Has my mirror got a terminal illness?

[JimmyH155]

Last weekend, after over 3 months of lousy weather, the clouds blew away, and out came my beaut 12 inch Lightbridge. When I undid the mirror cover, I saw a large black mark near the outer edge:doh:
Out came the rubbing alcohol, but it made no difference. The mark was completely black and 6 mm in diameter. Around this black mark was a 2 mm wide whitish blemish - all told 10 mm diameter - OUCH:shrug:
The mark seems to be under the surface - not on it. Would this be the silvering deteriorating? How do they put on the silvering? I would have thought it would go on top of the glass:shrug:
Anybody got any ideas? Is it likely to grow? Remember, this mark is not on the edge of the mirror - about 15 mm in from the edge.

[PCH]

Those of us with LBs can scarcely wait for someone to shed some light on this one.

Thankfully mine is still spotlessly clean after about nine months - even after I dropped an allen key on it whilst collimating/adjusting the secondary - duh:P

Cheers,

[Suzy_A]

A 10 mm spot on a 250 mm mirror won't affect the performance very much - it's only 0.16% of the total surface area. Your secondary will block probably 20% or so of the light.

So just ignore it for the time being.

If you want to resurface your mirror, there are a few places that do it. The standard thing is to have it re-aluminised and then silicon monoxide overcoated. Cost you maybe $100 or so.

Astro Optical in Sydney used to do it, I don't know if they still do. Have a look on th net.

You can also get it silvered for quite a bit less from a local mirror maker. Have a look in the yellowpages. Make sure you give them instructions how you want it done - front surface, no protective paint! However silvering only lasts 6 months or so before it needs to be done again, but in the few months that it's fresh, its quite a bit better than aluminium.

You can even silver it yourself using the 'silver mirror test'. Have a look in a chemistry book or on the net. From memory you use silver nitrate, caustic soda and ammonia solution. I've done it a few times and if you take care it works well. If you don't take care, it blows up in your face - the solutions slowly forms explosive silver azide solution.

[JimmyH155]

Thanks for that, Suzy. Actually I just looked up on the web for Astro Supplies and for my 12 inch, it is $200. Very tempting, and I know it is not a big deal percentage wise, but OOOOHH :sadeyes: the sight of that mark will drive me crackers. I am going to take the thing apart this weekend and will report next week what I find. Thanks for your reply. :)

[rmcpb]

Ignore it at the moment, its not a significant proportion of the mirror's area.

Cheers

[JimmyH155]

Actually, I took the thing apart at the weekend, and discovered that it is the aluminising coating completely gone from that spot:sadeyes: So now I will wait and see if it "grows"
The worry is now, for all those Lightbridges we have here in Oz, is mine just a bad one in the bunch, or have they all had sub-standard coatings put on them??:shrug:
I guess, all you Lightbridge owners, have a close look at your mirror:mad2:

[Garyh]

If no more spots appear on your mirror, I wouldn`t worry about recoating for sometime. You are only losing a 1% or 2% of your light gathering.
But I would get some good flat back paint and coat the blemish well past all the oxidization. That should stop it growing to its like a 50c piece in size.
cheers

[JimmyH155]

Thanks, Garyh, the paint goes on next weekend:)