USNO Great Equatorial telescope. United States Naval Observatory (USNO) 26-inch (66 centimetre) Great Equatorial refracting telescope on its new mount after moving to the Observatory at Washington DC, USA. The move was made in 1893. The new mounting incorporates a floor that raises 3.6 metres to allow access to the eyepiece. In 1877 this telescope was used to discover Mars' two moons Phobos and Deimos. It is still in use today, measuring the parameters of double stars.