Optical coatings

Does anyone know what kind of optical coatings are used for metal mirrors in optical applications to prevent the surface from oxidizing and that also does not degrade the reflectance of the aluminum itself?

I am guessing the aluminum would have to be cut to shape then placed in a vacuum to repolish the surface and apply the coating. What is used to "polish" the oxide coating off the aluminum without messing up the surface and the shape while it is under vacuum?
 
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A6061 is typically used for mirrors. It is ok without coatings because it produce naturally very thin and transparent oxide coating just on contact with air.
 
A6061 is typically used for mirrors. It is ok without coatings because it produce naturally very thin and transparent oxide coating just on contact with air.
Is reflectance better if it is coated though or is the difference negligible?
 
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Is reflectance better if it is coated though or is the difference negligible?
negligible, as typical for very thin coatings.
 

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