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.