Interferometry

Rather than a traditional big telescope, interferometers instead use many smaller individual elements which are connected together in an array, with their data combined digitally using powerful computers. This technique is known as "aperture synthesis" and is used in many modern radio telescopes.

Aperture synthesis enables an array to act like one big telescope equivalent in size to the distance between the two furthest antennas, something that would be impossible to build as a single structure. The SKA telescopes both follow this design, linked by thousands of kilometres of optical fibre. Both SKA-Mid and SKA-Low can also be divided into sub-sets of antennas which can be used for different observations, making them extremely versatile. 

The technique

Higher resolution is achieved by increasing the separation between the antennas - doing this is the equivalent of creating a zoom lens. How? Imagine radio waves of the same wavelength coming from an object in space, and two distant antennas on the ground. The waveform is made up of cycles up and down. When it hits antenna number one, it will include part of that up and down cycle; when it hits antenna number two, further away, it will include a different part of the cycle. This difference is known as a phase offset, and it tells us about the position of an object in the sky.

If the two antennas were next to each other, there would be no phase offset detectable; in other words, the two antennas would see the same thing. The more distant the antennas are, the greater amount of phase offset can be detected, and the more precisely positional information can be recorded. This is why aperture synthesis creates higher resolution/a greater amount of detail than is possible compared to collecting radio waves with one dish. These different versions of the wave form are then combined and synchronised (to counter the time delay).

Another benefit of using an aperture synthesis array, is that adding more small antennas increases the collecting area of the telescope, meaning it can collect more photons; this is known as sensitivity. Increased sensitivity enables a telescope to reveal fainter details in the sky. This is similar to our own night vision; if you stay outside a while, your pupil dilates, and you can see more stars. 

Aperture synthesis gave rise to many of today’s iconic radio telescopes, including the 27-dish Very Large Array (VLA) in New Mexico, USA, the 66-dish ALMA telescope in Chile (currently the world’s largest radio telescope array), and many SKA pathfinders. When operational, SKA-Mid and SKA-Low will be the largest radio telescope arrays on Earth.

Aperture synthesis and the SKA telescopes

Getting hundreds or even thousands of antennas to operate coherently as a single telescope isn't as simple as just connecting them together. Both SKA telescopes have long distances between antennas - the most distant are separated by baselines of 150km for SKA-Mid and 65km for SKA-Low. This achieves very high resolution, but in order to make sense of the signals you need an extremely precise timing and synchronisation system to correct for the different arrival times of astronomical signals, and for environmental factors which affect the transmission of those signals along such long distances. For the SKA telescopes, the requirements were so stringent that this demanded the development of bespoke technology. Read more about these innovations on our dedicated pre-construction website.

A key benefit of aperture synthesis arrays is that they are intrinsically scalable; increase the number of antennas and you increase the size and capabilities of your telescope. The long-term ambition for the SKAO is to eventually expand both telescopes by increasing the number of dishes across other African countries and the number of antennas in Australia. This scalable nature also benefits the construction process, as smaller portions of the full arrays can be operated long before the telescopes are completed.