Taking the First Picture of a Black Hole  - How do Radio Telescopes work?
Can you imagine yourself hearing only the bass of a music recording? Or only seeing objects of a particular colour? Well, in a way, you experience this every day. The human eye can only detect a narrow part of the electromagnetic spectrum: a section we call visible light. But a broad range of electromagnetic waves exist with the same nature — for example radio waves, which have much longer wavelengths than the light we can detect with our eyes. Radio wavelengths range from 1 millimetre to over 10 metres, while visible light wavelengths are only a few hundred nanometres — one nanometre is 1/10 000th the thickness of a piece of paper!
Radio waves are not visible to us directly, but in 1867 their existence was predicted by James Clerk Maxwell. By the end of the 19th century, scientists had developed instruments that could transmit and detect electromagnetic waves at the radio end of the spectrum. A few decades later, it was discovered that these instruments could not only be used for communication, but could also be directed towards space — hidden parts of the Universe were suddenly revealed!
The first detection of radio waves from an astronomical object was in 1932, when Karl Jansky observed radiation coming from the Milky Way. Then came the phenomenal discovery of the cosmic microwave background in 1964, worthy of a Nobel Prize in Physics. Soon afterwards, Jocelyn Bell Burnell observed the first pulsar with an array of radio aerials in 1967, which led to another Nobel Prize. And this was only the beginning — a dazzling array of discoveries have been made since.
But how do radio telescopes work?
In order to detect signals from astronomical objects, every radio telescope requires an antenna and at least one receiver. They come in a variety of shapes and sizes, reflecting the need to be able to detect a great breadth of radio waves across many wavelengths.
The antennas of most radio telescopes working at wavelengths shorter than 1 metre are paraboloidal dishes. The curved reflector concentrates incoming radio waves at a focal point. For shorter wavelengths, such as millimetre waves collected by ALMA and VLBI networks like the EHT and GMVA, the perfection of the dish’s surface is critical: any warp, bump, or dent in the parabola will scatter these tiny waves away from the focus, and valuable information is lost.
In addition to the main dish, most radio telescopes have secondary reflectors that send the concentrated waves to receivers. These receivers select, detect and amplify the radio signals of the desired frequencies. The receiver delivers these signals in an analogue format, which is converted into a digital signal and fed into a computer. Astronomers can then stitch these signals together to create a map of the sky measured by radio brightness.
Radio telescopes point at a radio source for hours in order to detect the faintest signals coming from the near and distant Universe. This technique is a similar to keeping the shutter of a camera open for a long exposure at night. After combining these signals with a computer, astronomers can analyse the radiation emitted by many astronomical phenomena — such as stars, galaxies, nebulae and supermassive black holes.
Here’s the problem in radio astronomy: because radio wavelengths are so long, it is difficult to achieve a high resolution of the objects being observed. Even the shortest radio wavelengths observed by the largest single telescopes only result in an angular resolution slightly better than that of the unaided eye. The resolution (or degree of detail in the image) of a single telescope can be calculated by dividing the length of the radio wave by the diameter of the antenna. When this ratio is small, the angular resolution is large and therefore finer details can be observed. The larger the diameter of the telescope, the better the resolution, therefore radio telescopes tend to be much larger than telescopes suited for other, shorter wavelengths like visible light.
The longest wavelengths, on scales of metres, pose a particular challenge because it is hard to achieve good resolution from a single dish. The largest moveable dish is the Green Bank Telescope (100 metres across). Dishes that don’t move can be much, much larger. The world’s biggest radio dish is the newly-constructed Five-hundred-meter Aperture Spherical Telescope (FAST) in China: a fixed dish supported by a natural basin in the landscape. FAST can observe radio waves up to 4.3 metres in wavelength. There are also other similar dishes, such as the historic 300-metre Arecibo Observatory, which was the largest telescope for five decades until FAST was completed in 2016.
But building antennas any larger than this is not feasible, so here we reach a limit when it comes to observing at longer and longer wavelengths. But what can be improved is the angular resolution, opening the door of investigation into the finest details of the low-energy Universe.
A Nobel Prize winning technique called interferometry opened this door: if the signals from many antennas spread over a large area are combined, then the antennas can operate together like a gigantic telescope — an array. Modern arrays usually bring the signals together at a central location in digital form using optical fibres, and then process them in a special-purpose supercomputer called a correlator.
One such array is the Atacama Large Millimeter/submillimeter Array on the Chajnantor plateau in the Atacama Desert. ALMA comprises 66 high-precision antennas up to 16 kilometres apart, working together as an interferometer. The resolution of an interferometer depends not on the diameter of individual antennas, but on the maximum separation between them. Moving the antennas further apart increases the resolution.
The signals from the antennas are brought together and processed by the ALMA correlator. The antennas work together in unison, giving ALMA a maximum resolution which is even better than that achieved at visible wavelengths by the NASA/ESA Hubble Space Telescope. This is because the maximum distance between the antennas can be very large, increasing the resolving power of the interferometer and allowing it to detect smaller details.
The ability to link antennas over baselines of many kilometres is crucial to obtain extremely good resolution and a high degree of detail in the images. This gives astronomers the possibility to go even further than arrays like ALMA; by combining the signals from radio telescopes all across the world, the distances between the antennas can be Earth-sized — and even larger, in the case of space-based antennas like Spektr-R.
The telescopes do not have to be physically connected; rather, the signals recorded at each telescope are later “played back” in the correlator. This technique, called very-long-baseline interferometry (VLBI), provides exquisite angular resolution and paves the way for phenomenal new discoveries — including the detailed observation of the supermassive black hole at the centre of our galaxy.
This is the fifth post of a blog series following the Event Horizon Telescope and the Global mm-VLBI Array projects. Next time, we’ll talk about how to build an Earth-sized radio telescope.