Radio waves are a form of electromagnetic radiation – like visible light waves only with much longer wavelengths. Visible light waves are oscillating electromagnetic signals with wavelengths of less than one millionth of a metre, whereas radio waves have wavelengths in the range of a few millimetres to several metres. Radio telescopes gather and concentrate the radio waves from an astronomical source. The signal received is then electronically processed so that it can be measured by a computer. When a picture of an area of the sky is to be built up the signals are stored in a computer and displayed as a radio-wave intensity map of the region being observed. The Australia Telescope National Facility is a world leader in the technology associated with radio astronomy and in the astrophysics of sources that emit radio waves. 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.