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Chromatic Aberration

by Michael and Diane Porter

A glass lens works by bending light. This phenomenon is called refraction. But there's an inherent problem with color and refraction. Glass bends each color in a beam of light to a different degree. This is why a prism will spread a ray of sunshine so that it becomes a rainbow on the wall: each color in the sunlight comes out of the prism at a slightly different angle.

The same thing happens with a telescope lens. If a telescope's image were focused by a single ordinary lens, no two colors in the image would be in focus at the same time. The red feathers of an elegant trogon might be sharp, but all the green feathers would be out of focus.

The problem is called chromatic aberration, and it affects all lenses. Understanding how telescope designers try to correct chromatic aberration will help you understand what you're getting when you're buying a scope.

The basic fix for chromatic aberration is to use a double lens. Two lenses, cemented together, each made from a different kind of glass, can make the chromatic errors tend to cancel each other out. The correction is not perfect, but it's a great improvement. The resulting two-part, color-corrected lens is called an achromatic doublet.

Another important development is the use of exotic materials, such as fluorite crystal, ED glass, or SD glass, in the objective lens. These are low-dispersion materials, which don't spread the color spectrum so much as ordinary glass. They create less chromatic aberration to begin with. They are costly, but they can make a difference at high magnifications.

Copyright 2009 Michael and Diane Porter

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