[ATM] The standard aluminium coating: How much scatter?

[Guntram Lampert <glampert@aon.at>]

Hello to all who contributed!

Thank you all again for your thoughts and contributions.

Anthony Stillman: The graph you mentioned was instructive. If we assume
the 0.5% scatter they measure after aluminizing are correct, it would be
what I consider a worst case. The increase in scatter in about two years 
is nearly inexplainable to me. I have a 160mm primary, coated when
president Reagan was early in his first term. I still use it, with the
same coating, with no visible deterioration aside from some sleeks and
minor scratches due to infrequent cleaning. And this mirror is kept in
the open since many years, only protected from rain, but exposed to dew,
dust, fog, pollen, insects, whatever. I live far from the salty ocean,
perhaps this makes some difference.

Last night, I compared the scattered halo of light of a coated vs. an
uncoated mirror. The uncoated mirror was put on a sheet of black plastic
and some glycerin between glass and plastic eliminated reflections from
the back of the mirror. A LED flashlight served as the lightsource, used
at full brightness for the uncoated mirror, and dimmed with ND filter
and stopping-down, for the coated mirror.
The reflection of the LED was occulted by my thumb as "occulting bar".

Now, such a setup is not remotely quantitative, and questionable even
for qualitative estimations.
However, it was obvious that, after mental subtraction of the light
scattered by dust, and filmy dirt residuals, very little light is
scattered (obviously) by the coating alone. After some close looking, a 
faint
halo of scattered light could also be seen on the uncoated mirror,
likely caused by residual pits and roughness on the surface.

Personally, I now feel more comfortable with the standard coating than ever
before. It is, IMHO, not detrimental to the images of typical
reflectors. And I use a four-mirror design!
So, the scatter due to the coating is, in my opinion, one of those
telescope myths that deserve to be buried.
In fact, before aluminizing the three mirrors I made for the four mirror
telescope, I silvered them to check out the performance before having
them (expensively) coated. And the silver coatings produced really 
significant scattering!
So, I appreciate today´s standard coating even more than before I asked
the friendly and helpful members of this list!

Best regards,

Guntram Lampert







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