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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 residues.
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