Does Aluminum Coating on Telescope Mirrors Turn into Aluminum Oxide?

[anorlunda]

I saw on Nova thst large telescope mirrors have aluminum reflective coatings. Wouldn't that rapidly become an aluminum oxide costing?

Are the coatings coated to keep oxygen out?

 

[Drakkith]

Yes, when aluminum corrodes it forms a layer of aluminum oxide on the surface that protects the rest of the underlying aluminum from corroding. Aluminum oxide is transparent to visible light, so it makes an excellent coating for telescope mirrors.

 

[davenn]

anorlunda

pretty much all mirrors on reflector telescopes are aluminised even small scopes used for amateur use

I have 3 reflecting scopes with mirrors of 6", 8' and 9.25 " They all have an aluminium coating and then other coatings on top of that to protect the aluminium coating
and that is pretty much the standard

cheers
Dave

 

[Chronos]

Aluminum coatings are usually overcoated with silicon monoxide, or something similar. This is a good idea for a couple of reasons - increased reflectivity and improved coating life.