Telescopes from the Ground Up

The convex lens would bring light rays together, whereas the concave lens would spread light rays apart. The combination of different shapes and substances made chromatic aberration disappear: the way one lens splits the colors of light is cancelled out by the way the other lens combines them.

Another Englishman, John Dolland, extended Hall’s technique to create a similar set of lenses. By changing the curve of the lenses and fitting them together, Dolland managed to bend the light enough to cancel out spherical aberration, another major problem in refracting telescopes, as well.

Still, glassmaking techniques couldn’t produce a useful primary lens larger than 4 inches in diameter. Large pieces of glass contained imperfections and bubbles that refracted light unevenly, making them unsuitable for telescopes.

Bigger and better

In the late 1700s and early 1800s, a Swiss artisan, Pierre Louis Guinand, teamed up with a German optician, Joseph von Fraunhofer to work on the process of casting glass for lenses. By adding certain chemicals to the molten glass and inventing new stirring techniques, Guinand and von Fraunhofer were able to create large pieces of glass that contained fewer flaws and refracted light uniformly.

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