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What is Spherical Aberration in a Telescope?

Spherical aberration is a common optical aberration that occurs when different rays of light reflect off of a spherical surface and do not all meet at the same image point. As a result, the edges of your image may appear stretched or blurred.

What Causes Spherical Aberration?

Spherical aberration occurs when your telescope focuses light at different points on a mirror. This causes the image to focus on different focal planes. Therefore, the center will be focused furthest, distorting the image as you near the edges. In reflector telescopes, spherical aberration is caused by hyperbolic or ellipsoidal mirrors. In catadioptrics it’s caused by the mirror deviating from its ideal shape. In refractors it’s caused by the curvature of the objective lens.

Which Types of Telescopes Experience Spherical Aberration?

Spherical aberration is most prevalent in refractor and catadioptric telescopes because they use rounded lenses. Most modern reflector telescopes use the Ritchey–Chrétien design, which eliminates this aberration. Other telescopic designs, like the Newtonian telescope, employ a single, parabolic lens to remove any spherical aberration.

How to Fix Spherical Aberration?

The easiest way to resolve this aberration is by using an aspherical lens. This is because spherical aberration only presents itself in rounded surfaces. You can also minimize this optical error by employing additional glass elements, like doublets or triplets. These allow you to adjust the thickness and curvature of your lens.

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