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COATING PROCESS James uses a form of physical
vapor deposition (PVD) that lays down a uniform thin film layer of aluminum
on a mirror's surface. The process involves the use of a controlled
high vacuum environment.
James offers a bare aluminum alloy coating that in general has a higher
reflectivity than aluminum with an SiO2 overcoat. Our experience has been
that if stored indoors in a climate controlled environment these coatings
can last many years. Of course prolonged exposure to humid environments,
including marine areas will degrade these coatings faster than if they had
an overcoat. Read about aluminum's
resistance to corrosion and properties in general.
For those people interested in higher reflectivity and contrast in their
observations and who don't mind having to recoat their mirrors more often
than with SiO2 overcoats then the bare aluminum coating is a good option.
Aluminum + SiO2 overcoat (not currently offered)
Silicon dioxide (SiO2) overcoats are thin films (< 600nm) of transparent material
that help protect aluminum coatings. Although they are extremely thin, overcoats do
diminish the reflectivity of bare coatings by up to 4%. If you need an
overcoat we recommend you get your coating done at Optical Mechanics.
Stripping off old coatings
There is no charge for stripping off old coatings. There are however some
types of strange coatings out there that can not be stripped. If James can't
coat your mirror because of a strange coating he won't charge you.
Secondary Mirrors
A secondary mirror can be recoated with a primary mirror for an extra $15.
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