Optical Aberrations

Optical Aberrations

In their basic design, optical systems are held to the standard of first order or Gaussian optics: a monochromatic "point" light source located at infinity and centered on the optical axis will appear as a "point" image at the center of a focal plane that is flat and perpendicular to the optical axis. This standard is then extended off axis to include the image of any point visible anywhere within the telescope image area or eyepiece field of view.

Any departure from this optical perfection is an aberration. The most important of these were identified and analyzed in the mid 19th century — empirically by the Hungarian optician Joseph Petzval, and theoretically by the German mathematician Philipp Ludwig von Seidel (pronounced ZY·dul). They are usually called the Seidel errors, and in optical systems that are symmetrical around an axis of rotation that is identical with the optical axis, the Seidel errors are significant because they have both the greatest impact on image quality and the greatest utility as guides to improving an optical design.

The five Seidel errors, in traditional order, are: (1) spherical aberration, (2) coma, (3) astigmatism, (4) field curvature and (5) distortion. Two types of first order (6,7) chromatic aberration (caused when the image is not monochromatic) are consistently included among the important aberrations; and (8) spherical aberration of the exit pupil is a flaw often encountered in wide angle eyepieces.