A well-known problem for space researchers is polymerization of volatile hydrocarbons onto optical surfaces. Strong ultraviolet radiation from the Sun causes the formation of films that are remarkably opaque to short-wavelength radiation. These films have a brownish golden color in transmission. For spaceborne solar instrumentation it is necessary to take heroic measures to avoid forming these films. The same problem also affects the vacuum optics of the KPVT.
For the first several years of operation of the KPVT, there was no film problem. Then an experiment was run that involved filling the vacuum tank with helium. Evidently this helium was contaminated with hydrocarbons because, shortly after the experiment, nasty films formed on the optics. It also did not help that the vacuum pump system malfunctioned, resulting in vacuum pump oil being sucked into the tank. Since these events, it has been necessary to open the tank and clean the optics rather frequently. Attempts to clean the walls of the tank have not helped the problem.
In an effort to reduce these labor-intensive cleanings, we plan to fight the problem by reducing the amount of ultraviolet light that enters the vacuum tank. Currently, two aluminized coelostat mirrors feed sunlight through a fused silica vacuum window. The unfettered flow of ultraviolet light could be reduced at any of these elements. The simplest solution seems to be to coat the second mirror with gold. This will reduce the ultraviolet light by two-thirds and should extend the life of the vacuum optics accordingly. We did not want to remove ultraviolet light completely because an instrument is operated from the light feed that uses the Calcium K-line at 393 nm. A bonus will be more light at 868 and 1083 nm where nearly all of the KPVT observations are made. The gold coating may also prove to be more durable than aluminum, and may also produce less mirror heating.
Jack Harvey, Jeremy Wagner