150th Anniversary of an Historic Telescope

12 November 2023 commemorates the sesquicentennial of first light for the 26” Great Refractor of the U.S. Naval Observatory (USNO). At the time, and for the next seven years, this was the largest telescope in the world. The Navy needed a large telescope to determine positions of the moons of the outer planets, which would allow the planets masses to be determined. Those masses were important for accurate predictions for the position of Jupiter, an important body frequently used in celestial navigation. A few years after first light, Asaph Hall, in August 1877 discovered the two moons of Mars: Phobos and Deimos. Solar System work continued but slowly decreased into the 1960s with the most recent observation taken in 2003.

Since First Light the other major observing program of the 26” is double stars. In addition to his success observing Solar System objects, Hall was also a prolific observer of double stars and while there were other successful observers of double stars, the program began its greatest period of productivity in the 1960s when two programs began regular operation. Charles Worley began his filar micrometry program observing on close pairs. Also, the eyepiece end of the telescope was modified to allow for a “Hertzsprung Style” photographic double star camera due to the efforts of Kai Strand, Jerry Josties and others for precise measurement of wide pairs. The photographic program continued into the 1980s before giving all double star time to micrometry. In the early 1990s Worley abandoned micrometry for the relatively new technique of speckle interferometry. The speckle program continues to this day and as a result of these dedicated programs conducted over a long time the 26” telescope has the most double star observations of any telescope.