The refractor is an achromatic two-lens telescope (free refractor diameter 244 mm, focal length 4320 mm) and has a parallactic mounting to compensate for the rotation of the earth during observations: one of the two axes of rotation ist oriented parallel to the earth’s axis of rotation; the device is equipped with a weight-driven tracking mechanism.
Joseph von Fraunhofer (1787-1826) began developement of the large and powerful refracting telescopes on a parallactic mounting in 1818. Fraunhofer’s refractors were unequalled both in the size and lens quality, as well as in the precision of the mechanism’s mounting.
At the instigation of Alexander von Humboldt (1791-1855), the new Berlin Royal Observatory acquired this second large refractor from the Fraunhofer Workshops in 1829, three years after Fraunhofer’s death. Fraunhofer had deliverd the first refractor to the Imperial Russian Observatory at Dorpat (now Tartu in Estonia) in 1824. Using the telescope in Berlin, Johann Gottfried Galle (1812-1910) suceeded in discovering the Planet Neptune in 1846 on the basis of theoretical calculations of its position made by French astronomer Jean Joseph Le Verrier (1811-1877). Le Verrier had postulated the existence of an as yet undiscovered planet and calculated its position base on irregularities in the orbit of the planet Uranus.