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Telescope Mirror-- reconditioning?

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Coyote Wilde

Thu Feb 23 2006, 07:43PM

Registered Member #175 Joined: Tue Feb 14 2006, 09:32PM
Location: Sudbury, ON
Posts: 111

I recently came into ownership of a six-inch first-surface parabolic mirror-- the primary for what I hope to be a rather lovely reflector. But, alas, there is a problem (isn't there always?) : two small dings, a bit off center, scratches in the silver. The larger is ~1cm, the smaller slightly less than half of that. They appear as black marks on the mirror, and do not seem to have damaged the surface of the glass itself, only the silver coating.
So, my query-- does this even need repaired for amature use? I suspect the consensus there will be "yes," so it then follows that I should ask how do I fix this? Can I resilver the scratches, or would the entire mirror need polished off and redone?
There is a derth of information on google about the making of such mirrors; thus far, I've found nothing on the maintenance. I suppose they assume once having made such an investment of time/effort or cash, that one would be very careful. Unfortunatley, this one came from the bowels of a local high school, and we all know just how careful most students of such can be...

I can post a photo of the mirror itself if anyone is interested, but my digital camera gets confused by the virtual image and refuses to focus in on the scratches.

HV Enthusiast

Thu Feb 23 2006, 08:07PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

I'd recommend searching out the Cloudy Nights forum. There are a lot of ATMs on that group that probably can provide you with a lot more info than most here. Also, one of the local groups i belong to, www.starastronomy.org, has a strong ATM base as well. They have forums in which you can sign up as guest.

Carbon_Rod

Fri Feb 24 2006, 12:14AM

Registered Member #65 Joined: Thu Feb 09 2006, 06:43AM
Location:
Posts: 1155

Hmmm, I think the ding is really going to make it difficult. But there is a local fellow that runs a shop specializing in telescope equipment for the local universities and labs if you are interested.

On a larger mirror small dark defects like bugs etc are usually not too big of an issue. The vertical spun liquid mercury mirrors are also pretty interesting. If you canâ€™t get a replacement and you do have access to a mill as you were going to make a specialty prototype anyway. Why not put your mirror mount on an old large hard drive axis and spin it slowly (or split the scope body in half connected via a narrow ball-race in the front, and a spring loaded needle bearing on graphite in the back.) Mind you the thing would have to be perfectly aligned and balanced. It would be very difficult to do considering most mirrors have relatively delicate foam rubber backings to apply even force against the machined seat to keep consistent focal length.

Did you check for optical surplus at edsci or ebay etc.?
I think you may have to find other uses for the one you have now.

johnf

Fri Feb 24 2006, 05:48AM

Registered Member #230 Joined: Tue Feb 21 2006, 08:01PM
Location: Gracefield lower Hutt
Posts: 284

Check out your local Physics department at your local university. Suck up to them and ask if they could recoat your mirror with /aluminium. If they say yes you will need to remove the silver with a mixture of conc Nitric acid and conc sulphuric (aqua regia) this should also remove the silver sulphide (black spots) and generally clean the surface (they might do this for you). If you are very lucky they might put a layer of silicon dioxide over the aluminium to protect it.
I did several mirrors for amateurs while I worked at a university the biggest being a 12 inch primary for a reflector

Bjørn

Fri Feb 24 2006, 03:52PM

Registered Member #27 Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058

Black marks on a telescope mirror does little to reduce the quality. The area of reflecting mirror and the diameter is mostly responsible for the quality of the resulting picture. If you have dings in it that makes it reflect light int he wrong direction you may get reduced contrast and possibly other problems. You may improve it by painting the problem areas black. If 90% of the area is in very good shape and 10% is black then it could still give exellent results.

For point sources like stars you need very good alignment to get optimal results. For planets and other uses it is less critical.

When it comes to taking pictures of mirrors, try to focus on something that is the same distance away as the mirror, push the button halfway down then move the camera to point at the mirror and press the button all the way down. That will work on all but the simplest of cameras.

Coyote Wilde

Fri Feb 24 2006, 10:37PM

Registered Member #175 Joined: Tue Feb 14 2006, 09:32PM
Location: Sudbury, ON
Posts: 111

I think perhaps then I'll try it as is, based on Bjorn's statements/
A couple 1cm marks on a 6" mirror only make up a very small percentage of the overall surface... but for reference, a photo. the marks appear whitish in the highly-exposed photo, but black at all angles to the naked eye. The two main blemishes are marked-- close inspection reveals 3-5 other specks which may or may not be of the same nature. (I have not yet had the chance to properly clean the mirror; what you see on the photo uncircled is the result of dust)

Sulaiman

Sat Feb 25 2006, 09:06AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141

I agree that a few non-reflective areas can be ignored
(as far as physics goes, not aesthetics)
For naked-eye viewing I found 4.5" too small to see the colours in Saturn's rings
and a 12" gave beautiful sights but was a pain to mount/steer.
6" sounds like a good size!

Proud Mary

Sun Sept 13 2009, 05:17PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

So far as I can see, "error" would be proportionate to the surface area damaged, which in the case of a few small areas would be negligible. Think of the Newtonian, which has a mirror and its supports right across the aperture of the tube.

I suppose it would be possible to re-silver the speculum using the silver nitrate process, but frankly I would have thought it more likely to produce further faults than leaving the mirror as it is.

I believe that mirrors without any silvering at all are used for solar observations, so don't be too quick to put it in the dustbin.

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