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 certain designs.  The elimination of spherical aberration is critical to how certain telescopes such as Schmidt-Cassegrains and Newtonians are designed.

Above:  How a spherical mirror creates spherical aberration

A simple spherical mirror cannot focus light to a single point.  As the diagram above shows, light from the edge of the field is focused closer to the mirror along the optical axis than is light from the center of the field.  This means it is not possible to find a single point of best focus, only a point where the image is smallest but still not sharp.  The simplest way to eliminate coma with a single mirror is to change the shape from spherical to parabolic.  A parabolic mirror does not suffer from spherical aberration and can focus all light to a single point.

Above:  A parabolic mirror focus all light to a single point

Note that this is the same principle used in radar and satellite dishes.  Radio waves are simply electromagnetic radiation, just like visible light only with much longer wavelengths.  Satellite dishes are parabolic in shape.  Even the sound-collecting dishes you see along the sidelines of NFL games are parabolic to focus the incoming sound waves onto the microphone located at the focal point of the dish.

How Telescope Designs Eliminate Spherical Aberration

Every telescope design sets out to eliminate spherical aberration.  In the case of the Newtonian telescope this is simply done by making the primary mirror parabolic (see the Optical Designs section on Newtonians for more details).  Cassegrain reflectors use a very specific combination of mirror shapes to eliminate spherical aberration, normally either a parabolic mirror combined with a hyperbolic mirror, or a pair of hyperbolic mirrors.  Commercial Schmidt-Cassegrain telescopes use spherical mirrors, which would, on their own, create spherical aberration.  The Schmidt corrector lens on the front of an SCT eliminates the spherical aberration inherent in the mirror design.  Most Maksutov-Cassegrains work the same way.

Refracting telescopes normally use spherical lenses, due to the extreme difficulty and cost associated with constructing aspherical lenses.  A single spherical lens of course suffers from spherical aberration.  However, a refractor eliminates spherical aberration by combining two lenses with equal but opposite amounts of spherical aberration.  More complex refractor designs may use three or four lenses, but the basic idea is the same.  These lenses must also work to eliminate a number of other aberrations, so the design process is tricky, but in the end spherical aberration must be the smallest residual aberration if the telescope is to provide a good image.  See the Optical Designs section on refractors for more details.