coma—An aberration affecting the sharpness of images off the axis, caused by the fact that rays from an object point off the axis passing through a given circular zone of the lens come to a focus in a circle rather than at a point, and the circles formed by rays through different zones are of different sizes and are located at different distances from the axis. The image of a point object is therefore comet-shaped. See also aberration of light (definition 2).

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Source: NSPS “Definitions of Surveying and Related Terms“, used with permission.

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