This page contains information that people often ask
        about their newly coated mirror.

    Never touch any optical glass surface with your bare finger. A mirror coated with aluminum has a soft surface which is very easily scratched. Also the oils from your finger are slightly corrosive and will react with the mirror coating. If you unwrap your new mirror in a dusty area the mirror may have a few bits of dust on it within seconds. Resist the temptation to blow or wipe the dust off. Either ignore the dust or buy an ear syringe to huff the dust away with air. Alignment of your Newtonian Telescope will be aided by sticking an adhesive paper dot to the center of your mirror. When storing your telescope cover the top with a shower cap or a piece of aluminum foil. Do not store your telescope near swimming pool chemicals. Do not store the telescope with the tube straight up and down because dust will settle on the mirror. At the star party you may spend the night with your telescope outdoors. Before retiring for the night be sure the telescope is turned so that at sunrise the sunlight will first strike the bottom of the mirror cell. If you do not do this the moisture from the tube can evaporate in the warm sun and frost your cold mirror. 

CAUTION  Never turn your telescope near the direction of the sun. Someone may be blinded by the concentrated light of the sun or the tube may be set afire.

Mirror cleaning

    A dusty mirror can perform so that the average person at a star party won't be able to tell. So before embarking on a mirror cleaning project make sure it is worth the trouble. First wash the mirror under the hose to wash away sand and grit. Note that your mirror will be much happier laying face up on damp grass than in the kitchen sink. Just watch out for the sun's reflection. Next immerse the mirror in dilute dish washing detergent in warm water in a plastic pan. Wipe lightly with paper towels to remove grit without scratching the soft aluminum. If the coating is new you may want to pre soak the paper towels. Wash and rinse with warm water. Final drying is most critical. All drops must be absorbed into paper. Blow drying will leave drop marks. Use caution because stiff paper may scratch the coating and soft paper can leave lint. Blotter paper may be pulled across the mirror for drying. If the coating is new do not press down as you pull the paper. At a star party a friend removed the water from his mirror with 70 percent rubbing alcohol and dried the mirror with toilet tissue. If the mirror has been painted then drying the mirror with alcohol or acetone may leave streaks on the mirror. If the mirror is a small diagonal it may be necessary to warm the alcohol in the sun before use. If the telescope has a dust cover and dew is kept from the mirror the mirror should not need to be cleaned for several years. A small amount of dust on the mirror is best ignored or you can huff the dust off with an ear syringe. 
More about mirror cleaning

Beyond mirror cleaning

    Eventually the coating will become dull even with the best of care. It may take ten years or longer before the average person can notice a loss in the image. Salty or acid air will deteriorate the coating sooner. The mirror will then need to be recoated. 

De-coating in preparation for recoating 

    De-coating in preparation for recoating is best done by Aluminum Coating.



Aluminum coating is done by Aluminum Coating in a high vacuum tank. Aluminum metal is evaporated from a wire near the bottom of the tank and coats the mirror which is being rotated at the top of the tank. A very thin SiO transparent coating is coated over the aluminum coating. The mirror never becomes hot. A recoated mirror looks just as good as new.  Information on installing the mirror.

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----- Original Message -----
From: coating_customer

I have vintage 8 from a dobsonian that needs recoating - here's a picture,
if you think there is hope. Ignore the reflection of clouds and tree.
Please let me know. thanks, J. W.

I see a lot of old mirrors that look like this.
They look like new after I'm done.
Occasionally there is one that the old coating
cannot be completely removed but not often.
Should work in the telescope in any case.
Bob Fies

----- Original Message -----
From: coating_customer
Sent: Thursday, July 27, 2006 1:13 PM
Subject: RE: Aluminizing

Hi Bob,  

Thanks for your prompt aluminizing service on my telescope mirrors.  

I finally got around to installing, collimating, and testing them,
and while the images are much improved they still fall far short of their original acuity. 
Stars de-focus into pillow-shaped squares instead of concentric rings.  

I'd appreciate your advice.

( Editor note, mirror had been in a house fire. )

The problem you describe is not typical of a coating problem. 
Could it be caused by bad collimation?
Coating problems might more likely result in loss of contrast.

Be sure when star testing to get something comfortable
to sit on so your head is always aligned the same
way for each comparison.  Otherwise you will have a
problem with two unknowns.  If your eye glasses have astigmic
correction then when your head is tilted the axis of astigmatism
will not be correct because your eyeball will rotate
relative to your head ( and glasses ).
Try rotating the eyepiece first since that is the easiest,
also try a different eyepiece since there might be
some unknown damage to the eyepiece.
If you are fairly certain the problem is not in the eyepieces
then look at a double star, e Lyre for instance, and note the
diameter of the abberated star image compared to the
dark separation between the stars.  If the dark separation
increases more than the abberated image with higher power
then the higher power eyepiece is of higher quality or the
limiting aberration is in your eye.

Try defocusing the eyepiece inside and outside of focus.
How does the image compare inside and outside of focus?

Check the diagonal support vanes to see if they seem

Try blowing some air into the tube with a hair drier
set on cold and watch the aberration to see if it settles back
into the same pattern.

Make a drawing of the problem and tape it to the scope then put an index mark on the
primary mirror and rotate it 90 degrees and test again after re collimating
or or easier than that look for astigmatism in a stars image when focusing
inside and outside of focus.

If the diagonal is of unknown vintage and you have another diagonal,
even if it is the wrong size,
try with that.

On a Newtonian scope the things above are the most probable things that can
account for your problem other than some type of atmospheric refraction near
the horizon. 

The old Amateur Telescope Making books talk about warped
but I have not found any of these yet.  Maybe modern
glass is better annealed.  
Removing an old coating with a polishing lap is a good way
to remove a stubborn coating such as a coating with a chrome
or nichrome primer coating but could easily cause the aberrations

Bob Fies


Woo-hoo!  We finally finished, after 2.5 years, and the telescope is

As a word of warning to those still in process, though, do not get lazy
in your polishing.  Our mirror has a powdery look, and is not as shiny
as ones I have seen in the past.  Bob said this was because we had
insufficiently polished the rascal.  Now, I am regretfully considering
re-polishing, but you can spare yourself this anxiety by doing it right
the first time. 

Happy holiday wishes to all,

-----Original Message-----
On Behalf Of Kenneth F
Sent: Saturday, December 16, 2006 9:01 AM
To:; John Dobson telescope making class fall 2006;


Hi Bob,
I thought you might be interested to see the results of knife edge tests before and after you coated my mirror.  It looks like, if there was any change, it is within the measurement error. All the numbers are in mm and referenced to zone 5.
zone   |   before, 0 deg.  |   before, 90 deg.      |      after ? deg. |   after ?+90 deg.  |     ideal
1               0.11                      0.19                       0.17                  -0.05                 0.19
2               0.81                      0.71                       0.71                   0.67                 0.63
3               1.15                      1.15                       1.12                   1.06                 1.05
4               1.48                      1.41                       1.42                   1.47                 1.47
5               1.96                      1.96                       1.96                   1.96                 1.96
So it looks like the coating is very uniform!


Hello Bob,
I received the mirror and diagonal and they were in fine shape.  Thank you for the coating service.  Great job!
Season's Greetings.



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Page last edited November 20, 2016