Radio waves converted to images
Thousands of dishes and hundreds of thousands of radio antennas on two continents will capture radio waves from space, to be converted into images that we can see.
In remote Western Australia, around 800km north of Perth on the ancestral lands of the Wajarri Yamaji people, thousands of ‘pine-like’ antennas are emerging from the dry landscape. At the same time, some 10,000km away in the barren Karoo region of South Africa, white mushroom-like dishes are being installed in their hundreds.
Thousands of astronomers and engineers have spent 30 years conceiving and designing the single telescope that will encompass these two distant regions, the dishes and antennas combining to make up the SKA (Square Kilometre Array) telescope, the construction of which was finally given the go-ahead in 2021. The SKA will be the biggest radio telescope in history, and it should provide us with the answers to some of the universe’s biggest mysteries.
“Mankind will take another giant leap by undertaking to build what will become the biggest scientific structure of its kind on the planet,” says Professor Philip Diamond, the Director-General of SKAO, the organisation behind the telescope.
With routine observations hoped to start in the late 2020s, the costs of construction plus the first decade of running the telescope will total close to A$3 billion, but astronomers expect results that will justify such expense. The radio telescope’s sensitivity will be unprecedented, able to reveal images of the universe that are 50 times more detailed than those from the Hubble space telescope, providing new knowledge about almost every important field of astronomy: the expansion of the universe, the birth of the first galaxies, the nature of gravity, even the distribution of life in space.
One of its key missions will be to explore close to the beginning of the universe some 13+ billion years ago, or about 380,000 years after the Big Bang, when the first galaxies lit up a pitch-dark universe.
Optical telescope weaknesses
Astronomers hope that the SKA telescope will capture radiation that has spent 13+ billion years travelling from the remotest regions of the universe to Earth. But instead of visible light, the SKA will record radio waves. Both visible light and radio waves are types of electromagnetic radiation, but with different wavelengths. Optical telescopes capture visible light with wavelengths of 380-700 nanometres, and if they are located on dry land on Earth, a cloudless night sky is essential for good results. Hence the biggest optical telescopes are often located on mountain peaks, such as in Chile, Hawaii and the Canary Islands.
Optical telescopes are smaller than radio telescopes because their mirrors must be made to ultrafine specifications, able to detect weak cosmic light with the minimum of distortion. Even so, the light can be blocked by dust and other obstructions on its way towards Earth, or distorted by Earth’s atmosphere, where turbulence varies the air’s refractive index. This can make the image of a remote star or galaxy soft and blurry instead of razor sharp.
But this is not true for radio waves, which have much lower frequencies and hence longer wavelengths – from millimetres to
PHILIP DIAMOND DIRECTOR-GENERAL OF SKAO, THE ORGANISATION BEHIND SKA
“Mankind will take another giant leap by undertaking to build the SKA telescope!”
several kilometres. These are not influenced by the atmosphere in the same way.
Radio stars
Many objects in the universe – galaxies, stars, pulsars and black holes – emit radio energy that can be detected by radio telescopes. The first time this happened was in 1931, when American physicist Karl Jansky detected radio energy from the Milky Way.
When the radio waves are translated into wavelengths that we can see, they can reveal things about objects of the universe that we could not perceive just from the visible light they are emitting.
One problem with using radio telescopes, however, is that they are sensitive to radio signals other than those they were designed to capture. And that’s why the SKA telescope’s discs and antennas are being placed in sparsely-populated regions, as far from the hum of radio, TV and phone signals as possible. A location in the Southern Hemisphere also provides a good view of the Milky Way’s band across in the sky, while the two points of view in Australia and South Africa allow scientists to study the same object for most of the day.
Telescopes generally get better as they get bigger, because a larger receiving area means higher sensitivity for very weak signals. The size of the SKA will also cover more of the sky at a time, and so be able to scan it faster. A third size advantage is that the image resolution rises.
The world’s biggest radio satellite dish in current operation is located at the FAST observatory in China. Its satellite dish is 500 metres wide, with a receiving area of 71,000m2, which is some 14 times less