Is the coating on my mirror stuffed?

[DavidTrap (David)]

Hi,

I bought a second hand scope recently and have had problems with the mirror dewing up at the drop of a hat. The scope has fans to help cool the mirror and I wondered if these fans are cooling the mirror below the dew point.

The mirror was rather dusty and I have also pondered if the dust was acting as a nidus to facilitate the dew forming.

Today, I decided to clean the mirror. When rinsing the mirror, I noticed that rather than the water forming 2-3mm beads, there were thousand of microscopic beads across the surface. These evaporated quickly to leave a clean surface. However, I am now concerned there could be fine pits across the surface which could also be acting as a nidus for dew formation.

When I first saw the beads, I thought it might be an oily film on the mirror surface, but it did not change with repeated washing (soap and running water only, not rubbing)

Obviously, my first step will be to try the mirror outside and see if it dews up, but the dramatic warming of the weather over the last couple of weeks might invalidate that test.

Any suggestions?

DT

[alocky (Andrew lockwood)]

Best way to check for pinholes is to shine a torch at the mirror and have a look from the other side. Beware - even a newish mirror will look a bit scary when you do this. The signs that the coating has tarnished will be large-scale blotching, or even bits of the coating flaxing off.
The dewing sounds a little odd - is the mirror enclosed at one end of a tube? It's unusual in my experience for a primary in an enclosed system to dew at all, but it has happened on nights of extreme humidity when the temperature has dropped sharply - doesn't sound like Brisbane in spring at all! I don't think there's an awful lot you can do, and I'd be surprised if the surface of the mirror made any difference.
cheers,
Andrew.

[RickS (Rick)]

David,

Dunno about the water beading but my GSO RC10 has never dewed up on the primary with the fans blowing. On several occasions there has been a significant amount of dew on the outside of the tube. I do run it with a long dew protector though...

Cheers,
Rick.

[DavidTrap (David)]

Thanks Gents,

I have left the mirror to dry for a while and under indirect light there are fine marks across the entire surface.

I'll try to take some photos of it tomorrow.

DT