Telescopes from the Ground Up

The daily grind

Hadley ground his mirror by trial and error, using a test he developed to figure out how well the mirror was focusing light. Hadley illuminated the mirror in such a way that its surface would light up evenly if the light rays were focusing correctly. If part of the mirror wasn’t evenly lit, he would just grind that part some more.

It was a long and difficult task, but Hadley was determined. His finished telescope, with a mirror 6 inches in diameter, worked almost as well as the 123-foot (37.5-meter)-long refractor designed by Christopher Huygens. Although Hadley’s mirror was only a little smaller than Huygens’ lens, his telescope was just 6 feet (1.8 meters) long.

Hadley’s shorter telescope could be completely enclosed in a metal tube and easily moved to view the sky. Because Huygens’ telescope was so long, it could not be enclosed in a tube, and it often lost images due to nearby light. Also, it was difficult to adjust and maneuver.

Spreading the word

After his initial success, Hadley made several more telescopes and drew up guidelines that would help other astronomers decide how large their mirrors had to be. He didn’t write down his technique for grinding and polishing mirrors, but he taught it to others who spread the word.

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Early Reflectors