The Archenhold Observatory (German: Archenhold-Sternwarte) was named in honor of Friedrich Simon Archenhold, an observatory in Berlin-Treptow. It houses the Großer Refraktor (Great Refractor), which is the longest pointable telescope in the world. It is also called the Himmelskanone (Celestial Cannon).
The telescope has an aperture of 68 cm, a focal length of 21 meters, and a movable mass of 130 metric tons. Note that the telescope and its unique mount are not protected by a dome. Even a hundred-plus years after its construction, the “Himmelskanone” is still the longest (by length, not by aperture) pointable telescope in the world.
The Archenhold Observatory began as a temporary installation at the 1896 Great Industrial Exposition of Berlin. At the initiative of Wilhelm Foerster and Max Wilhelm Meyer, it was intended to present scientific and astronomical knowledge to the general public. When the industrial exposition opened on 1 May 1896, the Great Refractor was still unfinished; it began operation in September 1896.
Archenhold Observatory was operated by a society, the Verein Treptow-Sternwarte e. V. with Archenhold as president. This is now considered the oldest and largest public observatory in Germany. In 1908, the original wooden building was demolished and on 4 April 1909, a new building was inaugurated.
According to the Wikipedia