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
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
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
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 (Paul)]

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

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 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 (Rob)]

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 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??
I guess, all you Lightbridge owners, have a close look at your mirror

[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