As the separation between the dishes increases, the angular size of source that the telescope can detect decreases. For sources of angular size much larger than the resolution, the output ceases to vary and less information can be obtained about the sources. To study large-scale structure, astronomers need to use a lower resolution ie a more closely spaced pair of dishes. An interferometer works most efficiently, in the sense of returning the most information about the intensity and size of the source being observed, for a source whose size is comparable with the telescope's resolution. Using many dishes together in an interferometer array allows astronomers to form more complete images of objects.
Most sources have a complicated shape, with both large and small-scale structure.
Better images of observed sources can be formed if more than two dishes are used at once, and as more powerful computers have become available signals from more dishes have been able to be combined electronically. The computer must carefully coordinate the movements of all the dishes. Each dish can be combined as a pair with every other dish to maximise the number of spacings between dishes. This maximises the size-scales in the source about which the interferometer can obtain information. The rotation of the Earth under the source then changes the distances from the source to each dish.