Radio Source Size and Resolution of an Interferometer

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

If a source being observed, covers an angle in the sky much smaller than the angular resolution ${\lambda}/D$ (see section 1.2.1), then the combined output from the pair of dishes observing the source oscillates strongly with time. As the separation between the dishes increases, the angular size of source that the interferometer can resolve decreases. For sources of angular size much larger than the resolution, the output ceases to vary and less information on the source can be obtained. In order to study the large-angular-scale structure, astronomers need to use a lower resolution i.e. a more closely spaced pair of dishes. An interferometer works most efficiently, in the sense of providing the maximum information of the intensity and the size, for a source that is observed, when its size is comparable to the resolution of the interferometer or a single telescope, as the case may be.