Telescopes from the Ground Up

A well-crafted refracting telescope sets a record

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.

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Great Refractors
Map showing the location of the Naval Observatory in Washington, D.C., USA.
Image of th 26 inche refractor located at the Naval Observatory.Enlarge picture
The Naval Observatory 26-inch Refractor
Year completed: 1873
Telescope type: Refractor
Light collector: Glass lenses
Lens diameter: 26 inches
(66 cm)
Light observed: Visible
Discovery Highlights:
  • While being used to study the motion of the planets, discovered the two moons of Mars, Phobos and Deimos.