Radio astronomy

 ( Observational Astronomy ) 

Radio astronomy is a subfield of astronomy that studies celestial objects using . It started in 1933, when Karl Jansky at Bell Telephone Laboratories reported radiation coming from the Milky Way. Subsequent observations have identified a number of different sources of radio emission. These include and galaxy, as well as entirely new classes of objects, such as Radio galaxy, , , and masers. The discovery of the cosmic microwave background radiation, regarded as evidence for the Big Bang, was made through radio astronomy.

Radio astronomy is conducted using large radio antennas referred to as , that are either used alone, or with multiple linked telescopes utilizing the techniques of radio interferometry and aperture synthesis. The use of interferometry allows radio astronomy to achieve high angular resolution, as the resolving power of an interferometer is set by the distance between its components, rather than the size of its components.

Radio astronomy differs from radar astronomy in that the former is a passive observation (i.e., receiving only) and the latter an active one (transmitting and receiving).

Karl Jansky made the discovery of the first astronomical radio source Serendipity in the early 1930s. As a newly hired radio engineer with Bell Labs, he was assigned the task to investigate static that might interfere with short wave transatlantic voice transmissions. Using a large directional antenna, Jansky noticed that his analog signal pen-and-paper recording system kept recording a persistent repeating signal or "hiss" of unknown origin. Since the signal peaked about every 24 hours, Jansky first suspected the source of the interference was the Sun crossing the view of his directional antenna. Continued analysis, however, showed that the source was not following the 24-hour daily cycle of the Sun exactly but instead repeating on a cycle of 23 hours and 56 minutes. Jansky discussed the puzzling phenomena with his friend, astrophysicist Albert Melvin Skellett, who pointed out that the observed time between the signal peaks was the exact length of a sidereal time: the time it took for "fixed" astronomical objects, such as a star, to pass in front of the antenna every time the Earth rotated. By comparing his observations with optical astronomical maps, Jansky eventually concluded that the radiation source peaked when his antenna was aimed at the densest part of the Milky Way in the constellation of Sagittarius.

Jansky announced his discovery at a meeting in Washington, D.C., in April 1933 and the field of radio astronomy was born. In October 1933, his discovery was published in a journal article entitled "Electrical disturbances apparently of extraterrestrial origin" in the Proceedings of the Institute of Radio Engineers. Reprinted 65 years later as along with an explanatory preface in . Jansky concluded that since the Sun (and therefore other stars) were not large emitters of radio noise, the strange radio interference may be generated by interstellar gas and dust in the galaxy, in particular, by "thermal agitation of charged particles." (Jansky's peak radio source, one of the brightest in the sky, was designated Sagittarius A in the 1950s and was later hypothesized to be emitted by electrons in a strong magnetic field. Current thinking is that these are ions in orbit around a massive black hole at the center of the galaxy at a point now designated as Sagittarius A*. The asterisk indicates that the particles at Sagittarius A are ionized.)

After 1935, Jansky wanted to investigate the radio waves from the Milky Way in further detail, but Bell Labs reassigned him to another project, so he did no further work in the field of astronomy. His pioneering efforts in the field of radio astronomy have been recognized by the naming of the fundamental unit of flux density, the jansky (Jy), after him.

Radio amateur Grote Reber was inspired by Jansky's work, and built a parabolic radio telescope 9 meters in diameter in his backyard in Wheaton, Illinois in 1937. He began by repeating Jansky's observations, and then conducted the first sky survey in the radio frequencies. On February 27, 1942, James Stanley Hey, a British Army research officer, made the first detection of radio waves emitted by the Sun.

