Hamburger Sternwarte - Buildings & Telescopes: Mirror coating facilities
With the construction of the
Hamburg Schmidt mirror,
mirror coating facilities were installed by Leybold in the same building
(today the
Oskar Luehning telescope
is in the building).
In the facilities both the Schmidt mirror, with a diameter of
123 cm, and the
1m Reflector Telescope
were coated. The facilities were also used for
the coating of the Oskar Luehning telescope, which has a diameter of 120 cm.
In 1987/1988 the facilities were overhauled with a grant from the
DFG (German research council).
Specifically, the vacuum pumps had to be replaced.
The largest mirror coated with aluminum was the
132 cm mirror of the
University of Crete in Heraklion.
Additionally the facilities were used intensively in the
BACH experiment.
Not only can mirrors with glass bodies be coated, but also aluminum mirrors can
be coated with highly reflecive aluminum. This ability is used frequently by the
Insitute for Atmospheric Physics.
Telescope mirrors normally consist of a glass body, which is precisely polished
parabolically or spherically on one side. On this glass body a layer of highly
reflective aluminum is then applied. In a vacuum chamber
with a diameter of 153 cm a pressure of 5·10 Torr
(60 billionths the normal pressure of air)
is produced. Inside is the glass body
with the polished surface facing upwards. When a coating is replaced, the old
aluminum layer is removed first using a caustic solution.
With each coating approximately 1 ccm=2.7 gram
aluminum is then heated to a temperature of 2500° C and evaporated within approx. 2 minutes.
The thickness of the highly reflective aluminum layer formed on the
glass body
thereby amounts to about 0.5 thousandths of a millimeter. With mirrors, which have
a diameter smaller than 1m, a silicon layer
can later be applied. It protects the mirror against
small scratches and environmental influences.
Even today the facilities are in operation.
Other than the Hamburg facilities, the only other comparable facilities in Germany are
commerical facilities run by Zeiss in Jena. Many German observatories therefore
let their mirrors be recoated in Hamburg. There are regular requests from
institutes outside of Germany as well, such as from Denmark.
For amateurs and other organizations telescope mirrors can be coated as
well, at cost (contact 040/42891 4112).
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