What Is a Radio Telescope? Seeing the Sky at Wavelengths We Cannot Feel

A radio telescope is a directional antenna with a receiver. The familiar form is a parabolic dish, which reflects incoming waves to a focus where a feed antenna collects them, exactly as a mirror does in an optical reflector. The signal is extraordinarily weak, so it is amplified immediately by a receiver cooled with liquid helium to a few degrees above absolute zero, because the thermal noise of the electronics themselves would otherwise drown the astronomical signal. The amplified signal is shifted to a lower frequency, digitised, and processed to produce either a spectrum, showing power at each frequency, or an image built by scanning the dish across the sky. The dish surface must be accurate to a fraction of the observed wavelength, which is far easier at long wavelengths than for optical mirrors, which is why a radio dish can be made of mesh and still work perfectly for metre waves while needing a solid, precisely shaped surface for millimetre observations.

The ability of any telescope to distinguish fine detail depends on the ratio of the observed wavelength to the aperture, and radio waves are enormously longer than light waves, by factors of a million or more. A dish the size of a football pitch therefore has worse angular resolution at metre wavelengths than a pair of binoculars has in visible light, which is why early radio astronomy produced maps of blobs. Building bigger dishes runs into gravity, since a steerable structure deforms under its own weight, and the largest fully steerable dish is around a hundred metres. Fixed dishes can be larger, built into natural hollows, as at Arecibo in Puerto Rico, which was three hundred metres across before its collapse in 2020, and the five-hundred-metre Chinese telescope that succeeded it, both of which trade steering for size and observe by moving the receiver rather than the dish.

The solution to resolution was to stop trying to build one enormous dish and instead combine the signals from many separated ones. Correlating the waves arriving at two antennas gives information about structure on the angular scale set by their separation, so an array acts, for resolution purposes, like a single telescope as wide as the greatest distance between its elements:

• •Aperture synthesis, developed by Martin Ryle at Cambridge, uses the earth's rotation to sweep the antennas through many effective positions and reconstruct an image, work that won a Nobel prize in 1974
• •Arrays of dozens of dishes, including the Very Large Array in New Mexico, whose antennas are moved along rail tracks to trade resolution against sensitivity to large structures
• •Very long baseline interferometry, in which antennas on different continents record data with atomic clock timestamps and the signals are combined later, giving baselines the size of the earth
• •That technique produced the first image of a black hole's shadow in 2019, using a network of observatories operating as a single instrument with resolution fine enough to read a newspaper in another city
• •Millimetre and submillimetre arrays such as ALMA in Chile, sited high and dry because water vapour absorbs those wavelengths
• •Low frequency arrays of simple fixed antennas combined entirely in software, which have no moving parts at all and point by computation

The field began by accident in 1933, when Karl Jansky, investigating sources of static affecting transatlantic telephone calls for Bell Labs, identified a hiss that rose and set with the stars and traced it to the centre of the galaxy. What followed changed cosmology repeatedly. The twenty-one centimetre line of neutral hydrogen made it possible to map the structure of the Milky Way through dust that blocks visible light entirely. Quasars were identified as extraordinarily distant and luminous objects. Pulsars were discovered in 1967 by Jocelyn Bell Burnell as regular pulses initially labelled with a joking reference to little green men. The cosmic microwave background was found in 1964 by Penzias and Wilson while trying to eliminate noise from an antenna, and is the strongest evidence for the big bang. More recently, fast radio bursts, millisecond flashes from far outside the galaxy, remain only partly explained. The field's main practical threat is radio frequency interference, which is why major observatories sit inside legally protected quiet zones and why satellite constellations transmitting in nearby bands have become a serious concern.

A radio telescope focuses long-wavelength radiation onto a feed and amplifies an extremely weak signal with receivers cooled to a few degrees above absolute zero. Because resolution depends on aperture divided by wavelength, single dishes see poorly, which is why astronomers combine separated antennas into interferometers that resolve as if they were one instrument as wide as the array. The field began with Jansky tracing telephone static to the galactic centre, and produced pulsars, quasars and the cosmic microwave background.