At Cambridge University, where ionospheric research had taken place during World War II, J. A. Ratcliffe along with other members of the Telecommunications Research Establishment that had carried out wartime research into radar, created a radiophysics group at the university where radio wave emissions from the Sun were observed and studied. This early research soon branched out into the observation of other celestial radio sources and interferometry techniques were pioneered to isolate the angular source of the detected emissions. Martin Ryle and Antony Hewish at the Cavendish Astrophysics Group developed the technique of Earth-rotation aperture synthesis. The radio astronomy group in Cambridge went on to found the Mullard Radio Astronomy Observatory near Cambridge in the 1950s. During the late 1960s and early 1970s, as computers (such as the Titan) became capable of handling the computationally intensive Fourier transform inversions required, they used aperture synthesis to create a 'One-Mile' and later a '5 km' effective aperture using the One-Mile and Ryle telescopes, respectively. They used the Cambridge Interferometer to map the radio sky, producing the Second (2C) and Third (3C) Cambridge Catalogues of Radio Sources.

Observations from the Earth's surface are limited to wavelengths that can pass through the atmosphere. At low frequencies or long wavelengths, transmission is limited by the ionosphere, which reflects waves with frequencies less than its characteristic plasma frequency. Water vapor interferes with radio astronomy at higher frequencies, which has led to building radio observatories that conduct observations at millimeter wavelengths at very high and dry sites to minimize the water vapor content in the line of sight. Finally, transmitting devices on Earth may cause radio-frequency interference. Because of this, many radio observatories are built at remote places.

The Cambridge group of Ryle and Vonberg observed the Sun at 175 MHz for the first time in mid-July 1946 with a Michelson interferometer consisting of two radio antennas with spacings of some tens of meters up to 240 meters. They showed that the radio radiation was smaller than 10 arc minutes in size and also detected circular polarization in the Type I bursts. Two other groups had also detected circular polarization at about the same time (David Martyn in Australia and Edward Appleton with James Stanley Hey in the UK).

Modern radio interferometers consist of widely separated radio telescopes observing the same object that are connected together using coaxial cable, waveguide, optical fiber, or other type of transmission line. This not only increases the total signal collected, but it can also be used in a process called aperture synthesis to vastly increase resolution. This technique works by superposing ("interfering") the signal from the different telescopes on the principle that that coincide with the same phase will add to each other while two waves that have opposite phases will cancel each other out. This creates a combined telescope that is the size of the antennas furthest apart in the array. To produce a high-quality image, a large number of different separations between different telescopes are required (the projected separation between any two telescopes as seen from the radio source is called a "baseline") – as many different baselines as possible are required in order to get a good quality image. For example, the Very Large Array has 27 telescopes giving 351 independent baselines at once.

The pre-eminent VLBI arrays operating today are the Very Long Baseline Array (with telescopes located across North America) and the European VLBI Network (telescopes in Europe, China, South Africa and Puerto Rico). Each array usually operates separately, but occasional projects are observed together producing increased sensitivity. This is referred to as Global VLBI. There are also a VLBI networks, operating in Australia and New Zealand called the LBA (Long Baseline Array), and arrays in Japan, China and South Korea which observe together to form the East-Asian VLBI Network (EAVN).

Since its inception, recording data onto hard media was the only way to bring the data recorded at each telescope together for later correlation. However, the availability today of worldwide, high-bandwidth networks makes it possible to do VLBI in real time. This technique (referred to as e-VLBI) was originally pioneered in Japan, and more recently adopted in Australia and in Europe by the EVN (European VLBI Network) who perform an increasing number of scientific e-VLBI projects per year.

The cosmic microwave background radiation was also first detected using radio telescopes. However, radio telescopes have also been used to investigate objects much closer to home, including observations of the Sun and solar activity, and radar mapping of the Solar System.

Other sources include:

• Sun
• Jupiter
• Sagittarius A, the Galactic Center of the Milky Way, with one portion Sagittarius A* thought to be a radio wave–emitting supermassive black hole
• Active galactic nuclei and have jets of charged particles which emit synchrotron radiation
• Merging often show diffuse radio emission
• Supernova remnants can also show diffuse radio emission; are a type of supernova remnant that shows highly synchronous emission.
• The cosmic microwave background is blackbody radio/microwave emission

Earth's radio signal is mostly natural and stronger than for example Jupiter's but is produced by Earth's and bounces at the ionosphere back into space.

