How Radio Telescopes Detect Signals From Deep Space

Radio telescopes do not look like optical telescopes, but they work on the same basic principle: collect light, focus it, and analyze it. The difference is the wavelength. According to the US National Radio Astronomy Observatory (NRAO), radio telescopes observe wavelengths roughly between 1 millimeter and 10 meters, far longer than the few hundred nanometers of visible light, which is why their 'lenses' are metal dishes rather than glass.

Turning a faint whisper into usable data

NRAO describes cosmic radio signals as astonishingly weak, noting that a single cell-phone signal carries roughly a billion billion times more power than the faint cosmic radio waves these telescopes are built to detect. To catch anything useful, a radio telescope needs a large collecting area and sensitive electronics.

• A parabolic dish, whose curved shape reflects incoming radio waves to a single focus point above it
• A feed horn at that focus, a metal funnel sized to the wavelength being studied, which channels the waves into the receiver
• A receiver, often supercooled, that detects the signal and passes it on
• An amplifier that boosts the signal before it is recorded for analysis

Linking telescopes together: interferometry

A single dish, however large, cannot match the fine detail an optical telescope can achieve. NRAO explains that radio astronomers solve this with interferometry: pointing several antennas at the same object and using a supercomputer called a correlator to combine their signals as if they came from one giant telescope. Because the same cosmic signal arrives at each antenna at a slightly different instant, the resulting timing differences let astronomers reconstruct far more detail than any single dish could capture alone. NRAO notes correlators can run at femtosecond speeds, performing up to 16 quadrillion operations per second, and that its Very Long Baseline Array spreads antennas across continents, synchronized by hydrogen maser atomic clocks accurate to a few billionths of a second.

Imaging a black hole with an Earth-sized telescope

The clearest demonstration of this technique is the Event Horizon Telescope (EHT). According to NRAO, it links eight ground-based radio telescopes spread across the globe, which observed together during a 2017 campaign using very long baseline interferometry so that they acted as a single instrument roughly the size of Earth. Each telescope recorded roughly 350 terabytes of data per day; the recordings were later flown to specialized correlators at the Max Planck Institute for Radio Astronomy and the MIT Haystack Observatory to be combined.

The target was the supermassive black hole at the center of the galaxy M87. Its event horizon spans roughly 40 billion kilometers, yet from Earth the resulting bright ring around its shadow measured only about 40 microarcseconds across — a scale NRAO compares to 'measuring the length of a credit card on the surface of the Moon.' The combined data produced the first-ever image of a black hole's shadow.

Catching split-second flashes from across the universe

Not every radio telescope relies on a dish at all. CHIME, the Canadian Hydrogen Intensity Mapping Experiment at the Dominion Radio Astrophysical Observatory in British Columbia, uses four fixed, half-pipe-shaped cylindrical reflectors with no moving parts, according to MIT Physics. Instead of steering hardware, its correlator processes roughly 7 terabits of data per second to digitally reconstruct signals arriving from thousands of directions in the sky simultaneously.

That design made CHIME extraordinarily effective at catching fast radio bursts (FRBs), millisecond-long flashes of radio energy from deep space. In its first year of operation, 2018–2019, CHIME detected 535 new FRBs, nearly quadrupling the roughly 140 bursts known worldwide up to that point, MIT Physics reported. The haul included 18 sources that burst repeatedly, a pattern that suggested repeating and one-off FRBs may arise from different astrophysical mechanisms.

The world's largest dish, and what comes next

China's Five-hundred-meter Aperture Spherical radio Telescope (FAST), built in a natural valley in Guizhou province, has a 500-meter dish made of roughly 4,400 adjustable aluminum panels, giving it an effective aperture of about 300 meters at any given time, according to EarthSky. It observes wavelengths from 10 centimeters to 4.3 meters and holds roughly twice the collecting area of the 305-meter Arecibo dish it surpassed in size. FAST became fully operational in January 2020 and began allocating about 10% of its observing time to international researchers in August 2021.

That sensitivity has translated into a record haul of pulsars — fast-spinning neutron stars that sweep beams of radio waves past Earth like a lighthouse. As of October 2025, FAST had identified 1,152 pulsars, more than the combined total found by other telescopes worldwide over the same period, Chinese state broadcaster CGTN reported.

The next leap in scale is already under construction. The SKA Observatory is building two complementary arrays, according to its own description of the project: SKA-Low in Western Australia, with 131,072 antennas spread across a maximum baseline of 74 kilometers, and SKA-Mid in South Africa, with 197 dish antennas spanning a maximum baseline of 150 kilometers. Together the two sites are intended to let astronomers study galaxy structure, the environments around black holes, and gravitational waves in greater detail than existing instruments allow.