Telescope Mirror Re-Coating

This mirror has lots of pinholes. We never see pinholes after re coating a mirror. I suspect that they may be caused by salt air. If the mirror is kept in the damp in this condition for a few years the mirror at the edge at the pin holes may be marked by a reaction with the aluminum. If you want to preserve a mirror like this without recoating I would wash and dry it then seal it with something. Possibly cover it with acid free paper or cloth then seal it in aluminum foil.

Other mirrors we get for re coating appear to have been spray painted with aluminum paint. We think this is caused by trying to clean the mirror with vinegar. Sometimes the mirrors have spider web or snail tracks. These need to be cleaned with paint thinner or maybe even ethyl acetate before removing the old coating.


This mirror is more typical of mirrors we see that are ready for re-coating.  The hazy film scatters light reducing contrast of fine detail in the image.  If the haze is caused by plant resins or nicotine and dust then cleaning will often help.  If caused by pool chemicals then cleaning won't help.
This coating is almost three years old but deteriorating faster than expected.  The cardboard tube for this scope had been painted with the green wood preservative which supposedly never dries.  The scope was stored in the vertical position and apparently over time some of the preservative settled on the mirror and polymerized with inclusion of dust.  Holding the mirror up to a bright light as a sun filter shows the aluminum was corroded as in this page background.  Cleaning did not help.  When the marker pen marks were removed with methanol the coating where the marker pen marks had been was not deteriorated.  Also no deterioration was noted on the diagonal mirror.

 A week after recoating and installing we tested the telescope at Fremont Peak on a night with excellent seeing.  The mirror focal length is 62.5 inches and we used a  2 x  'Shorty Barlow' with an old Edcorp  9 mm eyepiece giving about about 350 magnification.  The diagonal is a good Edmund diagonal. Testing on e-Lyre, the double double, the resolution appeared to be less than 1 arc second.  A mottled first diffraction ring was visible around the Ary disk at best focus.  Resolving the double double was not a real challenge for the scope, more challenging was keeping the image in the field of view with the Dobsonian mount pointed almost straight up.

From: Coating customer
Sent: Tuesday, November 03, 2015 5:59 PM
To: Bob Fies
Subject: Re: 16 inch mirror

The mirror is about 12 years old. It seems to be in pretty good shape.
Does the coating deteriorate with age or does it last indefinitely?


The condition of an old coating depends a lot on how it was stored.
If it was kept covered and out of the weather all that time it might
look like new. Old coatings oxidize over the years and become
harder and also more difficult to remove.
Various things can deteriorate the coating faster.
Dew is often a factor. If there is any type of acid in the air
it will deposit on the mirror with the dew.
Other factors, storage with pool chemicals, salty ocean air,
vapor from green wood preservative, some types of paint
if not allowed to bake out for some time.
If the mirror is left out in the rain for several years it can be
almost impossible to remove the old coating.
One customer had a cat that slept in the telescope
and he failed to clean the mirror for some time after that.
Best not to let chemicals dry on a mirror or fail to
wash them off.
If space restrictions require storing the tube straight up its
best to make a cap for the mirror of lint free acid free paper
such as lens paper held in place with aluminum foil.
More info at:
Bob Fies

At one time observatories re-coated every two years.
Then it went to three years and in some cases six
years. They don't seem to have any problem de-coating
with 'green river' ( HCl + Copper sulfate ).
In the past I have been getting mirrors 30 years old.
These won't de-coat with HCl in most cases.
But I found FeCl3, iron chloride, printed circuit board
etchant would work. 
But now I'm getting mirrors with 50 year old coatings.
If these mirrors have been kept damp most of that
time the old coating won't come off.
If they have been dry all that time most of the old
coating can be removed and if the customer tries
the re-coated mirror in a telescope it will yet perform.

If the aluminum is thick enough it will protect the
glass so long as moisture and corrosion have
not penetrated to the Al/glass interface.
But aluminum is more reactive than the metal
ions in the glass and the Al will gradually take
their place.  Acid dew or acid rain can accelerate
this process.
When we get a mirror for re-coating its hard to
tell if it will de-coat completely and we do not
do any polishing or expensive refiguring so
we can only do our standard de-coating
procedure with possibly some adjustment for
older mirrors.
But we cannot warrantee the outcome of the
process. It usually works but not always.
Bob Fies

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Page edited February 20, 2022