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The USNO 12-inch refracting telescope is located on the west end "tower" of the Observatory's historic Building 1 in Washington, DC.  
The telescope mounting was made by George Saegmüller of Washington, DC in 1892 and originally fitted with the 9.6-inch Merz lens from the Observatory's first large telescope, installed at the Foggy Bottom site in 1844.  It was finally 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 clockwork 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 1952 the telescope began dedicated observations of the Moon with a "dual-rate Moon camera" designed by USNO astronomer William Markowitz.  Several of these cameras were deployed to other observatories to record 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.  In 1957 a new, standardized 8-inch telescope was developed to provide consistency in observations from the Moon camera program, forcing the relocation of the venerable 12-inch.
 
From 1957 until 1967 the 12-inch telescope was relocated to the dome that had previously housed 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 parts stored at various locations on the grounds of the Observatory.
 
In 1980, with the end of the Moon camera 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 observatory.  It is used by staff members for their personal research programs and recreational observing.
 
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