I would like to know how long one can expect an ordinary al-coating to last. I have heard people quoting values like 5 to 10 years in normal use (whatever that means).
I have two scopes one of which is stored in an unheated shed. Does anyone know whether this kind of storage shortens the life-expectancy of the al-coating as opposed to indoor storage. I always place the mirror box in a vertical position to avoid moisture condencing on the surface.
How does an SiO2-overcoating affect the situation?
I have heard of overcoatings (aluminum oxide? MgF?) which are supposedly more durable than SiO2 but which are nearly impossible to remove because the usual chemicals (ferrichloride, sodium hydroxide, HCl etc) won't permeate through the layer. I have even heard of people who have had to re-aluminize their mirrors right on top of the old coating(!) Anyone have any background info, or is this just an atm urban legend...?
Jaakko Oksa