How does a radio-frequency telescope array such as the VLA (Very Large Array, in New Mexico) or ALMA (Atacama Large Millimeter Array, in Chile) form an image, here of a pair of interacting galaxies? As Earth rotates, the object:array viewing geometry changes to record different Fourier coefficients of spatial frequencies
that make up an image. Each pairing of dish antennas produces a single curved track in the
plane. Although many coefficients are not measured by the sparse array of dishes, enough are recorded to produce a reasonable fidelity image whenever structure is not over-resolved. The three pictures, from left to right, are: the
plane Fourier coefficients mapped out by Earth rotation during the total exposure "track", the object being observed, and the result of imaging with the linked array of radio dishes. Dishes in both radio telescopes are moved every few months to pads of different separation, allowing the telescope to "zoom" in or out to cover different fields of view on the sky at different angular resolutions.