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.