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Fine scratches on my primary mirror

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#1 leetje

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Posted 21 August 2019 - 10:25 PM

Hello, I picked up a used Obsession classic 15 scope, fully loaded with Argo Navis and ServoCat, for about 1/3 the price of new. I noticed several very fine scratches on the primary mirror, most likely from improper cleaning/wiping from the previous owner, and assumed it would have minimal effect on my views. In hindsight I wonder if I be concerned about this? At my first light with the scope, I looked at the Andromeda galaxy, Lagoon, Trifid and Veil nebulae and the views under dark Wyoming skies were spectacular. The mirror may need some gentle rinsing with distilled water and I plan to do so with no wiping. So my question to the forum is should I worry about these fine scratches? At what point does one consider recoating? I don’t think I need to do the latter as the views seem great but still wondering. Thanks!

Edited by leetje, 21 August 2019 - 10:47 PM.


#2 Pinbout

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Posted 21 August 2019 - 10:38 PM

They won’t hurt anything

 

Look at these ashy coatings 

 

https://youtu.be/7e_Rpiaj1fo

 

 

Remember this - contrast is in the glass 


Edited by Pinbout, 21 August 2019 - 10:39 PM.

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#3 ngc7319_20

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Posted 21 August 2019 - 11:06 PM

 So my question to the forum is should I worry about these fine scratches? At what point does one consider recoating? 

Don't worry about the scratches.  The tendency is to see the scratches, but not the 99% of the mirror that is perfect.

 

You should recoat the mirror if you can hold the mirror up to a light and see through it pretty much everywhere.  Some pinholes and scratches are nothing to worry about.


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#4 sg6

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Posted 22 August 2019 - 01:01 AM

No worries, the marks will be on the overcoating not the mirror coating.

Looking at mirrors and objectives in general is never a good idea. None are perfect and therefore anything seen raises worries.


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#5 macdonjh

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Posted 22 August 2019 - 08:26 AM

None are perfect and therefore anything seen raises worries.

I don't know about that.  When it arrived, my newest scope had mirrors that were "clean room perfect" to the eye.  They were so clean and shiny I almost hated to install the scope.  The dust that accumulates on it drives me nuts, but then I take a deep breath and look through the eye piece...

 

As has been said in previous posts, you won't notice the small scratches or some dust.


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#6 Starman1

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Posted 23 August 2019 - 01:53 PM

Gentle cleaning so you don't add to the issue:

https://www.youtube....h?v=9Y8xFnXFVGQ

You may not need it now, but this technique really works.


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#7 leetje

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Posted 23 August 2019 - 10:39 PM

No worries, the marks will be on the overcoating not the mirror coating.
Looking at mirrors and objectives in general is never a good idea. None are perfect and therefore anything seen raises worries.



#8 leetje

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Posted 23 August 2019 - 10:45 PM

So is the top coating that protects the aluminum layer of the mirror that has the scratches simply a layer of silicon dioxide? Note the scratches are pretty much following the same direction so I’m pretty sure it resulted from improper cleaning techniques. I’ll be super careful going forward

#9 ngc7319_20

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Posted 24 August 2019 - 12:37 AM

Yes, silicon dioxide is a popular over-coating for aluminum. It is harder than aluminum.  Whether any scratches went through the overcoat and into / through the aluminum is hard to say.  You could study this by shining a light through the back of the mirror, and see if light comes through the scratches.  In any case, I wouldn't worry about the scratches.


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#10 leetje

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Posted 26 August 2019 - 11:22 PM

I did shine my red light behind the mirror and there were a couple of scratches where the light shined through, not many. My views of deep sky objects look fine but I’m still bothered by the scratches. At what point would I consider recoating and would it even be worth it? Not sure if recoating would even remove the scratches. Thanks for reading.

#11 ngc7319_20

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Posted 27 August 2019 - 02:10 AM

I would not base any re-coat decision on the scratches.  I would only re-coat when the other 99.9% of the coating area has some issue.  Like when your flashlight shines through every where on the mirror, and you can (almost) read a newspaper through it -- then it needs a re-coat.  Probably it just needs a gentle cleaning at this point.

 

If the scratches are all the way through the coating and into the glass, then yes, they will still be there after the re-coat.  In fact they will be even worse -- instead of merely being dark regions on the mirror, they will be 95% reflective and scatter even more light.

 

Maybe also seek advice from some local expert who can look at the mirror and make some judgement directly.  But from what you have said, I would not re-coat this.


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#12 earlyriser

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Posted 27 August 2019 - 03:47 AM

Don't worry. Be happy. 


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#13 leetje

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Posted 27 August 2019 - 06:12 PM

I would not base any re-coat decision on the scratches.  I would only re-coat when the other 99.9% of the coating area has some issue.  Like when your flashlight shines through every where on the mirror, and you can (almost) read a newspaper through it -- then it needs a re-coat.  Probably it just needs a gentle cleaning at this point.

