[Wavytone]

Mark, Alex,

There is, I suggest, an obvious answer - if the mirrors are over coated with SiO they shouldn't need recoating for many years, and there's no point trying to recoat them as it can't be chemically stripped and has to be polished off, meaning the mirror would have to be refigured before recoating. Which will cost more than replacing the mirror if its a straightforward Newtonian. Hence the reply I got from Royce.

Alex if those mirrors are SiO over-coated then OK, but users should be warned they can't be recoated and should treat them with care to get a long life (i.e. protect from dust and cleaning).

Mark - I guess it depends on what you think the useful life is. From an engineer's view everything has a finite life and the choice of repair/replacement is a simple decision that comes down to lowest cost, and unfortunately the chinese mass-produced optics are absurdly cheap compared to what it takes for an artisan like yourself to make them. While I'm well aware a good mirror should be "forever", like many of yours, when you consider how many cheap dobs are sold commercially every year/month it implies (like so many other things) a hefty percentage end up in landfill sooner than later courtesy of someone who neither cares about such things, nor knows any better. A small fraction appear on Cloudynights.

Frankly for a consumer scope I'm not sure what is best. If you told a newbie customer (one with little idea and no experience) buying a 12" netownian for the first time that every 2-3 years the mirrors need recoating and the likely cost I suggest they'd balk. Alternatively tell them an over-coated mirror has perhaps 10 years life, then trash it ? Conversely an old-time ATM would be perfectly happy with a plain metallic coating (possibly BerAl or one of the other high-reflectivty blends). One with deep pockets might stretch to multilayer dielectric coatings (effectively lasting forever).

I'm more intrigued by the optics that are matched pairs and exposed to air, such as the Vixen/Tak cassegrain astrographs or the GSO RC's. On one hand it would make some sense of these mirrors were plain aluminium (no SiO coating) and had to be recoated every 2-3 years. But I suspect they're SiO over-coated, too. Would be interesting to know.

Might make an interesting chat with the guys at Bintel one day.