Aperture synthesis is a method of combining signals from a collection of individual antennae or telescopes to provide an image with a resolution equivalent to that of a single telescope with a size roughly equal to the maximum distance between the individual antennae. The technique was first used with arrays of radio telescopes but is now also used routinely for making high-resolution optical, infrared, and submillimeter observations.
In the simplest case, two antennas are used as a radio interferometer, and the phase and amplitude of the combined radio signal are measured continuously. As the Earth rotates in the course of a day, one antenna is automatically carried around the other, effectively sweeping out a ring. On successive days the separation between the two antennas is changed, so that a large elliptical area is gradually covered. When the records are combined in a computer, it is possible to produce a radio map of the section of sky under observation with the detail resolved as if the telescope aperture were the size of the total area swept out.
In practice, more than two antennas are normally used to speed up the process and give greater flexibility. It is also possible to combine observations made at different sites, separated by thousands of kilometers.