Grey mirror deposits that won't shift

[Pathfoot]

I know this will not be a new question, so I've tried finding solutions and recommended treatments first before calling for help and cluttering the help forum, but what I've tried so far doesn't work.

 

After a long gap, I recently dusted off (metaphorically and literally) my old 15cm telescope. The main mirror has lost its sparkle, though that's not the subject of my query. It's the secondary. It has a grey-looking film, almost like dried salt, on it. I took it from the spider and it was obvious this thing was not reflecting many photons. I could barely see my face in it. 

 

I tried rinsing in clean water, I tried mild detergent which, when wet, looked like it was working, but when the mirror dried, the film returned, which I'm guessing may be some kind of oxide. I tried clear window-cleaning liquid. I tried PEC photographic cleaning solution, no joy. I've tried prayer, magic dances, incantations, libations to the gods, and sacrificed a small goat. The gods are not listening either.

 

Perhaps my only option is to buy another flat, but I fear I may have to replace the primary too, and at the moment, my inclination is to spend any such money on decent binoculars instead. However, if there are any suggestions for things I've not tried, I'd be grateful.

[estwing]

how old is the mirror?....and I use pure soap flakes....but it does sound you might need a re-coating

[Pathfoot]

It's old. (How that time has passed so quickly is alarming.) I think I bought it in the early 90s, from Orion Optics in Cheshire. Are there recommendations for places to have re-coats carried out?

[Peter Drew]

The appearance is due to the coatings having oxidised. Unfortunately this can't be cleaned off. Recoating is the solution, Orion Optics could do you a nice Hilux recoat which is long lasting, it's quite expensive but cheaper than new mirrors. Other firms that recoat are available.

[Physopto]

You could always ask John Nichol. He is a mine of info on mirrors and very helpful, great bloke. Just don't tell him I said the last bit 

Derek

[Pathfoot]

Many thanks, re-coating is the route I'll pursue, as it's obvious both mirrors should be treated at the same time. Looks like around £100 should cover it.

 

Well, I'd only spend it on beer and wicked women anyway.