Clear Night Skies: Cleaning the Telescope Mirror at Stellarium Gornergrat
Even a telescope mirror has to be cleaned – once it reaches a certain level of dirtiness, the main mirror needs a bath: On Gornergrat, local weather events (e.g., Saharan dust), condensation, and dripping from the interior of the dome, among other things, cloud the mirror’s shine. Easier said than done, because this task requires separating the mirror cell from the telescope structure – a delicate undertaking. With combined forces, we mastered this exciting task.
Pack the bags and off to Gornergrat!
During the preparations for the cleaning operation, we were able to rely on the expertise of Beat Hiltbrunner, the recently retired workshop manager at the Astronomical Institute of the University of Bern (AIUB, https://www.aiub.unibe.ch/index_ger.html). Peter Schlatter, also a former AIUB employee and telescope specialist, provided us with support over the phone. We traveled to the Gornergrat with pure gas (for blowing off dust and drying the mirror surface after wet cleaning), de-ionized water (which evaporates without leaving any residue) kindly provided by the Space (https://www.space.unibe.ch/) department of the University of Bern, and everyday aids such as dishwashing detergent, sterile cotton, and household paper.
The preliminary work is challenging
The real challenge arose right at the start: how could we detach the mirror cell from the rest of the telescope while ensuring that it (and we ourselves!) remained undamaged? We also wanted to prevent any interference with the telescope’s fine adjustments. After a few detours, we were able to solve this problem. The procedure was documented on video to simplify the work next time.
Let’s get cleaning!
Once exposed, the mirror is carefully cleaned. First, high-purity water is sprayed onto the mirror, which is then blown dry with gas. We dab away stubborn dirt with cotton balls soaked in a solution of water and dishwashing detergent. With a lot of patience, we achieve impressive results after several cycles.
Deconstruction is followed by reconstruction
Thanks to the experience gained at the beginning, reconnecting the mirror cell to the structure is easier than the reverse step. Using the existing lifting device and a few tensioning cables, we attach the mirror cell to the housing at the correct angle.
Waiting anxiously for the proof of concept
All’s well that ends well? We complete our project, knowing that only the next clear night will reveal the success or failure of our efforts. And lo and behold: the images from the depths of space are of outstanding quality.
The images were taken on Gornergrat using the “RiFast telescope” from the Italian company “Officina Stellare” and a “Finger Lakes Instrumentation” CCD camera (16 megapixels) with an “Astrodon” filter wheel, with exposure times between 30 and 185 seconds. The structure at the top left is the Cirrus Nebula, a collection of clouds of gas and dust (the latter consisting of small molecules, e.g., carbon) that emit light themselves or scatter light from neighboring stars. The emission nebula (catalog number NGC 6853) at the top right shows the final stage of a star similar to our sun: A white dwarf remains at the center, causing the ejected stellar envelope to glow. The Trifid Nebula (M 20) at the bottom left, on the other hand, is a star-forming region: parts of a gigantic hydrogen gas cloud are contracting under the influence of gravity. When the pressure and temperature are high enough, nuclear fusion occurs in such a “gas ball” – a star is born. At the bottom right is the globular cluster M 13. In such clusters, ancient stars have gathered as a result of their mutual gravitational attraction and orbit our galaxy together. So take a look to the edge of the Milky Way and beyond from Gornergrat!
Monika Hager, July 2025