The USNO 26-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 cloudy.
This telescope has a rich history. Completed in 1873 at a cost of $50,000, it was the largest refracting telescope in the world for a decade. The lens and mounting were made by the renowned firm of Alvan Clark & Sons of Cambridgeport, MA, and the great telescope was duly erected on the grounds of the old Naval Observatory site in the Foggy Bottom section of Washington.
It was from this site, in August of 1877, that astronomer Asaph Hall discovered the two moons of Mars, Phobos and Deimos, with the "Great Equatorial Telescope", bringing the attention of the world to the USNO.
The move to the Observatory's present site in 1893 allowed the 26-inch lens to be re-mounted in a new dome with a new mounting designed by the Warner & Swasey Company of Cleveland, OH. This design incorporated a rising floor to facilitate access to the eyepiece. This floor is still the largest elevator in the city!
Today, the telescope is used on every clear night to measure the parameters of double stars. Over the years, visual observations by astronomers using micrometers have ben replaced by electronic imaging techniques. By taking very short exposures with a Charge-Coupled Device (CCD) camera, astronomers can actually use the blurring effect of Earth's atmosphere to their advantage to measure the separations and position angles of double star components. The technique, known as "speckle interferometry" is ideally suited to the 125 year-old optics of the great telescope, and relatively unaffected by the urban location of the Observatory. Several thousand stars are measured annually, and the database of such observations, added to the visual observations dating back over a century, provide for one of the most concise double star catalogs in the world.
The telescope is also used to measure the positions of the moons of the outer planets to help refine their orbital parameters. These data are vital in planning missions to such distant worlds.