Archenhold Observatory, Astronomical observatory in Alt-Treptow, Berlin, Germany

Archenhold Observatory is a facility in Berlin-Treptow housing multiple telescopes for studying the sky. Its main refracting telescope stretches 21 meters long and ranks among the world's longest instruments of this type.

The observatory grew out of a giant telescope built for the 1896 Berlin Industrial Exposition in Treptower Park; the instrument was unfinished at the opening on 1 May and only began operating in September. When no funds remained to move it, the city let it stay, and Friedrich Simon Archenhold turned the site into a public observatory. The wooden building was replaced in 1908 by a classicist structure inaugurated on 4 April 1909. On 2 June 1915 Albert Einstein gave his first public lecture on general relativity in its lecture hall.

The name honours Friedrich Simon Archenhold, who ran the site from 1896 and was its first director; the city renamed it Archenhold-Sternwarte in 1946. The Great Refractor is nicknamed the "Himmelskanone" (Celestial Cannon). A society, the Verein Treptow-Sternwarte e. V., was founded in 1896 with Archenhold as president to run the observatory and its exhibitions.

Visitors can join guided tours and nighttime observations through various telescopes on the grounds. It's wise to check opening times beforehand, as observations depend heavily on clear skies.

The Great Refractor was decommissioned in 1958 and kept as a monument of technology, so it no longer observes the sky. Its lens aperture of 68 cm was stopped down to 58 cm after damage. The 21 m tube, 130 t moving mass, is the longest pointable telescope in the world.