Telescope cleaning at the Stuttgart Observatory
It is possible to make out around 10 billion stars in the night sky using modern large telescopes – not quite so many can be seen from the Stuttgart Uhlandshöhe hill, where the Stuttgart Observatory has stood since 1921. On the hill in the middle of the South German state capital, traffic noise and train station announcements echo faintly in the valley and light from the street lamps and city lights linger in the night sky like a bell over the entire city. This light pollution only makes it possible for the voluntary team at the observatory to see a fraction of the 10 billion stars through their telescopes. But it is not just light pollution that obscures the view of the celestial objects: deposits of fine dust, insects, pollen and weathering have made the large mirror of one of the telescopes so dirty that the stars appear wan and dull.
Ulrich Teufel, engineer and passionate amateur astronomer, loved to gaze at the stars through the attic window of his bedroom as a boy, which is why he joined the society that looks after and maintains the observatory early on. He was no longer able to see through the dirty telescope. The 49-year-old is standing in the sunshine on the plateau of the water container one Tuesday and pulls the 150 kg, rail-mounted telescope out of the metal shelter. "Today we are going to start cleaning the telescope mirror," says Teufel. And what sounds like a simple task proves to be somewhat more challenging.
The heart of the telescope is a 30 kg, 50 cm wide, high-precision mirror. It is attached to the lower end of the telescope and reflects the incident light of the stars onto a second mirror. This directs the view via an eyepiece, then into the eye of the observer. The surface of the large mirror is handled with extreme care so that irregularities only occur in the nanometre range. Every irregularity ultimately leads to an inaccurate image. Such a high-precision surface is naturally sensitive. If you were simply to wipe it with a cloth, the fibres would leave countless tiny scratches. "The image would be even worse than with a dirty mirror," says Teufel. "So it is better to work with a dirty mirror than a poorly cleaned one."
But the dirt has to come off all the same, and given the delicate nature of the mission, it is best left to the professionals. Ulrich Teufel and two of his colleagues in the society have taken a day off for this, as they expect the project to last several hours. Once the telescope has been moved onto the courtyard, they get to work. First of all, the rear cover is unscrewed and the mirror is carefully removed together with its mount. As soon as the mirror has been removed, it becomes clear how heavy the dirt is that has built up over the years. A clearly visible grey film and numerous dark spots cover the surface. And no wonder: after all, the telescope was last cleaned in 2008.
Patience and the right technique
A delicate touch is needed now. The mirror is placed in a large container and the dirt is soaked in distilled water. "Standard tap water would leave lime stains – and that is something we want to avoid," says Teufel. A small amount of cleaning agent that has been specially developed for solar installations is then applied. It is extremely gentle and designed to remove stubborn dirt like pollen, soot and dust. And now patience is needed, as the mixture has to be left to work for two hours.
Meanwhile, Ulrich Teufel gets to work on the telescope tube. One or two spiderwebs, dust and pollen have collected on the inside of the black velour. The material which lines the inside of the telescope also serves to absorb the scattered light in order to ensure a clear view of the stars. Without hesitation, he reaches for the multi-purpose vacuum cleaner. Using the suction brush, he sets about removing the dirt.
Back to the mirror: the dirt is now well soaked and has come off largely without having to touch the sensitive surface. The remnants are removed by gently wiping the surface with a lint-free cloth. The final treatment involves a mixture of alcohols to combat greasy residues.
The team smiles as the heavy mirror is taken out of the container again and they are met with a dazzling sight. A delicate touch is needed once again: using velvet gloves, the delicate piece of equipment is screwed back into its mount and fitted in the tube of the telescope.
Clear view of the stars
The evening has now arrived, the vibrant urban valley has long lain in the shadows of the surrounding hills and nearby the illuminated Stuttgart TV Tower protrudes into the night sky. The team sets to work on the final task of the day: recollimating the telescope.
This involves precisely adjusting the different mirrors of the telescope: the main mirror axis, secondary mirror and focuser are aligned with one another and every tilt eliminated. After around one hour, this task is complete and the stars can once again be observed with clarity. "That was really worth it," conclude the enthusiastic amateur astronomers. All of the school classes and visitors who regularly visit the Uhlandshöhe hill can now get a clear view of at least some of the more than 10 billion stars. And Ulrich Teufel can now see his favourite constellation, Orion, again.