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Whats wrong with this primary mirror and is it salvageable?

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#1 Henry from NZ

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Posted 22 August 2019 - 05:04 AM

Saw an 8" Dob on a local auction site for a song, but the mirror has some spots as attached photo. What are these spots and are they removable?


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Edited by Henry from NZ, 22 August 2019 - 05:06 AM.

#2 Darren Drake

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Posted 22 August 2019 - 05:11 AM

Those are in the coatings and permanent until it gets recoated.  The effect on image quality however is negligible and not worth worrying about...

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#3 Henry from NZ

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Posted 22 August 2019 - 05:13 AM

Thanks Darren
Supplementary question - what are they caused by? fungus? 

#4 sg6



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Posted 22 August 2019 - 05:44 AM

I would say it is pinholes where the coating has failed and the aluminium underneath is now oxidised.

Not nice but until a fair % of the surface fails it is likely that you will in real terms not notice the problem. But it will get worse in time.


In a way what you are seeing are the initial signs that the coating layer is failing.


Solution is to get the mirror resurfaced and recoated. However that is generally not a small cost as it involves removing the over coat and the aluminium then a gentle regrind of the glass - assumes they determine the curvature. Other problem is locating someone that does such work.


If you can find at this time someone that performs such work I would contact them and get some idea of cost and what they think. Say this as you likely do not need to sort it out immediatly but in say 2 years you might, and by then the actual glass may have received damage and so be unable to have a simple regrind and resurface. So finding out now may mean not throwing it in the bin in a couple of years.

#5 NeroStar


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Posted 22 August 2019 - 06:19 AM

That is likely the result of oxidization caused either by being stored for extended periods in a high humidity environment (such as a basement) or by by the previous owner(s) not drying the scope sufficiently prior to storage.  It could also be the result of a bad coating process during manufacturing.  As stated by Darren Drake, it's really nothing to worry about in terms of real effects on image quality unless the affected area spreads significantly.  Store the scope in a stable temperature environment with some desiccant bags and you shouldn't have any more problems.    

#6 Henry from NZ

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Posted 22 August 2019 - 06:29 AM

Thank you all!



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Posted 22 August 2019 - 06:54 AM

If the mirror has a good figure to begin with, it certainly doesn't need regrinding! --- All it needs is recoating... and even there, it is actually perfectly usable the way it is now.    Tom

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#8 ngc7319_20



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Posted 22 August 2019 - 07:49 AM

The centers of the spots look very dark -- as if the coating might be gone there.  If you can shine a light into rear or side of the mirror, you might be able to confirm this suspicion.  If the mirror is transparent there, then the coating is gone in the spots. The spots are small enough so as to have little effect on anything.   You might want to clean it, just to get whatever is eating the coatings off of there.  It could be fungus, or some chemical that came into contact with the mirror.  A salt crystal, bit of metal, fungus, etc.

#9 Star Geezer

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Posted 22 August 2019 - 08:58 AM

This is somewhat of a question as I don't know what I'm talking about.


I picked up a used Orion StarBlast at the flea market a couple years ago. The scope had no cover and while enjoying my new scopes the StarBlast set in the garage for a couple months. When I got the scope out under the stars again the view was nothing like I remembered. After pulling out the eyepiece, I looked down the focus tube and discovered that mice had soiled the secondary. The secondary cleaned up nicely but it was left with scratches around the edge. I could still see things I'm told I would need an 8”scope to view.


A few months back I picked up another used StarBlast from the the flea market for replacement parts. The first thing I did was dismantled and inspect the whole scope. The reflections from the secondary was sharp and bright. Looking down the tube the primary appeared in good condition. After setting it flat on a table and approaching it at a low angle with the sun behind me, I noticed it was cloudy in spots.


Am I wrong in thinking this would have a more detrimental affect then a few little blemishes? Would this be a figuring issue or deterioration?

Edited by Star Geezer, 22 August 2019 - 09:13 AM.

#10 Starman1


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Posted 23 August 2019 - 01:50 PM

Step one for any mirror that looks like this is:


Here is how:


THEN, AND ONLY THEN, can you really say whether or not the coatings are failing.

And if they are, then re-coating the mirror is the logical next step and there are a hundred mirror coaters to do it.

Here are a few:


#11 DocGP



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Posted 23 August 2019 - 02:36 PM

My thought also was a good cleaning before too much condemnation.


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