Is there an oxide layer on aluminum reflectors for telescopes? I was watching a Nottingham Science video linked here (periodic videos) and they mention modern reflectors are made of aluminum. 

I can understand why mirrors are preferred over lenses, but what inhibits the aluminum oxide coating that would naturally form on the surface of the mirror? Why do they work so well?
The aluminum oxide layer forms as soon as oxygen comes in contact with the mirror surface.

Aluminum oxide is tough, firmly attached and transparent;  it serves to protect the aluminum surface.  Rubies and sapphires are slightly impure aluminum oxide