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 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.
There are five monochromatic aberrations:
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 affect only telescopes with lenses, such as
but also telescopes which are primarily reflecting scopes but also incorporate
lenses, such as Schmidt-Cassegrains or
Maksutov-Newtonians. There are two
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.
Presentation of Optical Aberrations
You may read through the following sections in order, or click any aberration
name below to jump directly to its page.