This chapter explains special cleaning processes that are useful if classical methods do not work in unique situations (e.g., an unconventional or irreplaceable optic, limited access, or an unusual contaminant).
Eight methods are presented, many of which have been developed
and improved upon over time. For example, the drag method was
developed to clean laser optics where high-quality surface finishes must be maintained to reduce scatter. Some require tooling, specialized equipment, and special materials. The reference section has a list of white papers and books that provide additional information on each method, along with various equipment and material suppliers.
For decades, strippable materials have been used for cleaning or
to protect optical surfaces. Diethyl ether (collodion) is one type of
strippable coating. It was developed in the 1850s for use on
photographic plates. Amateur astronomers have used it to clean
their telescope optics (mirrors and lenses). However, collodion is
no longer used as a solvent because it has a low flash point and
leaves a residue on surfaces.10 In the early 1980s, a polymer coating
with alcohol solvent was developed, and further improvements since
then have produced strippable coating materials that are free of
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