Published by Australian Academy of Science
Simply astronomical – the Square Kilometre Array Box 1 | What is a radio telescope?

Many Australians are familiar with the radio telescope near Parkes in New South Wales, which featured in the movie The Dish. Another telescope, the Arecibo is so spectacular it played a role in the X-files and the James Bond movie GoldenEye. Radio telescopes such as these use a large dish to capture radio waves, which, like the light waves gathered by optical telescopes, are a part of the electromagnetic spectrum.

The electromagnetic spectrum
(Image: © NASA)

Radio waves have a longer wavelength than visible light so the telescopes need to be very large to produce sharp images of astronomical objects. The larger the collecting area of a telescope, the more sensitive it is and therefore able to detect faint objects at great distances. After reaching the dish (or reflector), radio waves are reflected onto a receiver. The receiver detects and amplifies the signals which can then be processed and stored in a computer ready for analysis.

Two or more dishes (antennas) can be linked together to form a telescope known as an interferometer. The signals from each antenna can be electronically combined to imitate a single dish equal in size to the distance between them – the greater the separation, the sharper the image produced. Images of the very highest resolution are produced by connecting telescopes across continental scales around the globe, a technique known as Very Long Baseline Interferometry.

Radio telescopes have several advantages over optical telescopes. The radio waves they detect can penetrate interstellar gas and dust that block visible light – so the universe is much more 'see-through' at radio wavelengths. Radio telescopes can also observe some objects such as pulsars that do not radiate visible light. A radio telescope can also operate during the daytime.

Optical telescopes must be sited well away from the lights of busy cities. Similarly, radio telescopes are highly susceptible to man-made interference from television, mobile phones, car ignition systems and industrial machinery. To reduce this interference, radio telescopes are often sited in remote and sparsely populated locations.

Australia has been a pioneer of radio telescopes since the earliest days. The first radio galaxies were discovered from a cliff-top at Dover Heights in Sydney using an antenna no more complicated than a television aerial. At present our largest telescope is the Australia Telescope in Narrabri, northern New South Wales, constructed as a bicentennial project in 1988.

Related sites

• What is radio astronomy? (Square Kilometre Array, Australia)

• How radio telescopes work (National Radio Astronomy Observatory, USA)

• Basics of radio astronomy (Jet Propulsion Laboratory, USA)

• Radio interferometry (CSIRO, Australia)

Other boxes

Box 2. Spectroscopy – a vital tool

Box 3. Is Einstein still right?