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blemishes on my primary mirror

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#1 proud uncle

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Posted 06 August 2014 - 10:24 PM

A little more than one week ago, I cleaned both mirrors since I had the telescope disassembled for other repair and modification work.  It was my first time cleaning the mirrors; the scope is approximately nine years old.  I mostly followed instructions I previously read on this forum.

I soaked both mirrors in a tub of distilled water with a small amount of dish soap for one or two hours.  A few times I swished the water and soap around with my hands or gently rocked the mirrors back and forth, to get some motion.  But I did not touch the reflective surfaces at all.  I then moved both mirrors to a clean, empty tub and rinsed them with two gallons of distilled water.  I then used a hair blow drier on low setting for a minute or two to expedite drying.  

Next is where I think I made a mistake.  I noticed a few spots remaining on the primary mirror.  I used a clean microfiber cloth over the end of a q-tip and blotted the spots with rubbing isopropyl alcohol.  That is when these spots or blemishes first appeared.  I immediately re-rinsed the mirror with one additional gallon of distilled water.  These blemishes are not always noticeable; the light has to be angled just right, as well as the angle of my eyes with respect to the mirror.  It does not appear to have adversely affected the reflectivity of the mirror.  The scope remains disassembled due to delays in completing the other work on the scope, so I have not been able to use the scope since my cleaning experience.

Have I done any damage to the coatings?  Is there anything I can do to minimize or repair that damage?  I am wondering now if I really read to use denatured alcohol rather than rubbing alcohol?

Here is a link to a photo in the CN photo gallery:


Any feedback you can give will be greatly appreciated!  Thanks much!


Edited by proud uncle, 06 August 2014 - 10:41 PM.

#2 choran



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Posted 06 August 2014 - 10:58 PM

Just a couple of points:  You do not need to leave the mirror in water for hours.  The whole cleaning process can be completed in a matter of a few minutes.  Soaking coatings for a protracted period of time is not necessary, and I do not believe it is desirable.  If the spots you mention only showed up after you used your microfiber cloth and not after the soaking, you may have just left a little residue of some sort in those areas. 

On your question as to whether a damaged coating can be repaired, the answer, I believe, is no--but it can be made worse.

I would do nothing until I reassembled the scope and tried the mirror.


When cleaning in the future, clean basically the way you did, but for a few minutes only, and omit the hair dryer--just use the edge of a paper towel to blot up, not rub, any distilled water that refuses to sheet off.  Unless you really rubbed (a no-no) with your micro fiber cloth, I don't think you are likely to have hurt the mirror in that fashion. Re: denatured alcohol, I have used it for many things, but never mirror cleaning.  I would not try it.  Rubbing alcohol is fine, but try to get 90% solution, not anything less concentrated.  Some folks use acetone, but it's usually not necessary, IMHO, but i would go that route before I would mess with denatured alcohol, as I've personally never seen anyone write about using it for the purpose you suggest.


After 9 years, it may be time for a recoating in any event.  Doesn't cost too much, and your mirror will look like new.  If you decide to go this route, my best advice is to consult a mirror maker such as Carl Zambuto or Mike Lockwood or others of similar reputation, and see who they recommend--your choice of a coater is very important.

Edited by choran, 06 August 2014 - 11:04 PM.

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#3 dragonstar4565


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Posted 06 August 2014 - 11:01 PM

I don't think you should worry about this, it's not in my opinion a real problem, and no I believe you have not, done anything detrimental to the coatings, after 9 years the coatings are probably really hard, and the harder they get, the more difficult it is to damage the coatings, however I think, the harder the coatings get, the more difficult it is to remove them, I'm not sure, but I think so, but I think you have a few more years, to go before a recoat, so I would just put it back in your scope and enjoy.  For what it's worth a telescope mirror has to be really dirty to effect the image at the eyepiece, and probably as you already know, the spider and secondary mirror, is like a huge scratch on the primary, that's why there is something called diffraction, on bright objects, and one more thing never shine a light on a telescope mirror, as even a freshly coated mirror will look worse then it really is, and show all sorts of irregularities, that you can't see without the light, so I will reiterate to not shine a light on a telescope mirror...clear skies always... :)

#4 sn1987a


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Posted 07 August 2014 - 02:53 AM

This might put your mind at rest https://www.youtube....h?v=9Y8xFnXFVGQ

#5 txmnjim


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Posted 10 August 2014 - 10:29 PM

i wouldnt worry too much about it. i guess see if you notice any degradation of the images in the field.

i remember the story of a disgruntled worker who shot several holes in a large mirror at the McDonald

Observatory in Texas and it didnt bother that telescope's image ;)  nine years isnt too long for a coating

but i guess it depends on who did it. i had a mirror done by QSP and it's coating was still acceptable after

nearly 20 years! 

Edited by txmnjim, 10 August 2014 - 10:30 PM.

#6 gnowellsct


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Posted 11 August 2014 - 09:12 PM

I had some blemishes on my primary.  Here is what it looked like held up with the back side to a strong light.  GN

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  • Optical Craftsman 10in-02 CN size.jpg

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