Australia Telescope National Facility

Introduction to Radio Astronomy and Interferometry

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Radio astronomy uses radio waves to study regions of space.

Single-dish radio telescopes produce blurry images.

When an optical image of an object is formed by reflecting light rays from a mirror onto a focus, quite sharp images of extended objects can be made. A single-dish radio telescope is not nearly as effective at detecting the fine detail in the objects it focuses on, and the radio image of an object from a single-dish radio telescope is very blurry. The blurring stems from a basic principle that affects all types of telescopes. In order to "resolve" images (ie make sharp images), the diameter of a telescope's collecting area must be many times greater than the wavelength of the radiation it detects. Light waves have wavelengths of less than one millionth of a metre, and so collecting mirrors are large enough compared to a wavelegth of light that they can resolve the details of objects observed. However, radio waves have wavelengths of roughly one tenth of a metre and so even large radio telescope dishes produce blurry images. A single radio dish would have to be many kilometres across to achieve a sharp image at radio wavelengths and such telescopes have been too difficult to build.

Interferometry is a technique used to overcome the blurring and produce sharper radio images.

The spacing between the radio telescope dishes in an interferometer determines the size of the objects that can be resolved by the interferometer.

Using many dishes together in an interferometer array allows astronomers to form more complete images of objects.

This is how an aperture synthesis telescope such as the Australia Telescope Compact Array works.

VLBI (Very Long Baseline Interferometry) enables very small objects to be clearly resolved.

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Last update by Michelle Storey. 14/2/99

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