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A null corrector is a feature of a mirror testing device, the feature designed for testing mirrors that are not spherical (aspheric mirrors, e.g., parabolic mirrors). Such testing devices generally use light passing through an interferometer (e.g., a Michelson interferometer), then to the mirror and back, using interference to detect discrepancies in the mirror surface. Concave spherical mirrors can be placed so that the entire surface is equidistant from a point, making such testing relatively straightforward, but other mirror shapes lack this quality, and a null corrector is an optical device (using lenses) that compensates for the mirror's shape, so that the interferometer can perform the test as if the mirror were spherical. Use of one is termed a null test, a step in preparing a telescope mirror for precision use. There are various kinds of null tests, checking for various aberrations that can be caused by the precision of the mirror shape.