Aluminum Coat

Up to last week the reflecting surface of Mount Wilson Observatory's 100-inch glass mirror, world's largest telescope reflector in use, was covered with a thin film of silver. If it were coated with aluminum, as all Mount Wilson astronomers have long wished, it would quickly acquire a protectively oxidized surface, it could be cleaned with ordinary soap and water and it would seldom if ever have to be removed from the telescope tube.

Last week Mount Wilson's big mirror got its first coat of aluminum, a film .00001 in. thick. The method of application was developed by Dr. John Donovan Strong and others from a plating process first hit upon by Thomas Edison. The glass disk is first thoroughly cleaned with blasts of electrons. It is then placed in a big sealed tank from which pumps suck almost all the air. Within the tank is a coil of tungsten wire covered with aluminum. When the wire is electrically heated the aluminum boils off as a vapor which, when it strikes the mirror's cool surface, condenses in a thin, even film.

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