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
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
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.
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 -----
Sent: Thursday, July 27, 2006 1:13 PM
Subject: RE: Aluminizing
for your prompt aluminizing service on my telescope mirrors.
finally got around to installing, collimating, and testing them,
while the images are much improved they still fall far short of their
Stars de-focus into pillow-shaped squares instead
of concentric rings.
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
to sit on so your head is always aligned the
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
relative to your head ( and glasses ).
Try rotating the eyepiece first since that is
also try a different eyepiece since there
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
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
Try blowing some air into the tube with a
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
then put an index mark on the
rotate it 90 degrees and test again after re
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,
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 old Amateur Telescope Making books talk
mirrors 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
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,
On Behalf Of Kenneth F
Sent: Saturday, December 16, 2006 9:01 AM
To: nancy@.com; John Dobson telescope making class fall 2006;
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. |
So it looks like the coating is very uniform!
I received the mirror and diagonal and they were in fine shape.
Thank you for the coating service. Great job!
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Page last edited November 20, 2016