Earth rotation synthesis

Earth rotation synthesis (or Earth rotation aperture synthesis) is the use of the Earth's rotation to provide multiple configurations of the radio telescopes comprising an interferometer. Interferometers require two or more telescopes to produce their data: at any point in time each pair of telescopes produces data for a one-dimensional image. For example, at one instant, four telescopes comprise six pairs, producing six such 1D images. Producing a two-dimensional image requires many such configurations of telescope pairs at many angles in relation to the celestial sphere (aperture synthesis). An interferometer may be given numerous telescopes to provide this (e.g., ALMA) but for longer baselines and more sensitive (expensive) telescopes, options are limited. Earth rotation synthesis consists of using the Earth's rotation, which generally constantly changes the direction of the baselines. Interferometric data taken from baselines over many directions can be collected merely by taking interferometric data from each single pair of radio telescopes over the course of some time. A limitation is that you don't get a "snapshot", i.e., the image is constructed from data taken over time, so there are limitations on its ability to image of movement or transient phenomena. Earth rotation synthesis is commonly used in very-long-baseline interferometry (VLBI) such as the Event Horizon Telescope (EHT). It also increases the time necessary to capture an image.