Australian Astronomical Observatory, Optical astronomy observatory in North Ryde, Australia.

The Australian Astronomical Observatory is a research facility perched on a mountain plateau at Siding Spring with two large telescopes dedicated to observing the night sky in visible light. The site houses the Anglo-Australian Telescope at 3.9 meters across and the UK Schmidt Telescope at 1.2 meters, which together enable thousands of observation nights each year.

The facility emerged from a partnership between British and Australian scientists in the 1960s and was completed in 1974. This international collaboration shaped optical astronomy research in the southern hemisphere for decades to follow.

The facility maintains strong connections with Macquarie University and the Australian National University, fostering research in astronomical sciences through educational partnerships.

The headquarters is in North Ryde, suburban Sydney, while the two telescopes stand at Siding Spring Observatory near Coonabarabran, New South Wales. The AAT building is 26 m high with seven floors of offices, labs and a mirror aluminising chamber. The telescope rests on a concrete pier with its own foundation, separate from the building, to reduce vibration.

The 3.9-metre primary mirror blank was cast in April 1969 from zero-expansion Cervit glass by Owens-Illinois in the USA, then shipped to Newcastle-upon-Tyne to be figured and polished by Grubb Parsons. The finished mirror measures 3.9 m across with a focal length of 12.7 m.