The Anglo-Australian Telescope (AAT) is a 3.9-metre equatorially mounted telescope operated by the Australian Astronomical Observatory and situated at the Siding Spring Observatory, Australia at an altitude of a little over 1,100 m. In 2009, the telescope was ranked as the fifth highest-impact of the world's optical telescopes. In 2001–2003, it was considered the most scientifically productive 4 metre-class optical telescope in the world based on scientific publications using data from the telescope.