The Pluto Discovery Telescope
Built in 1928-1929 expressly for the purpose of completing the search for "Planet X" – the name for the hypothetical ninth planet in the solar system that Percival Lowell thought must exist – the Pluto Discovery Telescope, like the Clark, is one of the most famous telescopes in the history of American astronomical research.
Known as an astrograph (or astrographic camera), the telescope has three lenses, each 13 inches (32.5 cm) in diameter. The light from (or reflected off) a celestial object was focused onto a 14" x 17" (35 cm x 42.5 cm) glass photographic plate. Time exposures of about an hour were made for each image. The glass negatives were then scrutinized using the Zeiss blink comparator, which now resides in the Rotunda Museum.
Reviewing a set of glass negatives on February 18, 1930, Observatory assistant Clyde Tombaugh made the first recognized sighting of the object later named Pluto.
After the discovery of Pluto, Henry Giclas used the telescope for his proper-motion survey, in which he studied the actual (not just apparent) motion of celestial objects. The telescope was later moved to the Observatory's Anderson Mesa dark-sky site and then returned to Mars Hill in the early 1990s so our visitors could see this historic instrument first-hand.
Principal funding for the instrument and dome came from Abbott Lawrence Lowell, Percival's younger brother and then president of Harvard University.