Telescopes from the Ground Up

Hale hired Alvan Clark and his son, the team of expert telescope builders that had designed the Naval Observatory, to build the telescope. The 40-inch (101-centimeter) lens doublet was placed in a 60-foot-long (18-meter) telescope. The whole thing weighs 20 tons and is balanced so well that it can be positioned with the slightest touch of a hand. However, the telescope is actually controlled by electric motors, which the observer operates. The telescope was renovated in 1969 to allow astronomers to position the telescope more quickly and to allow it to automatically follow objects in the sky. It is still used today.

The floor, designed to move up and down to let the observer reach the eyepiece no matter what the position of the telescope, is also powered by motors. The telescope is surrounded by a dome with shutters that can be opened by hand, although they, too, are powered by motors.

The 40-inch Yerkes refractor would be the last of the great refractors. The lenses had grown as large as they could. A bigger, thicker lens would either sag under its own weight or absorb too much of the light it collected.

Although refracting telescopes remained in use, for future advances, astronomers would turn to the refractor’s nearly constant competitor — the reflecting telescope.

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Great Refractors