The basic purpose of a telescope is to deflect the radiation incident on every part of its surface towards a focus, and to arrange that all the signals reaching this focus are of the same phase. Most astronomical objects only vary their sizes and brightnesses very slowly. If the emitting object remains unchanged for a sufficient time, we can dispose of our large telescope and use instead one which is much smaller. This can be moved so as to sample the signals incident on each part of the area that would have been covered by the large telescope that we want to synthesize. The signals have to be recorded carefully relative to some reference signal before the telescope is moved to the next position. If we consider the waves gathered by one collector of an interferometer, the reference signal is provided by the other collector.

In principle we can move the elements of this interferometer in any direction over the ground, but it is easier to understand an interferometer on an east-west line. Imagine that we are looking down upon it from the north pole as the Earth rotates. In twelve hours the two elements of the interferometer appear to trace out a complete ring about the mid-point of the interferometer baseline. If the elements of the interferometer are then moved closer together they will trace out the next ring inwards during another twelve-hour period. Eventually, the whole aperture can be filled in this way. The signals recorded throughout the many twelve hour periods are those that would be simultaneously added together by a single large dish of diameter equal to the largest interferometer spacing. However, in this case we take all the recorded signals, and feed them into a large computer. The computer is asked to synthesize the image we would have obtained with a single large dish. The computer then produces a radio picture of our source with the resolution and sensitivity of a single large dish. This technique is called EARTH-ROTATION SYNTHESIS and was pioneered by the Milliard Radio Astronomy Observatory at Cambridge, England. Their highest resolution instrument has a baseline of nearly 5km. When operating at a wavelength of 2cm,it has a resolution of about 1 arc sec, similar to that normally achieved by optical telescopes that are seeing-limited. Other important Earth -rotation synthesis instruments have been built at Westerbork, Netherlands, and in West Virginia, USA. A very large array of small telescopes is also being built in New Mexico to provide Earth -rotation synthesis observations of very faint objects.

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