Radio telescope facts for kids
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A radio telescope is a special kind of antenna and radio receiver. It helps scientists find radio waves coming from space. These waves come from things like planets, stars, and galaxies.
Radio telescopes are key tools in radio astronomy. This field studies the radio part of the electromagnetic spectrum. Just like regular optical telescopes see light, radio telescopes "see" radio waves. A cool thing about radio telescopes is that they work day and night!
Space objects are very far away. This means the radio waves they send are super weak. So, radio telescopes need huge antennas to collect enough energy. They also need very sensitive equipment. Most radio telescopes look like big dish antennas. They are similar to those used for satellites. Sometimes, many dishes are linked together. Radio observatories are often built far from cities. This helps avoid electromagnetic interference from TVs, radios, and cars.
The first time radio waves from space were found was in 1932. An engineer named Karl Guthe Jansky discovered them. He was working at Bell Telephone Laboratories in Holmdel, New Jersey. The first radio telescope built just for astronomy was a 9-meter dish. Grote Reber, a radio hobbyist, built it in his backyard in Wheaton, Illinois in 1937. His work is often seen as the start of radio astronomy.
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Early Radio Telescopes: How They Started
The first antenna to find radio waves from space was built by Karl Guthe Jansky. He was an engineer at Bell Telephone Laboratories in 1932. Jansky was trying to find out what caused static that messed with phone calls. His antenna was made of dipoles and reflectors. It was designed to pick up short wave signals at 20.5 MHz. This is a wavelength of about 14.6 meters.
His antenna could spin around. People called it "Jansky's merry-go-round." It was about 30 meters wide and 6 meters tall. By spinning the antenna, he could find where the static was coming from. After months of recording signals, Jansky found three types of static. Two were from thunderstorms. The third was a faint, steady hiss. This hiss was a mystery.
Jansky found that the "faint hiss" came back every 23 hours and 56 minutes. This is the length of an astronomical sidereal day. It's how long it takes for a "fixed" object in the sky to return to the same spot. Jansky thought the hiss came from outside our Solar System. He compared his findings with star maps. He then realized the radio waves came from the Milky Way Galaxy. The strongest signals came from the center of the galaxy, in the constellation of Sagittarius.
An amateur radio operator, Grote Reber, was another pioneer. He built the first parabolic "dish" radio telescope. It was 9 meters wide. He built it in his backyard in Wheaton, Illinois, in 1937. He repeated Jansky's work. He also found the Milky Way was the first radio source from space. Then, he made the first map of the sky using very high radio frequencies. He found other radio sources too. After World War II, new radar technology helped radio astronomy grow. Universities and research centers began building large radio telescopes.
Types of Radio Telescopes
The radio spectrum has a very wide range of frequencies. Because of this, radio telescopes come in many different designs and sizes. For long wavelengths (30 to 3 meters, or 10–100 MHz), they often look like large "TV antennas." Or they might be big fixed reflectors with movable parts. Since these wavelengths are so long, the "reflector" parts can be made from simple wire mesh, like chicken wire.
For shorter wavelengths, parabolic "dish" antennas are most common. How well a dish antenna can "see" details depends on its size compared to the wavelength it's observing. This means bigger dishes are needed for better detail. Radio telescopes that work with wavelengths from 3 meters to 30 cm (100 MHz to 1 GHz) are usually over 100 meters wide. Those working with wavelengths shorter than 30 cm (above 1 GHz) range from 3 to 90 meters wide.
Radio Frequencies Used
More and more, people use radio frequencies for communication. This makes it harder for astronomers to observe the sky. Scientists work together to protect parts of the radio spectrum for astronomy.
Here are some important frequencies used by radio telescopes:
- The United States National Radio Quiet Zone protects all frequencies within its area.
- Channel 37: This band is from 608 to 614 MHz.
- The "Hydrogen line" (also called the "21 centimeter line"): This is 1,420.40575177 MHz. Many radio telescopes use it, including The Big Ear when it found the Wow! signal.
- 1,406 MHz and 430 MHz.
- The Waterhole: This range is from 1,420 to 1,666 MHz.
- The Arecibo Observatory had receivers that covered the whole 1–10 GHz range.
- The Wilkinson Microwave Anisotropy Probe mapped the Cosmic microwave background radiation at 23 GHz, 33 GHz, 41 GHz, 61 GHz, and 94 GHz.
Giant Dish Telescopes
The biggest single-dish radio telescope in the world is the Five-hundred-meter Aperture Spherical Telescope (FAST). China finished it in 2016. Its dish is 500 meters wide, as big as 30 football fields! It's built into a natural hole in the ground in Guizhou province. The dish cannot move. Instead, the feed antenna hangs above the dish on cables.
The dish itself has 4,450 movable panels. A computer controls these panels. By changing the dish's shape and moving the feed antenna, the telescope can look at different parts of the sky. It can see up to 40° from directly overhead. Even though the dish is 500 meters wide, only a 300-meter circle is used at one time. Construction started in 2007 and finished in July 2016. It began working on September 25, 2016.
