Telescopes from the Ground Up

Get to the root of it

A matter of size

Remember, astronomers’ design goals had changed with improvements in the reflecting telescope. The point was to build larger mirrors. The mirror size determined how powerful the telescope would be. The length of the telescope just depended on the size of the mirror.

William’s first telescope had a mirror with a diameter of 6 inches (15 centimeters). It was 7 feet (2.1 meters) long and magnified 40 times, enough to clearly see Saturn’s rings.

But William knew he could build a bigger mirror, and followed that with a telescope with a mirror diameter of 9 inches (22.5 centimeters). As the mirror size increased, so did the length of the telescope, to 10 feet long.

He moved on to a 20-foot-long (6-meter) telescope with a mirror diameter of 18 inches (45 centimeters), and then tried to craft a mirror for a 30-foot-long telescope in a furnace he built in his basement. The first mirror cracked, and the second attempt sent molten metal flowing over the stone floor, blowing up the flagstones and causing workmen to run for their lives.

But even that experience wasn’t enough to stop him. Herschel’s crowning achievement was a telescope with a mirror diameter of 48 inches (122 centimeters). The telescope was 40 feet (12.2 meters) long.

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Early Reflectors


Portrait of William Herschel.
William Herschel
A family of musicans gives rise to a family of astronomers.
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