Radio telescope

Radio telescopes are specialised antennas and receivers designed to detect radio waves emitted by astronomical sources across the sky. They constitute the principal observational tools of radio astronomy, a branch of astrophysics concerned with the radio-frequency region of the electromagnetic spectrum. In contrast to optical telescopes, radio telescopes can observe both day and night because radio waves penetrate the Earth’s atmosphere and experience minimal interference from sunlight. Because the signals arriving from astronomical radio sources—such as planets, stars, nebulae and galaxies—are extremely weak, radio telescopes require large collecting areas and highly sensitive instrumentation.
Radio observatories are usually situated in remote locations far from centres of population to minimise electromagnetic interference from human activity, including broadcasting systems, radar and motor vehicles. The first detection of cosmic radio waves in 1932 laid the foundation for modern radio astronomy, which has since become essential for studying regions of the Universe invisible to optical instruments.

Early Developments

Karl Guthe Jansky, an engineer at Bell Telephone Laboratories, made the pioneering discovery of cosmic radio waves while investigating sources of interference in radiotelephone communication. In 1932 he built an array of dipole antennas mounted on a rotating turntable—informally called “Jansky’s merry-go-round”—to scan the sky at a frequency of 20.5 MHz (wavelength 14.6 metres). After months of systematic recording, he identified three categories of static: nearby thunderstorms, distant thunderstorms and a faint, persistent hiss. The timing of the hiss corresponded to the sidereal day, leading Jansky to conclude that the radiation originated from the Milky Way, strongest in the direction of Sagittarius.
Building upon Jansky’s work, radio amateur Grote Reber constructed the first purpose-built parabolic radio telescope in 1937 in Wheaton, Illinois. His 9-metre dish enabled him to repeat and extend Jansky’s observations, producing the first radio-frequency sky survey. The technological advancements made during the Second World War, especially in radar systems, subsequently accelerated the development of radio astronomy as a formal scientific discipline.

Types of Radio Telescopes

Because the radio spectrum spans wavelengths from millimetres to tens of metres, radio telescopes take a variety of forms, each suited to particular frequency ranges.

• Dipole and antenna arraysAt long wavelengths (30–3 metres; 10–100 MHz), radio telescopes often consist of large arrays of dipole elements or stationary reflectors with movable receivers. The reflector surfaces can be made of wire mesh, because the required precision is low relative to the wavelength.
• Parabolic dish telescopesAt centimetre and millimetre wavelengths, most radio telescopes employ large parabolic reflectors similar to satellite communication dishes. These may be fixed in position with a movable feed, or fully steerable to track objects across the sky.
• Phased arrays and low-frequency arraysArrays of many small antennas can be electronically combined to synthesise a large aperture, offering wide fields of view and flexible pointing capabilities.

Radio telescopes differ widely in scale—from backyard dishes built by early pioneers to national and international facilities spanning kilometres.

Frequencies and Observational Bands

Radio telescopes operate across a broad portion of the electromagnetic spectrum. Scientific committees negotiate international protection for key frequency bands essential to astronomy because human communications increasingly occupy the radio spectrum. Some notable bands include:

• 608–614 MHz – protected within the United States National Radio Quiet Zone.
• 1420 MHz – the hydrogen 21-centimetre line, critical for mapping neutral hydrogen in galaxies.
• 1420–1666 MHz – the “Waterhole” region, containing important spectral lines of hydrogen and hydroxyl.
• Receivers spanning 1–10 GHz – used in many major telescopes, including legacy systems at Arecibo.
• 23–94 GHz – employed by missions such as the Wilkinson Microwave Anisotropy Probe to map the cosmic microwave background.

These frequency bands reveal different physical processes, from molecular transitions to relic radiation from the early Universe.

Major Radio Telescopes

Some of the world’s largest scientific structures are radio telescopes:

• FAST (Five-hundred-metre Aperture Spherical Telescope)Located in Guizhou, China, FAST is the largest filled-aperture radio telescope ever built, with a 500-metre dish composed of 4,450 movable panels. Although only a 300-metre diameter region is illuminated at one time, its sensitivity makes it central to pulsar searches and hydrogen mapping.
• Arecibo Observatory (Puerto Rico)Formerly the world’s second-largest filled-aperture telescope until its collapse in 2020, Arecibo featured a 305-metre dish used both for passive radio astronomy and for active radar studies of near-Earth objects.
• RATAN-600 (Russia)The largest single radio telescope structure by diameter, consisting of a 576-metre ring of reflectors focusing radio waves onto a central receiver.
• Green Bank Telescope (United States)The largest fully steerable radio dish at 100 metres in diameter, located within the United States National Radio Quiet Zone.
• Effelsberg 100m Telescope (Germany)Operated by the Max Planck Institute for Radio Astronomy, it was the largest steerable dish for three decades.
• Lovell Telescope (United Kingdom)A 76-metre fully steerable dish at Jodrell Bank Observatory.
• RT-70 and Deep Space Network antennasA set of 70-metre dishes used for planetary radar and deep-space communication.
• Qitai Radio Telescope (China)A planned 110-metre fully steerable telescope expected to become the largest of its kind upon completion.

A typical mid-sized radio telescope has a 25-metre dish, and many such instruments operate worldwide.

Radio Telescopes in Space

Earth’s atmosphere absorbs and distorts some radio wavelengths, so space-based radio telescopes provide access to spectral regions unavailable from the ground. Three have been launched to date:

• KRT-10 – deployed on the Salyut 6 space station in 1979.
• HALCA – launched by Japan in 1997 for very-long-baseline interferometry.
• Spektr-R – launched by Russia in 2011, significantly extending baseline lengths for high-resolution imaging.

Radio Interferometry

One of the most transformative developments in radio astronomy is astronomical interferometry, first demonstrated in 1946. Interferometers combine signals from multiple telescopes to create a virtual aperture equivalent to the distance between them. This technique dramatically improves angular resolution, enabling radio astronomers to investigate fine structural details of galaxies, quasars and jets.
A prominent example is the Very Large Array (VLA) in New Mexico, comprising 27 steerable 25-metre dishes arranged in a Y-shaped configuration. Similar arrays exist worldwide, and long-baseline networks can link antennas across continents, forming Earth-sized interferometers.