Optical Aberrations

This section describes optical aberrations, why they occur, and how they can be eliminated or minimized. More specific information on how certain optical designs deal with aberrations is described in the Optical Designs section.

Spot Diagrams

Optical aberrations are normally displayed by showing a highly magnified image of a theoretically perfect star as imaged by the optical system. This is called a spot diagram. Software can calculate what a perfect star would look like after being imaged by a telescope. In theory, a perfect star image would be round and as small as possible (limited only by the physics of the diffraction of light). Any deviation from a small round spot indicates an aberration of some kind (or, often, several aberrations). Each aberration below is described along with a spot diagram to show what effect the aberration has on the star image.

Types of Optical Aberrations

There are seven basic optical aberrations in two classes. Chromatic aberrations are wavelength dependent, meaning each color of light is affected differently. Monochromatic aberrations are independent of wavelength, meaning they affect every color equally. Pure reflecting telescopes can only suffer from monochromatic aberrations. Telescopes with lenses can potentially be affected by all seven aberrations.

Monochromatic Aberrations

There are five monochromatic aberrations:

• Coma

• Spherical Aberration

• Astigmatism

• Field Curvature

• Distortion

Spherical aberration is an axial aberration, one that affects the entire field equally, including the center of the field. The other four aberrations are off-axis aberrations, affecting the star images increasingly toward the edge of the field but not at the center.

Chromatic Aberrations

Chromatic aberrations affect only telescopes with lenses, such as refractors, but also telescopes which are primarily reflecting scopes but also incorporate lenses, such as Schmidt-Cassegrains or Maksutov-Newtonians. There are two chromatic aberrations:

• Longitudinal Chromatic Aberration

• Lateral Chromatic Aberration

These are often called longitudinal color (or axial color) and lateral color. Longitudinal chromatic aberration is axial (affecting the entire field), while lateral color is an off-axis aberration.