Optical Aberrations

Lateral Chromatic Aberration

Lateral color is an off-axis aberration resulting from a difference of image scale at each wavelength. Above:  Lateral color arises from a difference in image scale depending on wavelength Lateral color causes colored fringes at the edge of the field.  The stars will appear to split into red and blue halves, pointed toward the center… More »

Longitudinal Chromatic Aberration

Longitudinal Chromatic Aberration Often called longitudinal color, or axial color, this is the primary aberration affecting refracting telescopes.  It can also arise in any system which uses lenses, although it tends to be most problematic in pure refracting instruments. How Longitudinal Color Arises When light rays pass from one medium to another–say, from air into… More »

Distortion

Distortion is one of the least problematic aberrations for telescopes.  Distortion is typically only large over very wide fields such as with wide-angle camera lenses.  Over the small fields of view associated with telescopes, distortion is rarely an issue.  Very wide field binoculars may show some noticeable distortion, especially less expensive pairs. Distortion arises from… More »

Field Curvature

The focal planes of most telescopes are not flat.  Off-axis light usually focuses closer to the objective than does on-axis light. Above:  A curved focal plane For visual observing, a slightly curved field is no big deal.  The eyepiece will compensate somewhat, and if the stars at the edge of the field are slightly out… More »

Astigmatism

Above:  Spot diagram of a star at the edge of the field affected by astigmatism Like coma, astigmatism is an off-axis aberration.  Unlike coma, it is a symmetrical aberration.  For professional astronomers this can be an important distinction, since asymmetrical aberrations do not allow accurate astrometric (positional) measurements to be made.  For amateur astronomers viewing… More »

Spherical Aberration

Spherical Aberration Spherical aberration is an axial aberration, affecting the entire field equally, including stars at the center.  All telescope designs strive to eliminate or minimize spherical aberration.  Normally, spherical aberration should not be visible in an optical system.  But it is important to understand how it arises to see how it is eliminated in… More »

Coma

Above:  Spot diagram of a star at the edge of the field affected by coma Coma is an off-axis aberration.  Stars in the center of the field are not affected by coma, but the effect grows stronger toward the edge of the field.  Stars affected by pure coma are shaped like little comets (hence the… More »