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Mirror coating question

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#1 Abhat

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Posted 23 September 2020 - 02:50 PM

Folks I have very old 6" F/8 Hardin Dob. When I took the mirror out and inspected it closely under light I do see coating loss on the mirror. Especially when looking at it from the back against the light I can see some light passing through instead of getting reflected. I think the secondary is probably OK. The views through the Dob are good but I do not have another 6" Dob I can do a side by side comparison against to quantify the loss of light.

 

How do I determine when the coating loss is too much and whether I need to get mirror replaced or re-coated. In case 6" F/8 its cheaper to buy a new mirror instead of getting it re-coated.



#2 Jim Waters

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Posted 23 September 2020 - 02:59 PM

"I can see some light passing through instead of getting reflected"   Time to recoat the mirror.


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#3 Keith Rivich

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Posted 23 September 2020 - 05:46 PM

An 8" is a pretty cheap to coat. Get R done!



#4 Starman1

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Posted 25 September 2020 - 05:17 PM

Folks I have very old 6" F/8 Hardin Dob. When I took the mirror out and inspected it closely under light I do see coating loss on the mirror. Especially when looking at it from the back against the light I can see some light passing through instead of getting reflected. I think the secondary is probably OK. The views through the Dob are good but I do not have another 6" Dob I can do a side by side comparison against to quantify the loss of light.

 

How do I determine when the coating loss is too much and whether I need to get mirror replaced or re-coated. In case 6" F/8 its cheaper to buy a new mirror instead of getting it re-coated.

Older than 10 years?  Recoat.



#5 PeteDCard81

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Posted 25 September 2020 - 10:25 PM

I would not determine a mirror needs to be re-coated based solely on the amount time since it was coated.

 

I base it mostly on how the mirror looks from the front side.

 

The coating may have been that way at the time is was coated.


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#6 Starman1

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Posted 26 September 2020 - 01:20 AM

With an optimistic deterioration, if kept clean, of 1% per year and a start rate of 90% on each mirror, the reflectivity of a 2 mirror system is down to 66% by 10 years.

If there are other issues with the coating, it will probably be lower.

You simply cannot base it on "well, it looks shiny to me", because it'll look shiny at 25% reflectivity.

And I learned the hard way that when a coating is failing, the substrate glass can also be damaged.


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#7 PeteDCard81

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Posted 26 September 2020 - 12:19 PM

Last week I cleaned a 4.25-inch RFT mirror I made in July 1976.

 

The mirror is made out of nice Pyrex blank. I had ground and  polished the back flat when I made the mirror so it is no problem to ascertain the condition of the coating from the back. The scope has had little use and kept in a closed tube.

 

I just took out the mirror and it's cell and held it up to a bright light. I could not see the light looking from the back of the mirror.

 

Just saying the coating could have exhibited seeing a bright light from the back from the start.

 

Certainly if the mirror used in a in a salt air environment (LA) could be susceptible to damage if the coating fails. Maye the salt air eats away the aluminum and the overcoat.



#8 Starman1

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Posted 26 September 2020 - 02:57 PM

Last week I cleaned a 4.25-inch RFT mirror I made in July 1976.

 

The mirror is made out of nice Pyrex blank. I had ground and  polished the back flat when I made the mirror so it is no problem to ascertain the condition of the coating from the back. The scope has had little use and kept in a closed tube.

 

I just took out the mirror and it's cell and held it up to a bright light. I could not see the light looking from the back of the mirror.

 

Just saying the coating could have exhibited seeing a bright light from the back from the start.

 

Certainly if the mirror used in a in a salt air environment (LA) could be susceptible to damage if the coating fails. Maybe the salt air eats away the aluminum and the overcoat.

No question about salt air, but the environment of use is at high altitude or the desert.

It's far more likely the sulfur compounds in the air and alkaline dust mixed with slight dew, in my case (and perhaps a poor quality initial coating).

 

But a 1% deterioration per year is lass than all the coaters guess (estimates rise as high as 3% per annum.

But the only on-line test of reflectivity over time I've seen is:

http://articles.adsa...000303.000.html

and it shows somewhere between 0.33% and 0.50% reflectivity (in our scotopic vision range) lost per year, with frequent cleaning (MUCH worse without cleaning).


Edited by Starman1, 26 September 2020 - 02:58 PM.


#9 Asbytec

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Posted 27 September 2020 - 05:07 AM

Great report, Don. Looks like a regular "wash" cleaning schedule is best, and contrary to my understanding of washing infrequently and only as needed. I cannot help myself, though, due to the dust in the local area. So, I wash it more than a few times a year. I hear water, however, is not good for the surface or substrate. So, I guess it should be dried soon after a wash. 


Edited by Asbytec, 27 September 2020 - 05:19 AM.


#10 Starman1

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Posted 27 September 2020 - 09:16 AM

Great report, Don. Looks like a regular "wash" cleaning schedule is best, and contrary to my understanding of washing infrequently and only as needed. I cannot help myself, though, due to the dust in the local area. So, I wash it more than a few times a year. I hear water, however, is not good for the surface or substrate. So, I guess it should be dried soon after a wash. 

I rinse it at an angle so the water runs off, then tip it so only a few tiny drops of distilled water are left, then place the mirror on edge on a towel and let dry.

A few drops of distilled water evaporate almost immediately.

It may be harmful to let the mirror soak in water for a long time, but not for a few moments and only a few drops.

 

The issue is tap water.  In my home, as in many places, the tap water contains dissolved minerals and salts.  These can easily interact chemically with the aluminum through the tiny

pinholes in the overcoating, so it is not a good idea to let the mirror sit in tap water, even when cleaning.

I've started using distilled water for the soapy bath as well as for the rinse to avoid any problems from the minerals.  We all know that tap water can leave spots when it dries.

Distilled water doesn't. 

 

And dew in many city environments is mildly acidic due to the presence of SO2 in the air, and ozone is a hyper oxygenator, so our mirror coatings are doomed in the long run.

At least aluminum oxide sticks to the aluminum, unlike iron oxide to iron.


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#11 Markovich

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Posted 27 September 2020 - 07:18 PM

Same here as Don, I wash my Obsession mirror about once a year, depending how cloudy it looks. I use 2 gallon jugs of distilled water. One jug mixed with ONE drop of original dawn ( no fragrances) and liberally pour it over the mirror. Then, tipped on its side, I rinse it thoroughly with the other jug of distilled water. Then it sits on my dining room table and dries off before I place it back in the scope. This has worked fine for years.


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