The Mount Wilson Observatory Optical Coating Facility

Astronomical telescopes are housed in large dome structures during the day, but are exposed to the elements every night that they are in used. The result is that the primary telescope mirror of a reflecting telescope will require periodic recoating to ensure that its reflectivity can perform at its peak. The Hooker telescope's 2.5 m (100 inch) diameter telescope mirror was the largest in the world until the Palomar 5m telescope was dedicated in 1948 and by today's standards is not particular large, however, a 100 inch diameter mirror that is 13 inches thick is a significant chunk of glass and weighs nearly 5 tons. Recoating the mirror requires that the mirror be lowered from the telescope deck to the ground floor mirror coating facility and placed on the lower portion of the coating chamber. The picture below shows Sean Hoss, the observatory superintended at the time and Scott Teare who was at Mount Wilson as a research associate with UIUC on the UnISIS laser guide star adaptive optics project. After a 24 hour marathon coating session they are standing behind the newly recoated mirror. Just behind them is the top half of the vacuum bell jar supported by a ceiling crane behind them. The panel on the bell jar is a series of copper knife switches used to apply the current to the heating filaments that deposit the aluminum.

The vacuum coating system was developed by John Strong and the 100 inch mirror was first coated in 1935. The original vacuum system used 5 mercury diffusion pumps to evacuate the bell jar. This was later replaced by a 16” oil diffusion pump which is still in service. To avoid oil back streaming a cold trap was constructed and installed in front of the diffusion pump.

The cold trap used in the 16-inch diameter CVC diffussion pump in the Mount Wilson Observatory Coating Lab. The steel ring was designed at Mount Wilson, turned by the machine shop at the University of Illinois and the copper work built and installed by Associated Vacuum in Covina, CA. The cold trap is shown below.

The vacuum pump used with the bell jar is a high capacity diffusion pump and every bit of that capacity is needed as it connects to a bell jar that is 108" x ~50" in size. That is a lot of volume to bring down to better than 2x10^-5 torr and like any vacuum system of this type does leak.

The cold trap mates up against the diffusion pump flange, and sits between the pump and the bell jar. The idea is for any hot diffusion pump oil to be condensed out of the atmosphere before it can get to the nice piece of clean glass (like the 100-inch mirror) in the bell jar. Due to a harsh winter and poor temperature control, the steel plate of the cold trap rusted to the point it had to be treated with Naval Jelly and Gempler's rust paint before it could be used this year. Moral of the story, don't leave bare metal around!

A number of documents related to this work were generated and are listed here. The expert on optical coating at the Mount Wilson Observatory is Larry Webster.