of molecules including organic molecules. Stars and planetary systems form in these molecular clouds. Therefore, radio telescopes are a vital tool in understanding the universe. I will discuss further the use of radio waves to explore the material between the stars and the structure of our galaxy in the interstellar medium chapter.
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last updated: September 14, 2011