An Interview with George Ball
George has been a member of the Royal Astronomical Society of Canada since dinosaurs roamed the planet freely. His major interest in astronomy, aside from looking at the sky, is building the equipment he needs to do it better. When George builds his telescopes, he really builds them, from grinding the mirrors and aluminizing his own mirrors, to manufacturing his mount and all the equipment that goes with it.
The observatory in his back yard contains a 12 ½" Schmidt Cassegrain, a 6" Schmidt Cassegrain and a number of auxiliary instruments all mounted on a massive modified German Equatorial Mount. George made the corrector lenses and mirrors for these telescopes. He has a well equipped machine shop in his basement and has also built a grinding machine and his own aluminizing chamber. The aluminizing chamber and the aluminizing process will be the main focus of this article.
George finished making the chamber in 1959. The vacuum chamber is a truncated cone of mild steel mounted on its side. The base is removable and holds the mirror during the aluminizing process. It can hold up to a 12 1/2" mirror. There are four tungsten filaments near the peak of the cone. The filaments are coils of about six half inch diameter turns.These hold the aluminum wire that is vaporized onto the mirror. On the bottom of the chamber are the high voltage conductors that ionize the vaporized aluminum. A layer of aluminum foil is attached to the inside surface of the chamber with magnets. This layer can be discarded every few years (or more frequently) and helps prevent contamination of the chamber with vaporized aluminum. The filaments are replaced after five or six aluminizations because they become coated with the vaporized aluminum and cannot work as efficiently. The chamber is pumped out with a small roughing pump to a rough vacuum. A diffusion pump of Georges making takes the vacuum down into the 10e-6 Torr range where the aluminizing is done.
To aluminize a mirror George first removes the old coating.
Removing the old aluminum coating is done with a solution of potassium
hydroxide. An overcoated mirror makes this task much more difficult.
He has found that the best way to clean the mirror before coating
it is to use a mild powdered soap followed by several rinses with
distilled water. This seems to produce a much tougher coating
than using alcohol or any other chemicals. Once the chamber is
loaded with aluminum, a test slide which was cleaned at the same
time as the mirror, is coated. The test slide is then checked
to make sure that every thing is OK and then the mirror is aluminized.
A good coating will last several years before it needs to be replaced.
George does not overcoat his mirrors and tries to avoid working
on overcoated mirrors as a general rule because of the difficulty
involved in removing the overcoating.