The world's second largest single-dish telescope was the Arecibo radio telescope. It was in Arecibo, Puerto Rico, but it sadly collapsed in December 2020. Arecibo was special because it could also send out radio waves to map nearby objects. Most other telescopes only receive signals. Arecibo was also a fixed dish, like FAST. Its 305-meter dish was built into a natural dip in the land. The antenna could be steered to look about 20° from directly overhead. This allowed it to use a 270-meter part of the dish.
The largest individual radio telescope of any kind is the RATAN-600. It is near Nizhny Arkhyz, Russia. It has a 576-meter circle of rectangular radio reflectors. Each reflector can point to a central receiver.
These fixed dishes cannot move to point anywhere in the sky. They can only look at areas near directly overhead. They cannot see sources near the horizon. The largest fully movable dish radio telescope is the 100-meter Green Bank Telescope. It is in West Virginia, United States, and was built in 2000. Europe's largest fully movable radio telescope is the Effelsberg 100-m Radio Telescope. It is near Bonn, Germany. It was the world's largest movable telescope for 30 years before Green Bank was built.
The third largest movable radio telescope is the 76-meter Lovell Telescope. It is at Jodrell Bank Observatory in Cheshire, England. It was finished in 1957. The fourth largest movable radio telescopes are six 70-meter dishes. Three are Russian RT-70 telescopes. Three are part of the NASA Deep Space Network. The planned Qitai Radio Telescope will be 110 meters wide. It is expected to be the world's largest fully movable single-dish radio telescope when finished in 2023.
A common radio telescope has one antenna about 25 meters wide. Many observatories around the world use dozens of these telescopes.
Gallery of Big Dishes
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The 500 meter Five hundred meter Aperture Spherical Telescope (FAST) being built in China (2016).
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The 100 meter Green Bank Telescope in West Virginia, US. It is the largest fully steerable radio telescope dish (2002).
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The 100 meter Effelsberg in Germany (1971).
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The 76 meter Lovell at Jodrell Bank Observatory, England (1957).
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The 70 meter DSS 14 "Mars" antenna at Goldstone Deep Space Communications Complex, California, US (1958).
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The 70 meter Yevpatoria RT-70 in Crimea. It was the first of three RT-70 telescopes in the former Soviet Union (1978).
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The 70 meter Galenki RT-70 in Russia. It was the second of three RT-70 telescopes in the former Soviet Union (1984).
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Radio Telescopes in Space
Since 1965, three radio telescopes have been launched into space. The first, KRT-10, was put on the Salyut 6 space station in 1979. In 1997, Japan launched the second one, HALCA. The most recent was Spektr-R, sent by Russia in 2011.
- Space Radio Telescopes
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KRT-10 dish of a Salyut-6 on a stamp.
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Japanese HALCA dish.
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Assembled Spektr-R dish (left).
Radio Interferometry: Working Together
A big step forward happened in 1946. Scientists started using a method called astronomical interferometry. This means combining signals from many antennas. This makes them act like one giant antenna. This helps them see much finer details.
Radio interferometers often use arrays of dish antennas, like the One-Mile Telescope. Or they might use arrays of one-dimensional antennas. All the telescopes in the array are spread far apart. They are usually connected by cables or optical fiber. Newer methods allow signals to be recorded separately. Then, they are combined later at a central computer. This is called Very Long Baseline Interferometry (VLBI).
Interferometry collects more signal. But its main goal is to greatly improve how clear the images are. This is done through a process called aperture synthesis. It works by combining the signal waves from different telescopes. Waves that are in the same phase add up. Waves that are in opposite phases cancel each other out. This creates a combined telescope. It has the same detail as a single antenna as wide as the distance between the farthest telescopes in the array.
To get a really good image, you need many different distances between telescopes. The distance between any two telescopes, as seen from the radio source, is called a baseline. For example, the Very Large Array (VLA) in Socorro, New Mexico has 27 telescopes. This creates 351 different baselines at once. This allows it to see details as small as 0.2 arc seconds at 3 cm wavelengths.
Martin Ryle's group won a Nobel Prize for their work on interferometry. The Lloyd's mirror interferometer was also developed in 1946 by Joseph Pawsey's group. In the early 1950s, the Cambridge Interferometer mapped the radio sky. This led to the famous 2C and 3C surveys of radio sources.
A large connected radio telescope array is the Giant Metrewave Radio Telescope. It is in Pune, India. The largest array is the Low-Frequency Array (LOFAR). Finished in 2012, it is in western Europe. It has about 81,000 small antennas in 48 stations. These are spread over hundreds of kilometers. It works with wavelengths between 1.25 and 30 m. VLBI systems use antennas thousands of miles apart. Radio interferometers have also been used to make detailed images of the Cosmic Microwave Background.
The world's largest connected telescope, the Square Kilometre Array (SKA), is planned to start working in 2025.
What Radio Telescopes Observe
Radio astronomy Many objects in space can be seen not only with visible light. They also send out radiation at radio wavelengths. Radio telescopes can observe energetic objects like pulsars and quasars. They can also "image" most space objects. This includes galaxies, nebulae, and even radio signals from planets.
See also