To improve harmonisation in spectrum utilisation, the majority of service-allocations stipulated in this document were incorporated in national Tables of Frequency Allocations and Utilisations which is within the responsibility of the appropriate national administration. The allocation might be primary, secondary, exclusive, and shared.

• primary allocation: indicated by writing in capital letters (see example below)
• secondary allocation: indicated by small letters
• exclusive or shared utilization: within the responsibility of administrations

In line to the appropriate ITU Region, the frequency bands are allocated (primary or secondary) to the radio astronomy service as follows.

• Atacama Large Millimeter Array
• Submillimeter Array
• Channel 37
• Gamma-ray astronomy
• Infrared astronomy
• Radar astronomy
• Time smearing
• X-ray astronomy
• Waves ( Juno) (radio instrument on the Juno Jupiter orbiter)
• Radio Galaxy Zoo
• Würzburg radar#Post-war use in astronomy

Journals

Books

• Gerrit Verschuur The Invisible Universe: The Story of Radio Astronomy Springer 2015
• Bruno Bertotti (ed.), Modern Cosmology in Retrospect. Cambridge University Press 1990.
• James J. Condon, et al.: Essential Radio Astronomy. Princeton University Press, Princeton 2016, .
• Robin Michael Green, Spherical Astronomy. Cambridge University Press, 1985.
• Raymond Haynes, Roslynn Haynes, and Richard McGee, Explorers of the Southern Sky: A History of Australian Astronomy. Cambridge University Press 1996.
• J.S. Hey, The Evolution of Radio Astronomy. Neale Watson Academic, 1973.
• David L. Jauncey, Radio Astronomy and Cosmology. Springer 1977.
• Roger Clifton Jennison, Introduction to Radio Astronomy. 1967.
• Jobn D. Kraus, Martt; E. Tiuri, and Antti V. Räisänen, Radio Astronomy, 2nd ed, Cygnus-Quasar Books, 1986.
• Albrecht Krüger, Introduction to Solar Radio Astronomy and Radio Physics. Springer 1979.
• David P.D. Munns, A Single Sky: How an International Community Forged the Science of Radio Astronomy. Cambridge, MA: MIT Press, 2013.
• Allan A. Needell, Science, Cold War and American State: Lloyd V. Berkner and the Balance of Professional Ideals. Routledge, 2000.
• Joseph Lade Pawsey and Ronald Newbold Bracewell, Radio Astronomy. Clarendon Press, 1955.
• Kristen Rohlfs, Thomas L Wilson, Tools of Radio Astronomy. Springer 2003.
• D.T. Wilkinson and P.J.E. Peebles, Serendipitous Discoveries in Radio Astronomy. Green Bank, WV: National Radio Astronomy Observatory, 1983.
• Woodruff T. Sullivan III, The Early Years of Radio Astronomy: Reflections Fifty Years after Jansky's Discovery. Cambridge, England: Cambridge University Press, 1984.
• Woodruff T. Sullivan III, Cosmic Noise: A History of Early Radio Astronomy. Cambridge University Press, 2009.
• Woodruff T. Sullivan III, Classics in Radio Astronomy. Reidel Publishing Company, Dordrecht, 1982.

• nrao.edu National Radio Astronomy Observatory
• The History of Radio Astronomy * Reber Radio Telescope – National Park Services
• Radio Telescope Developed – a brief history from NASA Goddard Space Flight Center
• Society of Amateur Radio Astronomers
• Visualization of Radio Telescope Data Using Google Earth
• UnwantedEmissions.com A general reference for radio spectrum allocations, including radio astronomy.
• Improving Radio Astronomy Images by Array Processing
• What is Radio Astronomy – Radioastrolab
• PICTOR: A free-to-use radio telescope