6. Aluminization

A. Once the mirror is done, you need to put a reflective coating on it. We have a device that does that: a military- and university-surplus vacuum chamber and aluminizer that we have been keeping alive for about 30 years or so, shown below. Briefly, this is what we do: 

B. We clean the mirror very, very thoroughly.

C. We put it into the vacuum bell jar in a special jig that holds it at the top, facing downwards;

D. Pump out almost all of the air with a mechanical pump. This gets the pressure from 760 mm of mercury (normal pressure, 760 Torr) down to about 0.1 mm of mercury (0.1 Torr).

E. While pumping, we zap the mirror surface with a high-voltage A/C plasma which further cleans and prepares the glass to accept the aluminum coating. This looks like the Aurora Borealis.

F. We then turn on the diffusion pump, which works by somehow capturing individual molecule of air in a condensing mist of expensive silicone oil that was boiled at very low pressure. It sounds like magic, but it works to get the pressure down from 0.1 mm of mercury all the way to 0.00008 mm of mercury, which is what we need for the last step.

G. Then we slowly heat up a special tungsten filament coil that has a slug of pure aluminum wire inside the coil. The coil melts the aluminum, which coats the tungsten coil and then boils off into the vacuum, in all directions. A small part of that aluminum vapor hits the mirror, and sticks to it in a very thin layer.

H. We then close down the diffusion pump, close and open various valves, and wait for the air inside the chamber to equalize with the air outside, so we can open up the chamber and remove your mirror.

I. You should be very gentle with the coating, especially at first. Resist the urge to wipe it clean if dust gets on it. It will be about 85-90% reflective, and will stay that way for years. If there were any nearly-invisible scratches left on your mirror after polishing and figuring, they will be highlighted by the shiny aluminum layer. They won’t have any optical effect, but they will make you feel bad. Keep the mirror protected from fingers, sneezes, dandruff, snot, and dust. Don’t let anything touch the surface, even soft cloths!

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