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The USNO's 26-inch "Great Equatorial" telescope, as seen in 1873The U.S. Naval Observatory's 26-inch "Great Equatorial" telescope
On November 12, 1873 a small group of U.S. Naval Observatory astronomers first peered through the eyepiece of the world’s largest refractor telescope, the 26-inch (66-cm) aperture “Great Equatorial” designed and built by the firm of Alvan Clark & Sons of Cambridge, Massachusetts.  Less than four years later the great telescope revealed the two moons of Mars, Phobos and Deimos, to astronomer Asaph Hall, a discovery ranked as one of the most important astronomical events of the 19th Century.
In 1893, hampered by the unfavorable climate of the Observatory’s original location in Washington’s Foggy Bottom district, the telescope was relocated to the Observatory’s new site in the clearer air of upper Georgetown.  With this move, the great telescope’s lens was re-mounted in a new tube and equatorial mounting built by the Warner & Swasey Company.  For much of the ensuing 100 years the telescope was used to measure the properties of double stars and the faint moons of the outer planets.  Photographs of these moons provided precise information on their orbits and physical characteristics.  These data were incorporated into the annual editions of the American Ephemeris and Nautical Almanac (re-named The Astronomical Almanac in 1981).
Over the years the Great Equatorial has undergone a number of improvements; its mechanical motors and controls have been replaced by electrical components, its visual micrometers replaced with cameras and spectrographs.  Despite the instrument’s advancing age, the Observatory has found a useful purpose for its continued operation, and now, well into its second century of use, it has been adapted to operate as a state-of-the-art 21st Century telescope.  Newly equipped with a remote computer-controlled pointing system and a specialized instrument called a Speckle Interferometer, the telescope is used to make precise measurements of the astrometric properties of double stars.  Understanding the motions of these systems through space is necessary for precise tracking by optical guidance systems on space-based assets that cannot rely on GPS for positioning.  In addition, important astrophysical data on the nature of the component stars can be derived from long-term observations.  The 26-inch telescope is a primary source of the data compiled in the
Washington Double Star Catalog (WDS), which is updated continuously with the latest observations made with the grand old telescope.  WDS is the most comprehensive catalog of double stars in the world, with information on over 150,000 systems.

The "Great Equatorial" will celebrate its sesquicentennial in November, 2023.  Here is an article on the history of the telescope from the February, 2023 issue of "Astronomy" magazine.

Dr. Rachel Matson of the USNO's Celestial Reference Frames Department gives a brief overview of Speckle Interferometry.


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