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coma, optical

optical coma
Coma is a defect of an objective mirror or lens in which rays of light, striking the objective away from the optical axis are not brought to focus in the same image plane. In the case of an astronomical telescope, for example, the result is that star images toward the edge of the field of view appear to have comet-like tails spreading radially out from the optical axis (negative coma) or in toward the axis (positive coma).

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