Another mirror cleaning thread

[midjam]

Hi,

I just brought a secondhand 12" LB and the mirror has tons of tiny white spots which on first glance looked like dust particles. On closer inspection it looks like it's under the surface of the mirror.

I soaked the mirror in warm water with a few drops of washing liquid for around 20 mins then very gently dragged cotton balls across it's surface (this worked great for my 4.5") but, they are still there.

I was thinking about using a water/alcohol mix but worried about damaging it. After doing some searching I'm starting to think this might be oxidation and it might need a re-coat, does anyone have any images of an oxidized mirror to compare?

It was relatively cheap for a 12" scope and the guy I brought it from says it will not effect the image quality, haven't looked through it myself yet but, I have never looked through a scope of this size so, have nothing to compare.

What would you guys do?

[Islander]

It might be the coating itself starting to oxidise. Have you tried holding the mirror up to a bright light - reflective face towards the light to see if there are lots of pinholes in the coating?

If it is the coating then it'll cost you around £100 to get both primary and secondary recoated by these people:

http://www.scientificmirrors.co.uk/Aluminising.html

I had my 8.75" done this year and they did a superb job. I found them to be friendly and helpful. Turnaround is fast but dependent on the time it takes to strip the old coating off.

[umadog]

The image is out of focus so it's hard to see, but it it's "underneath" then you need a re-coat. Do it soon, because it's possible the glass will be damaged by a decaying coating. FYI, alcohol and water won't damage the coating.

Islander, is your 8.75" a David Hinds? Vacuum Coatings currently have my 8.75" Hinds!

[midjam]

Thanks guys, looks like a re-coat is in order.

Do it soon, because it's possible the glass will be damaged by a decaying coating.

You have me worried now, I had to borrow some cash to buy it, won't have enough for a coating for quite some time.

Would it be best to keep it indoors until I can get it coated? really would like to test it out!

Also, it has 3 very small chips on the edge, looks like it might have come from the mirror clips don't think it's a major problem viewing wise but, do you know whether it would be a problem when it comes to re-coating?

[Steve Ward]

These people are very good.

http://www.galvoptics.fsnet.co.uk/

[umadog]

You have me worried now, I had to borrow some cash to buy it, won't have enough for a coating for quite some time.

Would it be best to keep it indoors until I can get it coated? really would like to test it out!

Also, it has 3 very small chips on the edge, looks like it might have come from the mirror clips don't think it's a major problem viewing wise but, do you know whether it would be a problem when it comes to re-coating?

The three chips sounds like coating clip marks. Not a problem: http://webstertelescopes.com/forum/viewtopic.php?f=7&t=599 The mirrors are sometimes suspended upside down in the coating chamber by three clips.

It's hard to tell from the photo but the mirror doesn't look too bad. The one I sent off recently was probably worse and they managed to strip it and re-coat. You just don't want to push it. By all means use it, but keep it as clean as possible. If you can get it re-coated over the next few months, that would be best. You should still get good views as it is. Probably you'll see better contrast following a re-coat.

[buzzc150]

How much does it cost to buy a new mirror? Much more than the £100 to recoat?

[midjam]

Thanks umadog,

I was thinking of maybe soaking it over night in water with a few drops of washing up liquid, incase it's just really stubburn dust.

Would this cause any more damage if it's oxidation do you think?

Yes a new mirror would be a ton more than recoating

[Islander]

The image is out of focus so it's hard to see, but it it's "underneath" then you need a re-coat. Do it soon, because it's possible the glass will be damaged by a decaying coating. FYI, alcohol and water won't damage the coating.

Islander, is your 8.75" a David Hinds? Vacuum Coatings currently have my 8.75" Hinds!

Yes - it's a lovely mirror. I bought it in the mid 90s and this was its first recoat - not bad at all really although it definitely needed doing!

[Islander]

Midjam, it won't hurt the coating to try cleaning it again but I wouldn't soak it overnight. You could try holding it diagonally under a running cold tap after a good soak - the flow might dislodge any stubborn particles and won't damage the mirror's surface. Then a quick rinse with distilled water as normal.