Optical Aberrations

Aberrations, if they occur, are physical optical errors caused by faulty optical design or manufacturing. A telescope will perform well if its optical errors do not exceed the Rayleigh Limit. Below is a summary of certain well-known aberrations:

Spherical Aberration

Coma

Chromatic Aberration

Astigmatism

Field Curvature

Collimation

Spherical Aberration

Spherical Aberration causes monochromatic light rays to focus at different points along the optical axis. If present, this aberration seriously upsets the energy distribution of the telescope's diffraction pattern. Because of this, all telescopes are designed to exclude spherical aberration.

Coma

Coma is an asymmetrical off-axis monochromatic aberration inherent in certain telescope designs. It is most often associated with fast Newtonians and is caused by the Offense Against the Sine Condition (OSC), expressed as the ratio of coma to field radius. For example, if OSC equals 0.0025, then the amount of coma present at the edge of a 40-degree apparent field is 3 minutes of arc, which is acceptable to the human eye. Coma correcting lenses substantially reduce this aberration when indicated.

Chromatic Aberration

Chromatic Aberration is the failure of a lens to bring light of all colors to a common focus. Ordinary telescope doublet objectives, called achromats, reduce chromatic aberration to a residual level known of as the Secondary Spectrum which leaves a faint violet halo around bright stars and planets. Fortunately, the eye soon learns to ignore it, making small achromats very popular and economical telescopes. Apochromats, which are exotic refractor designs, totally eliminate the Secondary Spectrum. But their high price puts them out of reach of many amateurs.

Astigmatism

Astigmatism is an aberration consisting of a cylindrical converging wave front. In it, two radii of optical curvature are oriented at right angles to one another. Astigmatism causes the image of a point light source to appear as an ellipse with the long axis of the ellipse shifting by 90° on opposite sides of the focal plane. The aberration is usually caused by pinched optics or collimation errors.

Field Curvature

Collimation

Collimation -- the correct alignment of the optical elements in a telescope. Special, inexpensive collimation tools are available for amateurs to ensure precise collimation of their instruments, which is critical for achieving optimum performance.