Spherical Aberration

An optical defect that causes light rays from an object, passing through an optical system at different distances from the optical center, to come to a focus at different points along the axis. On one side of focus, the Airy disk will virtually disappear and the outer diffraction ring will brighten. On the other side, the inner diffraction ring will be brightest. This may cause a slightly out of focus star, for example, to be seen as a discrete disk if the Airy disk and the inner ring blend together because of seeing conditions, but should not be confused with the star's normally smaller Airy disk. Spherical aberration is most often seen in small inexpensive imported reflectors, which use molded spherical mirrors rather than the costly and more difficult to make hand-figured parabolic mirrors found in a quality reflector.

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