Here in AZ I have coatings that are over 10yrs old and still in good shape! My 10.5-inch was coated with the 4-Meter Mayall mirror (Kitt Peak) in 1983 and except for a thumbprint on the edge, is still usable with few pinholes. I have a couple of other mirrors with the Beryl coatings that are even older!
> 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.
>
If its dry it will probably be O.K. What really does in a coating is
particulate urban pollutants landing on the mirror and having that
subjected to a humid environment. Cold/dry is the best environment
but hot dry has worked here in Tucson.
> How does an SiO2-overcoating affect the situation?
>
It helps to keep things from attacking the aluminum directly. The 10.5-inch mentioned above is NOT overcoated and in our typically 0-20% humidity is still going.
> 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...?
Heck, I could believe anything. But in order to retain the optical qualities of the primary you have to strip off the old coating.
-Rik