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The Square Kilometre Array

Thousands of antennas... one radio telescope

By Geoff Graham

Australia and New Zealand are engaged in a high stakes race that most people do not even know about. It is a race to host one of the largest scientific projects ever envisaged on the planet… the Square Kilometre Array.

Click for larger image
Potential SKA array station placement in Australia and New Zealand indicating the 5,500km 'baseline' or maximum distance between the array stations. Image courtesy CSIRO

The Square Kilometre Array (abbreviated to SKA) is an international initiative to build the largest radio telescope in the world.

The stakes are high. It will use technologies that have yet to be developed, will involve many countries from around the world and will cost billions of dollars.

The SKA consortium started with a list of four possible sites and has whittled that down to a short list of two; one in South Africa and the other in the Murchison region in Western Australia.

The Square Kilometre Array is a response to two of radio astronomy’s great issues: resolution and sensitivity.

With an optical telescope you are dealing with light that has a wavelength of the order of 600 nanometres and it is relatively easy to construct mirrors and lenses that can reflect and focus these short wavelengths.

Radio telescopes

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Artist's impression of dishes that will make up the SKA radio telescope. Each dish is approximately 15m in diameter. Courtesy Swinburne Astronomy Productions/SKA Program Development Office

In a radio telescope the wavelengths are much longer and so the “mirrors” (reflecting dishes) need to be correspondingly larger. This and the need for sensitivity has led to an “arms race” in radio telescopes with dish sizes growing from the 76-metre dish of Jodrell Bank in the UK (1957) to the 305-metre dish used by the Arecibo Observatory in Puerto Rico (1963).

With increasing size came improved resolution and sensitivity but it came at a cost. The Arecibo telescope is so large that it had to be built onto the walls of a valley and its view of the sky is determined by that part of the Earth is pointing to any particular time.

Another way of addressing the size issue of radio telescopes is to use an array of smaller dishes and employ complex electronics and powerful computers which correlate the signals to simulate one large dish.

The Very Large Array in New Mexico (USA) uses this technique, with the individual dishes spread out by up to 36km. This gives it the resolution of a single, very large steerable dish. While this was a great advance, the sensitivity of the telescope was still limited by the relatively small number of dishes and the resulting small collecting area.

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