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Telescopes from the Ground Up
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The achromatic refractor
Diagram demonstrating how combining a convex with a concave lense  removes chromatic aberration.

The Great Refractors took Kepler’s basic telescope design and used a lens doublet as the primary lens, resulting in the achromatic refractor.

A lens doublet consists of a pair of lenses of two different shapes and two different kinds of glass. Used together, the two different shapes and types of glass bring the light rays to a single focal point, eliminating both spherical aberration and chromatic aberration.

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