You are here: Home USNO About Us The USNO 12-inch Refractor

The USNO 12-inch Refractor


The USNO 12-inch refracting telescope is located on the grounds of the Observatory at Washington, DC and is included as part of the Monday night tour when skies are clear.

The telescope was made by George Saegmüller of Washington, DC in 1892 and fitted with a 12-inch f/15 objective lens made by the firm of Alvan Clark & Sons, Cambridge, MA in 1895, when it was installed in Building 1 atop a five-story masonry pier. It was used extensively in the early 20th Century for double star measurements and astrometry of planetary satellites and asteroids. Several asteroids were discovered by astronomer George Peters with this venerable telescope.

In 1957 the telescope was removed from the dome and replaced by a specialized camera which recorded the precise position of the Moon with respect to the background stars. These data were used to refine the Moon's orbit for the first lunar exploratory missions, culminating with the Apollo program. They were also used to define the relationship between the time-scale measured by the Earth's rotation and that measured by atomic clocks.

From 1962 until 1971 the 12-inch telescope was housed in the dome that now holds the 24-inch reflector, where it continued its work on double stars. When the 24-inch telescope was installed in 1971, the 12-inch was dismantled and its various parts stored at several locations on the grounds of the Observatory.

In 1980, with the end of the lunar program, the 12-inch was re-mounted in its original dome through the efforts of USNO astronomers Ted Rafferty and Rich Schmidt. Over 700 hours of volunteer time was required to restore the 2000-pound instrument. It still offers some of the finest visual views of the Moon, planets, double stars, and brighter Messier objects to be found at any publicly-accessible observatory.

USNO Master Clock Time
Javascript must be Enabled