University of Hawaii 2.2-meter telescope Primary mirror aluminization

Next time you are in your bathroom, take a close look at the mirror. If you look carefully, you will see that the reflective surface of the mirror is on the back side of the glass, and is protected. If you clean your bathroom mirror, you only touch the front surface of the glass - not the coating itself. If you look closely at reflections from your bathroom mirror, you will see that there are actually two reflections - the main reflection from the reflective surface at the back, and a second, fainter reflection, from the front surface of the glass.

Telescope mirrors are different. The reflective coating is on the front of the glass, so don't get two images of every star we look at. This means that the coating is much more fragile. The coating is exposed to the air, dust, and moisture. It is difficult to clean it without damaging it. All telescopes have to recoat their mirrors every few years.

Most telescopes coat their mirrors with a very thin layer of aluminum. The exceptions are the Gemini Telescopes, which recently coated their mirrors with silver.

The first step in recoating a telescope mirror is removal of the old coating, along with any dirt or other substances attached to the mirror.

In the photograph above, the engineer is using an acidic solution to dissolve the old coating.

After the coating is removed, the mirror must be thoroughly cleaned with pure water, and all dust and any other foreign material removed.

The mirror is then placed in a giant vacuum tank, and aluminum metal is placed on electric filaments inside the tank. The air is sucked out of the tank by a giant vacuum pump, and then the filaments are heated to white hot. The aluminum on the filaments melts, then is vaporized and coats everything inside the tank, including the mirror.

Air is then allowed back into the tank, and the mirror is removed. The mirror coating is then checked. It must be thick enough that light cannot be seen from behind, and technicians search for pinprick sized holes which might be a sign that the mirror wasn't cleaned properly. In the photograph above, an astronomer is checking the adherence of the coating by applying sticky tape to an unused part of the mirror near the center, and seeing whether the coating adheres to the mirror or the tape.

During the coating process shown in the photographs above, the resulting coating was good, and the mirror was put back into the telescope. If the coating is not satisfatory, the coating must be removed, and the entire process repeated.

The unusual shape of the 2.2-meter telescope enclosure is related to the aluminizing process. In most telescopes, the mirror is taken to the aluminizing chamber on a lower floor. At the 2.2-meter telescope, the aluminzing chamber is instead brought to the telescope. The protuberance on the 2.2-meter telescope dome actually houses a crane that is used to hoist the aluminizing chamber up to the observing floor of the telescope.

Copyright 2005, Richard J. Wainscoat