Radio telescopes are instruments for receiving and measuring radio frequency radiation coming from space or from special celestial objects. They are the most important tool of the so-called radio astronomy.
The antenna usually has the shape of a parabolic mirror. For shorter electromagnetic waves in the centimeter to decimeter range, the reflector must have a smooth surface; for longer waves, a grating structure is sufficient.
After the discovery of the first extraterrestrial radio source by Karl Guthe Jansky in 1932, radio telescopes were developed to observe the cosmos. The first radio telescope in parabolic form was built by Grote Reber, engineer and radio amateur in Wheaton, Illinois, as Jansky’s discovery was initially not given further attention by professional astronomy. In Germany, the first radio telescope, the Astropeiler Stockert on the Stockert near Bad Münstereifel, was built in 1956. It has been a listed building since 1999.
West-facing German airborne surveillance radar systems delivered false alarms whenever the constellation Swan (Cygnus) appeared on the horizon – caused by the radio source Cygnus A located there. In 1946, a research group at the Royal Radar Establishment in Malvern (England) discovered that a tiny region in the constellation Swan emitted intense radio radiation.
Most radio telescopes are parabolically shaped metal surfaces that focus the radio waves onto an antenna located at the focal point of the concave mirror. The antenna is also commonly referred to as the entire system. Today’s radio telescopes often consist of several parabolic antennas (arrays) as well as the evaluation station. The antennas of an array are coupled together to form an interferometer, effectively creating a larger diameter antenna. This technique can also be extended beyond the array to cover the entire globe: When radio telescopes distributed across the globe simultaneously observe the same source, the angular resolution of the radio telescopes can be increased quite substantially. The largest arrays exceed the resolution of optical telescopes by a factor of about 500, as can be seen in the adjacent image.
A distinction is made in radio telescopes between immovable and movable telescopes. Immovable telescopes are rare because they cannot be rotated in their orientation. They usually point their parabolic antenna at the zenith (for example, the Arecibo telescope, which is fixed in a lowland). Movable radio telescopes can be rotated so that they can “look” into the entire hemisphere.
In addition to the size of a radio telescope, which is a measure of its sensitivity, it also depends on the wavelength range it can cover. While the large telescopes can only observe wavelengths in the metre and centimetre range, smaller telescopes, such as the 30-m telescope from the Institute for Radio Astronomy in the Millimetre Range (IRAM) in Spain, the 3-m telescope KOSMA in Switzerland in the millimetre range or the 12-m telescope APEX (operated in the Chilean Atacama Desert by the Max Planck Institute for Radio Astronomy, millimetre and submillimetre waves) “listen” in shorter wavelength ranges. Since these frequencies are outside the atmospheric window, the sensitivity is greatly reduced by the overlying air envelope.
Besides observing celestial bodies, radio telescopes are also used to receive data from or send commands to distant space probes, or to search for extraterrestrial intelligence (see Project SETI).
There are several projects involving radio telescopes over long distances or even worldwide (global) in Very Long Baseline Interferometry (VLBI) imaging, such as the Very Long Baseline Array (VLBA), the Event Horizon Telescope or the Global mm-VLBI Array. Satellites are also used for this purpose (RadioAstron).
The largest radio telescope in the world is currently the Russian RATAN 600 near Selenchukskaya. The second largest is the FAST observatory in the Chinese province of Guizhou, which went into test operation on 25 September 2016.
Other large facilities are Atacama Large Millimeter/submillimeter Array, abbreviated ALMA, consisting of 66 antennas at an altitude of about 5000 m in the Atacama Desert in the northern Chilean Andes and, until December 2020, the Arecibo Observatory in Puerto Rico. The Arecibo radio telescope was destroyed on 1 December 2020 by falling parts due to material fatigue.
The largest German (and second largest moving) radio telescope in the world is the Effelsberg radio telescope in a valley in the Eifel, a 100 m diameter moving telescope operated by the Max Planck Institute for Radio Astronomy in Bonn. The largest moving radio telescope in the world is the 100 m × 110 m Robert C. Byrd Green Bank Telescope at the Green Bank Observatory in West Virginia, USA. The largest millimeter-wave radio telescope is the 50 m Large Millimeter Telescope in Puebla, Mexico.
Other large radio telescope arrays are the Giant Metrewave Radio Telescope (GMRT, 30 individual telescopes each 45 m, scattered at distances of up to 25 km, six frequency bands from 50 to 1500 MHz) in India, 80 km north of Pune in the state of Maharashtra, and the Very Large Array (VLA, 27 telescopes each 25 m in a Y-shaped configuration) in Socorro, New Mexico, USA.
Since 2006, a novel radio telescope for observing low-frequency radio waves in the meter-wave range, the Low Frequency Array (LOFAR), has been under construction in the Netherlands. At the time of its inauguration in June 2010, it had about 10,000 antennas across Europe. The first LOFAR station has been operating next to the Effelsberg 100-m telescope since 2007. LOFAR is a prototype for an even larger radio telescope, the Square Kilometre Array (SKA), construction of which is scheduled to begin in 2021. The first observations are expected to be possible in the mid-2020s.
An important project for the exploration of the universe, which is carried out with the help of radio telescopes, is HIPASS. This is a distance-sensitive search for the signature of hydrogen as an indicator of galaxies. The Southern Hemisphere region has already been completed. Most of the data were collected by the Parkes radio telescope in Australia.
- List of radio telescopes and research radio stations
- James W. Mar, Harold Liebowitz: Structures technology for large radio and radar telescope systems. MIT Press, Cambridge MA et al. 1969, OCLC 250925598.
- Jacob W.M. Baars et al: Radio Telescope Reflectors – Historical Development of Design and Construction. Springer, Cham 2018, ISBN 978-3-319-65147-7.
– Collection of images, videos and audio files