Cleaning Aluminized Telescope Mirrors
    Note that dry mirror wiping and polishing are not part of first surface mirror cleaning.


Big mirrors seem to have a fatal attraction to kitchen sinks. 
Mirrors are much happier on their back on the lawn, just watch out for the reflection of the sun.
Forget about that lens cleaning kit with the little bottles of stuff.  That won't do this job.
Other things that won't work, vinegar, silver polish, and  no, never use cleanser or similar.

We will presume this mirror has been in a flood or a tornado and is covered with grit glued down with plant resins.
So water spray alone will take some of the large grit away.
Note, yes this mirror is not coated but it was available at the moment.

If the mirror edge is sharp there is almost 100 percent chance that it will get flake type chips eventually.
If the mirror will need to be re-coated that is a good time to dull the edge with a wood block and
120 grit alumina sand paper. Be sure to get rid of all grit before wrapping.

The significant items in this picture are two pans of almost too hot water, dish washing detergent, paper towels and semiconductor grade 10 methyl alcohol warmed in the sun.  And no slip shoes and be very careful.
70 percent rubbing alcohol
cotton balls
some types of toilet paper
blotter paper

Put a bit of dish washing liquid in one pan and soak the paper towel until soft.
Wipe the mirror lightly, don't press down.
Move to the rinse pan.  That was easy, now the hard part.
Carefully remove the mirror from the pan and pour alcohol over it to get rid of the water.
Tip the mirror up to get rid of excess alcohol.   A bit of pre planning helps.
Before the drops of alcohol can dry drag a dry towel sheet across the mirror to spread out the alcohol.
Don't press down while dragging the towel.  
Then put the mirror in a safe place and lay a dry paper towel on it and take a break.

The mirror's coating becomes much harder after several years making the cleaning process easier.

Sky and Telescope March 2019 issue has a good article on mirror care.
Among other good suggestions they recommend using cotton balls for swabbing the mirror.
And they recommend using only one light sweep per cotton ball to avoid scratching the
coating with grit accumulated on the cotton ball.


Return to Coating Care

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    Other cleaning procedures worth mentioning.      (  I have never used these procedures .  )
         Cleaning with Collodion
             Collodion is poured on the mirror and as it dries it shrinks and separates taking the dust with it.
                    For instructions refer to a PAS members web-site.
         People who should know have told me that some observatory mirrors have been cleaned
              using carbon dioxide snow.  ( As in CO2  fire extinguisher. )
              I have never seen this done but I can predict at least two problems.
                   The cold  CO2 will condense lots of water out of the air.
                   The cold shock might cause green glass mirrors to crack in half.
                       ( Never put a green glass mirror blank in a pan of cold water in the sun.)

         Mirror cleaning using CO2

         If CO2 cleaning might be appropriate for your project try the link:

     Email from Astronomers

Hi there. I had a 10"mirror done about 4 weeks ago and I'd like to clean a "tiny" bit in the center
 (not your fault!!).
Can I use a Q-tip and reagent grade acetone to clean or will it damage the coating
 (considering it's rather new)??

OR?? Is there a better way to clean it?
It's just a little spot of white paper that had a bit of adhesive on it.

If the telescope is a Newtonian the center would be in the shadow of the secondary.
So doing anything to the center would have no effect on the performance as a telescope.
The mirror as a piece of jewelry is a different matter,
also would affect the value if the mirror were sold.
Home supply store acetone is not anything like reagent grade.
It will leave a residue.
Also if you use lacquer thinner and let it dry there will be a film that cannot be removed with methanol.
I have been using mineral spirits to remove finger prints, pitch residue, RTV silicon, duct tape and etc.
It dries very slowly but if allowed to dry it will leave a waxy residue.
I have been using mostly reagent grade methanol and occasionally reagent grade ethanol for drying.
The ethanol is denatured with propanol.
I have found that drug store 70 percent rubbing alcohol will also work. Its propanol.
The 90 percent rubbing alcohol seems to have calcium chloride in it for absorbing the
water so is not preferred.
Common sense says distilled water should work fine
but if the drops are allowed to slowly dry they will leave drop mark.
I have never figured that out.
So the water and alcohol and mirror should be warm, at least 70 F or above the due point
or it may frost with your breath.
After you have used whatever to remove the blemish use the alcohol for drying.
Tilt the mirror up and spread out the remaining drops with
Kimwipe, cotton balls, lens paper or some types of paper towel or toilet paper.
The remaining alcohol won't be there for long and then cover the mirror with a clean paper towel
until the humidity decreases.

Avoiding drop marks when cleaning:
Presuming the coated mirror has been washed as described at:
It should have been rinsed in very warm water.
Tap water should be adequate for the rinse unless the water is heavy with minerals.
The final drying is the tricky part. I use chemically pure methanol but 70 percent rubbing alcohol from the drug store will do.
Alcohol should be warmed in the sun.
With your covid mask on pour the alcohol over the mirror and tip the mirror up to get rid of the water/alcohol mix.
Then with the Kimwipe spread the drops out without pressing down.
The mirror face will be dry in seconds.
Cover it with a clean paper towel and leave it alone until it cools and the humidity dissipates.
If you are a mirror maker remember that mirror coating is the very last thing to do after the telescope is built. Its much easier to clean an uncoated mirror than a coated mirror.
Telescope will work fine on the moon and you can test for astigmatism using an artificial star.
As to the focuser, one that places the eyepiece close to the tube is better and there are many types to chose from. Focusers made for refracting telescopes won't work well on a Newtonian telescope.
Also see:
Bob Fies