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A comparator (aka blink comparator or blink microscope) is a device used to compare the position of astronomical objects imaged on two photographic plates. It allowed adjustment of their position to align them, and quick switching from viewing one plate or the other, so a moving object would appear to jump when the switch was made. The process was called blinking the plates. Its purpose is to identify and help track solar system objects in orbit, and the proper motion of stars. Pluto was discovered using this technique.