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William Herschel's 20-Foot Telescope

Herschel's 20-foot Telescope

In the hands of William Herschel, the telescope became a powerful scientific tool. Herschel had a talent for telescope making that matched his passion for studying the heavens. He began building them in the 1770s. Soon he was creating the most powerful telescopes in the world and looking deeper into the Universe than ever before. The telescope displayed here was Herschel's favorite: his 20-foot reflector.

Woodcut image courtesy Royal Astronomical Society, Burlington House, Picadilly, London

The Telescope Tube

This long wooden tube is from William Herschel's 20-foot telescope. He began observing with the telescope in 1783, and though he went on to build even larger ones, his 20-foot remained his favorite. In 1820 Herschel and his son John rebuilt the telescope, which had deteriorated after decades of use, salvaging what they could from the original. The tube displayed here is a product of that reconstruction.

Herschel constantly worked at improving the telescope, modifying its optics, framework, and controls many times. With it, he and his son discovered thousands of nebulae and star clusters and mapped the entire sky.

Lent by the National Maritime Museum, London
Image Courtesy of the Royal Astronomical Society

Inside 20-foot telescope tube
View inside 20-foot telescope tube

What Was It Like to Use the Telescope?

The telescope's eyepiece was mounted at the top of the tube, so Herschel observed from a platform that could be raised or lowered as needed. Caroline sat inside the house nearby at an open window, recording her brother's observations as he called them down to her. Of his nights at the telescope, Herschel wrote,

"Here an observer may sit for many hours, with constant entertainment, continually expecting new objects to present themselves, which he never could have perceived in common telescopes."

The work wasn't always so pleasant. Herschel observed whenever possible, sometimes even in bitter cold. One night, while using an earlier telescope, the temperature dropped to -12 ºC (11 ºF). His ink froze in its bottle and his best mirror "crack'd into two piece."

The Telescope Mirror
Herschel's Telescope Tube
This mirror was one of several crafted for the 20-foot telescope. Like other early telescope mirrors, it was made of metal (mostly copper and tin) and tarnished quickly, so it had to be repolished often. The base of the telescope could be opened and the mirror easily removed. Another mirror was always on hand to use while the first was being polished.

Lent by the National Maritime Museum, London

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