By Emily Levesque, University of Washington
With radio telescopes, we can achieve incredibly large diameters thanks to a technique known as interferometry. Imagine that you have an array of several radio dishes, separated by as little as a few feet or as much as a few thousand miles. You can picture those individual radio dishes as shiny spots spread across a giant virtual mirror.
The Interferometer Instrument
When combined, the dishes simulate the size of a single and incredibly large mirror. If all of those dishes are pointed at the same object, they can record data from that object and then combine the data using a mathematical signal processing technique known as aperture synthesis to compute and produce a final image, depicting what we’d see with radio-sensitive eyes.
This type of set-up is known as an interferometer, an instrument that combines multiple sources of light. Radio telescopes like these can combine data detected from multiple dishes to function as a single enormous telescope.
While each individual dish still only collects a small amount of light, the results combine into a beautifully sharp and clear image. The technique is much easier to describe than it is to execute. Interferometry is computationally complex, made easier at longer wavelengths but still incredibly challenging for radio telescopes.
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Powerful Radio Telescopes Use Interferometry
Radio interferometry makes it possible to build extremely powerful telescope arrays. One of the most famous is the Very Large Array, or VLA, in New Mexico. Originally built during the 1970s, the VLA consists of 27 dishes arranged in a giant Y.
Each dish is 82 feet across and designed to be moved along a set of railroad tracks by a specially-designed truck, increasing or decreasing the length of each arm in the Y. The result is a telescope that can actively change its size, with arms that can extend up to 22 miles long to simulate an enormous telescope.
Even larger radio interferometers operate across the globe. These include the Very Long Baseline Array—with antennae stretching from Hawaii in the west to the US Virgin Islands in the east to form an effective telescope the size of North America; and the Event Horizon Telescope, with telescopes ranging from Greenland to the South Pole to create a planet-wide telescope. The Event Horizon Telescope made headlines in 2019 when it captured the first-ever picture of a black hole.
All the Amazing Discoveries
From their earliest incarnation in the 1930s to the cutting-edge telescopes of today, radio telescopes have continually made incredible discoveries. During its decades of operation, the VLA has made discoveries ranging from ice on the surface of Mercury to the birth of a planetary system.
In 2012, the VLA was renamed: it’s now known as the Karl G. Jansky Very Large Array. A small radio telescope in England detected the pulsing signal that became humanity’s first observation of a neutron star, and the planet-sized Event Horizon Telescope captured a photograph of a black hole.
One astonishing discovery from radio telescopes came from early all-sky radio surveys. A 1959 survey of radio sources, done using a radio interferometer west of Cambridge, England, detected two radio sources, 3C 48 and 3C 273, that appeared bright in the radio spectrum, but which had no visible-light counterparts.
Follow-up observations with other radio telescopes proved that these objects appeared to be surprisingly tiny, compact, and bright. This finding puzzled visible-light astronomers, who promptly set out to explain why maps of the radio sky and the visible sky didn’t seem to match.
Mysterious Objects Named Quasars
In 1960, astronomers Allan Sandage and Thomas Matthews thought they had honed in on one of the radio light sources. They discovered what appeared to be a faint blue star at the same location, although to their surprise the star’s spectrum looked unlike any they had ever seen before, with strong broad emission lines rather than the usual neat absorption lines used to classify stars.
Puzzled, the objects gained the nickname quasars, an amalgam of their description as quasi-stellar radio sources. Most astronomers suspected that these sources weren’t really stars but couldn’t sort out what exactly they were. A breakthrough came a few years later, in late 1962, when astronomer Maarten Schmidt managed to capture a visible light spectrum of quasar 3C 273 using the Palomar 200-inch telescope.
To his astonishment, Schmidt measured an immense redshift for the quasar, corresponding to a distance of more than two billion lightyears. His discovery eventually revealed that 3C 273, and the other quasars discovered since were actually the incredibly luminous nuclei of very distant galaxies. Each of these galaxies had a supermassive black hole in its center, with a total mass of millions or even billions of suns.
These black holes were swallowing gas in these galaxies’ centers at astonishing rates, and as the gas poured into the black hole it was releasing huge amounts of energy as electromagnetic radiation, making these galaxies some of the most luminous objects in the universe. Their luminosity made them “radio-loud”, leading to their discovery in the radio surveys of the 1950s.
Common Questions about How Interferometry Helped Radio Telescopes Make Amazing Discoveries
Interferometry is a technique used by radio telescopes that combine data gathered by multiple dishes using a mathematical signal processing technique known as aperture synthesis, so even though each individual dish gathers a small amount of light, the result is a sharp image.
The VLA or Very Large Array telescope uses the radio interferometry techniques. It is made up of 27 dishes arranged in the shape of a Y. Each of its dishes is 82 feet across and can be moved to increase or decrease the length of one of the Y’s arms.
Using radio interferometry, radio telescopes can be built in massive sizes. The Very Long Baseline Array ranges from Hawaii to the US Virgin Islands, making up a telescope the size of North America. The Event Horizon Telescope is even bigger, ranging from Greenland to the South Pole, making it a planet-wide telescope.
