Astigmatism

An optical defect in which star images are elongated into ovals which change from a radial orientation (pointing towards the center of the field) to a tangential one (at right angles to the center) as the observer moves the telescope focuser from one side of the best focus to the other. Should not be confused with the star images of an out-of-collimation telescope, which may also be oval but which will not change orientation as the observer passes through best focus.


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