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Physics LibreTexts

10.25 Radio Telescopes

 


 

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Electromagnetic transmittance, or opacity, of the Earth's atmosphere. Click here for original source URL.

 

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Grote Reber. Click here for original source URL.

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Grote Reber's original Radio Antenna. Click here for original source URL.

 

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Replica of Karl Jansky's first radio telescope. Click here for original source URL.

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Karl Jansky. Click here for original source URL.

radio telescope is a device that gathers and concentrates radio waves; it is analogous to an optical telescope, which gathers and concentrates light waves. The American engineer Karl Jansky built the first radio telescope at Bell Telephone Labs in 1930. Jansky's radio antenna was 30 meters long, and it rotated on four wheels from a Model T Ford. He used it to detect radio "hiss" from the sky. In the process, he measured radio emission from the Milky Way, towards the constellation Sagittarius. Jansky was sure the hiss was from an astronomical object because it repeated with a cycle of 23 hours and 56 minutes, a sidereal day, whereas a terrestrial source of interference would repeat every 24 hours. 
 

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The Arecibo Radio Telescope. Click here for original source URL.

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Very Large Array radio telescopes in New Mexico, USA. Click here for original source URL.

American Grote Reber was an amateur astronomer who built a 10-meter radio telescope in his back yard. For nearly a decade after Jansky's invention, Reber was the only radio astronomer in the world! Like an optical astronomer, he curved the metallic surface to focus the radio waves. He observed almost every night from midnight to 6 a.m., then drove to his day job with a radio company. In 1940, Reber produced a beautiful map of radio waves coming from the Milky Way; this was the first map ever made beyond the optical spectrum. Radio astronomy got a huge boost from the development of radar technology during World War II. Many of the people who worked on radar became radio astronomy pioneers after the war ended.

Radio and optical telescopes work in basically the same way. Radio waves have much larger wavelengths than visible light waves, and they carry much less energy per photon. For these reasons, radio telescopes must have larger surfaces to collect sufficient energy to give a strong signal. On the other hand, since the wavelength is larger, the surface need not be as accurately shaped as the mirror of an optical telescope. Radio telescopes have a curved surface (or dish) of metal or wire mesh, which reflects the radio waves to a focus, where they are converted into an electrical signal. Radio astronomy is conducted over a wide range of wavelengths, from around a millimeter to ten meters, a factor of ten thousand. For comparison, the entire range of the visible spectrum of light from red to blue is only a factor of two in wavelength. One facility that combines 27 radio dishes is the Very Large Array (VLA) in central New Mexico. The largest telescope ever built is the 305-meter Arecibo dish, built into a natural topographic depression in Puerto Rico. It is not steerable, but can still track objects across a narrow strip of the sky using the Earth's rotation and movable "feed" antennas. The largest single radio dishes in the world are the 100-meter telescope at Effelsberg in Germany and the similar sized but fully steerable Green Bank Telescope in West Virginia.

Radio astronomers have their own techniques for detecting radiation and recording a signal. Recall that optical astronomy typically uses a CCD to convert photons into electrons in a detector that’s a semiconductor wafer. This signal is then amplified and digitized. Radio astronomers do not detect individual radio photons. The signal at the back of the telescope is a tiny electrical current that must be amplified millions of times and then converted from a analog signal to a digital signal. To keep radio noise low, the detectors and amplifiers are usually cooled with liquid nitrogen, a method also used in optical astronomy for the same reason, to keep noise low. Radio astronomers do not have to worry about light pollution the way optical astronomers do, but they have their own problems caused by interference from TV, radio, and radar signals. To combat radio noise pollution, radio observatories are far from population centers and usually in valleys. The National Radio Astronomy Observatory in West Virginia is in a special Radio Quiet zone. The nearby town of Green Bank is probably the only town in the U.S. where cell phones and WiFi routers are prohibited!

 

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Artist impression of the Spitzer Space Telescope rendered against an infrared sky. Click here for original source URL.