Re: Bug Spray Vs. Optics - dissolves coatings



chris1011@xxxxxxx wrote:

On Mar 31, 2:16 am, jerry warner <"warner(na)"@mchsi.com> wrote:
What is there in bug spray that dissolves coatings and some
paints on eyepieces? Does anyone know?

Thanks,
Jerry

There are many different kinds of coatings today, from simple
antireflection to simple aluminum reflectance coatings. The simplest
antireflection coatings consist of a micro-thin layer of magnesioum
fluoride. These can be quite soft or somewhat hard, depending on how
they were applied. If they were applied in a vacuum chamber where the
vacuum was not ideal, and if the substrate glass was not heated to a
high enough temperature, then the coating will be relatively fragile.
It can come off the glass easily and can be affected by strong
chemicals. Unfortunately, one cannot know how well the coatings were
applied by the coating lab, so it is best to treat them as gently as
possible and not let any contaminant touch the surface.

This is doubly true of aluminum mirror coatings. If they are not
applied properly and do not have a protective coating, they will be
easily degraded any number of ways, from judicial cleaning to simple
atmospheric dew settling on the surface.

Modern multi-coatings can be super rugged and hard. These are applied
in a more robust manner using ion beam acceleration, which imparts
enough energy to the particles to fuse them into the subsurface of the
glass. These type of coatings are tough enough to stand repeated
cleaning and will not allow water from dew drops to seep into the
structure (lesser coatings are porous which allows water and liquids
to penetrate and ultimately destroy the coating). Dew is especially
bad for any type of coating because it contains dissolved carbon
dioxide, and thus is a weak form of carbonic acid. These newest
coatings using ion assist implantation are pretty much impervious to
dew.

I have a large Mak-Cass permanently mounted in an observatory in a
climate that has heavy moisture content in the air, and had many
occasions of the internal optics drenched in dew, even though they are
in a closed tube. Even after 5 years of this, the coatings wipe
perfectly clean without any scratches or sleeks. Dew spots come off
clean without residue on all internal and external surfaces with
simple application of Balzer's #2 Substrate Cleaner. One of the guys
here who has a large Newtonian in his observatory can get scarcely 6
months out of his coating (simple aluminum coat from Clausing) before
it develops a serious case of measles. The only drawback to modern ion
implant coatings is the cost - approximately $800 per coating run. And
contrary to popular belief, these coatings do not change the figure of
the mirrors or lenses - my Mak-Cass tested the same before and after
coating, on the interferometer.

Rolando

Thanks Roland. This is very good. I'm gopignto keep this and
pass it around if you dont mind.
Sincerely,
Jerry



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Relevant Pages

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