The Hamburg Observatory offers aluminium coating services for mirrors up to a diameter of 150cm.
The vaporization plant
With the construction of the Large Hamburg Schmidt-Mirror a vaporization plant by Leybold was installed in the same building (today the building houses the Oskar-Lühning-telescope). The spacious plant can service Schmidt mirrors up to 123cm in diameter and parabolic mirrors up to 100cm. Thus it was suited for the later constructed 120cm Oskar-Lühning telescope. 1987/1988 the plant was refurbished with funds from DFG. In particular the vacuum pumps needed replacement. The thus far largest serviced item was the 132cm mirror of the Skinakas-Observatory of the University of Crete.
Telescope mirrors are typically made of glass that is highly precisely polished into a parabolic or spherical shape. On this glass foundation a highly reflective aluminium coat is applied. This involves a vaccuum bell jar of 153cm diameter that is evacuated to a pressure of 7·10-5 mbar (one fifteen millionth of normal air pressure with the glas bodies inside. In case of a re-coating process the old layer needs to be removed with lye, first. Each coating involves the vaporization of about 1 cm3 = 2,7 grams of aluminium at 2500° C within approximately 2 minutes. The layer of highly reflecting (88%) aluminium condensing on the glass reaches a thickness of roughly 50-100 nanometers.