The reconstruction, testing and improvement of the historical giant astronomical telescope in Lilienthal

Wolfgang Osten1,2* , Christof Pruss2, Alois Herkommer2
1Telescopium GmbH, Auf der Koppel 25, 28865 Lilienthal, Germany
2Institut für Technische Optik, Universität Stuttgart, Pfaffenwaldring 9, 70569 Stuttgart, Germany

* Corresponding author: wo@ito.uni-stuttgart.de

Abstract:

Around 1800, the small German city Lilienthal housed the best telescopes of the time and became the founding place of the world's first astronomical society in which scientists from 24 European observatories together patternd the firmament according to the lack of a planet between Mars and Jupiter. Lilienthal and his observatories were already in the late 18th and early 19th century sites of the encounter and the scientific exchange of numerous high-ranking scholars such as Bessel, Gauss, Olbers, and Harding. The central instrument was the 27 foot Giant Telescope where important discoveries were made. However, this first scientific institution of European format was to fall victim to a fire in 1813, as a result of which almost the entire scientific heritage of the founder, Johann Hieronymus Schroeter, was destroyed. The detailed reconstruction of the Schroeter Giant Telescope was only carried out in 2015 through exclusively private commitment from Lilienthal citizens and donors from Hamburg and Bremen. One of the most important contributions of the group around Schroeter was the development and implementation of a new technology for the fabrication of telescope mirrors. In contrast to those made by Herschel, these mirrors are characterized by a high surface quality, the reflective shine of which did not fade even after decades of use. In this paper we show a comparison between the image quality of the original spherical mirror made from a copper-tin-arsenic alloy with an aperture of 50 cm and a focal length of 27 foot that was fabricated in 1793 and a new mirror made from Schott borosilicate glass Supremax 33 with the same design data. Furthermore we show some optical concepts how to mimic and compare the historical image quality to the state of the art and to improve the imaging properties of the modern telescope.