Keeping Optics clean

[pmrid (Peter)]

One for the observatory-owners I suppose. With my permanentkly mounted scopes, over a surprisingly short time, I am seeing quite a buildup of a dusty film on the primary and secondary on my Newt and on the objective on my refractor. Over winter we've had quite a bit of fog but I tried to counter that by running an oil-heater in the obs between 10 pm and dawn every night. But seemingly, to no avail.
How do others protect theor optical surfaces from the dreaded dust and other films and so forth? Keeing dust caps on obviously, but that doesn't seem to have made much difference to my scopes at least.
ANy thoughts?
Peter

[DavidU (Dave)]

I used to find that I could rotate my newt (GEM mount) upside down, so the mirror is up. I used to cover the secondary with a cloth bag that came with a fishing reel. This helped quite a bit.
My refractor always has the lens caps on when not in use, the objective cap has dessicant bags taped to the inside (so I can peel them off and re dry them in a warm oven every so often).
Probably the best (but most expensive)for an observatory would be to have a smallish fan with a very good filter drawing filtered air into the Ob 24/7. This way there is always a slight but positive air pressure inside the Ob that naturally will keep dusty air out.