USNO Great Equatorial telescope. United States Naval Observatory (USNO) 26-inch (66 centimetre) Great Equatorial refracting telescope on its original mount at the Naval Observatory in Foggy Bottom, Washington DC, USA. In 1877 this telescope was used to discover Mars' two moons Phobos and Deimos. In 1893 the telescope was moved to the new USNO observatory on Massachusetts Avenue, Washington DC. It is still in use today, measuring the parameters of double stars.