 

If the scratches are all the way through the coating and into the glass, then yes, they will still be there after the re-coat.  In fact they will be even worse -- instead of merely being dark regions on the mirror, they will be 95% reflective and scatter even more light.

 

Maybe also seek advice from some local expert who can look at the mirror and make some judgement directly.  But from what you have said, I would not re-coat this.

Thanks for the comments. I don't know why, it just bugs me, and is probably the main reason I got the scope for the price I paid used. Based on the quality of my views so far I think I need to stop worrying about the scratches. This appears to be the consensus across many posts on the topic. 



#14 whizbang

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Posted 27 August 2019 - 06:18 PM

Scratches on your secondary will ruin your night.  The primary is much more forgiving.


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#15 macdonjh

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Posted 28 August 2019 - 01:14 PM

I don't know why, it just bugs me,  

Don't worry, we know the answer to that question, too: it's because you're an amateur astronomer and hard-wired to obsess over things that don't matter much in the real world.  We all suffer from minutia-itis to some extent.  You are not alone.

 

I agree with the others: don't worry about recoating until your mirror gets so dirty (with water spots or pollen or other stuff that won't wash off), or "tarnished"/ dim or "worn" (light shines through the coating) that you notice the difference in views between your scope and another scope of similar aperture with better coatings.  You know, in about ten years if your care for your gear.  Maybe longer.


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#16 TareqPhoto

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Posted 02 September 2019 - 02:33 PM

I have kind of scratches maybe on my secondary mirror, and I think now the view is bad, is there a rescue for that?



#17 ngc7319_20

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Posted 02 September 2019 - 03:37 PM

I have kind of scratches maybe on my secondary mirror, and I think now the view is bad, is there a rescue for that?

Can you post pictures of it?  Perhaps it just needs a careful cleaning?  Often the fix for simple Newtonian diagonal secondary mirrors is just to replace the mirror.  They are relatively inexpensive.



#18 TareqPhoto

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Posted 02 September 2019 - 05:35 PM

Can you post pictures of it?  Perhaps it just needs a careful cleaning?  Often the fix for simple Newtonian diagonal secondary mirrors is just to replace the mirror.  They are relatively inexpensive.

I can buy a replacement secondary mirror, but the problem is i don't know how to remove the stock one and add the replacement.

 

I will post later if i can.

 

I started a topic about collimation, and from the out of focus image i also talked about cleaning, and people told me about cleaning method, but it was mainly for the primary mirror, it has dust spots, but i won't bother cleaning it yet, but the secondary mirror i may need it to clean, because i remember when i looked at collimator cap of Cheshire it was clear, not i don't know what happened, and i used that ZEISS pre moist wipes on that secondary mirror.



#19 Starman1

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Posted 02 September 2019 - 06:35 PM

One rule about cleaning:

NEVER wipe anything on a dry mirror, even if the cloth is wet.

That is a prescription for scratches.


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#20 ngc7319_20

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Posted 02 September 2019 - 07:52 PM

...and i used that ZEISS pre moist wipes on that secondary mirror.

Ouch!  That is not the best cleaning way.  It is better to blow of any dust, and then rinse under a water stream or using a spray bottle.  Then once you are sure any dirt and grit is gone, then use some wet cotton balls, etc.  Wiping as a first step can move grit around the mirror surface and make scratches.

 

These mirrors are either attached with clips or a shell and small screws.  Or glue on the back of the mirror.  Sometimes you can saw through the glue with dental floss or something similar.



#21 TareqPhoto

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Posted 02 September 2019 - 08:00 PM

I will try to give it a try and see, i got suggestions or recommendations about using distilled water, i bought 2 bottles, but they said i can try with soap in water first and then rinse final with distilled water, using if possible a surgical cotton if i can find, some said with brush, but i doubt a brush is a good idea.

 

I will see if can replace the secondary mirror easily, not sure it has adhesive with glue or by screws.



#22 jeffmac

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Posted 03 September 2019 - 09:13 AM

No wiping with anything. Take the secondary mirror out, rinse it thoroughly with water, rinse it thoroughly with 99% pure isopropyl  alcohol. Lastly, rinse thoroughly with distilled water. This will take almost any contaminant off of the mirror, even spider secretion.  



#23 TareqPhoto

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Posted 03 September 2019 - 01:02 PM

No wiping with anything. Take the secondary mirror out, rinse it thoroughly with water, rinse it thoroughly with 99% pure isopropyl  alcohol. Lastly, rinse thoroughly with distilled water. This will take almost any contaminant off of the mirror, even spider secretion.  

And when dried the water won't leave that drying marks?



#24 cloudypatio

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Posted 03 September 2019 - 03:19 PM

And when dried the water won't leave that drying marks?

Distilled water won't leave marks, tap water will.


Edited by cloudypatio, 03 September 2019 - 03:20 PM.


#25 TareqPhoto

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Posted 03 September 2019 - 03:48 PM

Distilled water won't leave marks, tap water will.

I see, good to know, i hope that distilled water is truly distilled and pure, then i just need to find where i can get that isopropyl alcohol.




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