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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 mounting 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. The mounting was designed by USNO astronomer Prof. William Harkness and featured "star dials" that allowed the positions of celestial objects to be set from control wheels on the pier.  The mechanical sidereal drive could track at solar, lunar, and sidereal rates.  It was used extensively in the early 20th Century for double star measurements and astrometry of planetary satellites and asteroids.

In 1957 the telescope was removed from the dome and replaced by the Markowitz dual-rate Moon 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 "Ephemeris" time-scale measured by the Earth's orbit around the Sun and that measured by atomic clocks.

From 1957 until 1967 the 12-inch telescope was housed in the dome that once held the 1-meter Ritchey-Chretien reflector (removed to Flagstaff in 1955), where it continued its work on double stars. When a 24-inch Boller & Chivens Cassegrain telescope was installed in the dome in 1967, 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. Sadly, the original "star dials" and control wheels have been lost, but the telescope 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.