Telescope: Naval Observatory 26-inch Refractor

A well-crafted refracting telescope sets a record

Get to the root of it

What makes a good telescope?
The lens doublet corrects spherical aberrationinteractive
The lens doublet corrects chromatic aberrationinteractive

In 1870, the federal government gave the head of the United States Naval Observatory $50,000 to build itself the best telescope it could. The observatory director turned to Alvan Clark and his son, Alvan Graham Clark, of Massachusetts.

The pair had earned their reputation as expert telescope makers by grinding lenses powerful enough to resolve close double stars, and by significantly improving the lenses of other telescopes. The Clarks were the first Americans to grind lenses of the same quality as telescope-makers in Europe, becoming famous for their work after noted British astronomer William Rutter Dawes began using their instruments.

Moving on

The Clarks built the Naval Observatory an achromatic refractor with a 26-inch (66-centimeter) lens doublet in 1873. It was located on a hill north of the Lincoln Memorial, along the Potomac River, in Washington, DC. This 40-foot-long telescope would be the biggest refracting telescope in the world for a decade.

The observatory was eventually moved to a new location, outside the old city limits of Washington, because of the foggy conditions around the original site. The telescope was remounted in a new dome with a rising floor that brings viewers up to the eyepiece, like a giant elevator.

Picture-taking technology has been added to the telescope to bring it up to date, and it is still used today, mainly for observing double stars and planetary satellites.