From: "James Lerch" 
To: "ATM List" 
Reply-To: "James Lerch" 


----- Original Message -----
From: "Tom Krajci" 
> From: "John Sherman" 
>
> >..I have a couple questions about getting a 22" mirror coated.
>
> >For deep sky viewing, it seems to me that an enhanced coating is
> superior.
> >It seems that the image is not only brighter, but that there is also
> better
> >contrast. Is that correct?
>
> Enhanced coatings do reflect more light...but it's only about 5-10
> percent more than aluminum coatings.  Are you sure you can see a 5-10%
> difference?

Greetings Gentleman,

I'll chime in here with some observations I've made using my Bino-scope
with regards to Silver Vs Aluminum Alloy (Reynolds Wrap) Coatings.  The
observation is pretty simple, I can't SEE a difference.  M1, M33, M42 all
look the same, I am unable to SEE any change.

However there maybe one difference I can detect, and that's only during a
star test.  With Silver and a de-focused bright star, the rings appeared
kind of 'Fuzzy", with the Reynolds Wrap the rings are more contrasty
(This might also be a figment of my imagination, so Take it for what's its
worth!) My theory on why this is so, relates to how the coating was
deposited. Silver being chemically deposited did not conform to the surface
as well as the evaporatively deposited Aluminum Alloy.  However I reserve
the right to be wrong :)

While I was building my evaporative coating rig, I also did a lot of
research on coating characteristics.  For some good reading, check this
link out:
http://www.oriel.com/down/pdf/12010.pdf

Correct me if I'm wrong, but below is my summation of various coatings and
their intrinsic performance.

Typical reflective coatings come in 4 flavors,

#1 Single metallic layer
#2 Single metallic layer, Plus Single Dielectric overcoat
#3 Single metallic layer, Plus multiple dielectric overcoats,
#4 Multiple dielectric coatings on a transparent or metallic substrate.

Each flavor of coatings have unique upsides and downsides with regards to
performance,  again I'll try and summarize what I've read.

#1 Single metallic layer
Upside:
    A) High reflectivity over the largest frequency range
    B) Simple to apply (relatively speaking)
Downside:
    A) Fragile, easily damaged during cleaning
    B) Tarnishing reduces lifetime (For amateur use, this is a questionable
when applied to Al, IMHO)

#2 Single metallic layer, Plus single dielectric overcoat (usually MgF2 or
SiO)
Upside:
    A) Increased Durability
    B) Longer useful lifetime
Downside:
    A) Lower reflectivity than #1
    B) Overcoating needs to be 1/2 wave thick.  Since a 1/2 wave measurement
means nothing with out relating to a specific frequency, One person's 1/2
wave is not necessarily another's 1/2 wave :o
    C) More difficult to Apply (More Scatter? {Shrug})

#3 Single metallic layer, plus 2 dielectric overcoats of differing
refractive index(usually SiO + Ta2O5 or HfO2) Upside
    A) Incredible reflectivity
    B) Durable
    C) Selection of overcoats can adjust the frequency where peak
reflectivity occurs
    D) Higher contrast due to the non broadband reflectivity? (An
assumption!)

Downside:
    A) Peak reflectivity is not broadband
    B) The Selection of overcoats and their thickness can significantly
change performance characteristics (The point being you NEED a highly
skilled operator to properly apply such coatings)
    C) Diffuse reflectivity (Scatter) is most likely a function of the skill
of the operator
    D) Possibly subject to light polarization effect and angle of incidence

#4 (the mother of all coatings) Multiple Dielectric coatings on either
transparent or single/multi layer metallic substrates Upside
    A) Can have incredible reflectivity over a Narrow Band or Broad band
    B) Very customizable
    C) Freaking Incredible Durability
    D) You gotta see page 8 of this document: (the whole thing is pretty
cool)
             http://www-eng.llnl.gov/silver_mirror/pdf/URCL-JC-136508.pdf

Downside:
    A) Really screws over the KISS principle
    B) So many variables, its amazing anyone is capable of doing it :)
    C) A UV-shifted, Broadband, High reflectivity coating consists of 15
layers of 7 different ingredients (WOW!)

> The $64 dollar question.  How does the typical ATM measure scatter from
> an optical surface or coating?  One test was described by Suiter (in
> ATMJ?)...that tested the entire telescope system contrast (baffling,
> dirt, coating scatter, etc.).  It required a CCD, computer, and an
> inexpensive light trap.
>
> Who has conducted this (or similar) tests?
>
> I'll make a bold, daring prediction:  fewer than ten, repeat ten, folks
> on this list will reply that they have conducted this sort of test.

Heck, I'll wager your prediction is overly optimistic :)  So lets solve
this problem.  If we can come up with a testing methodology, I can produce
"Ugly" metallic coatings to test it with!

Take Care,
James Lerch
http://lerch.no-ip.com/atm (My ATM Fabrication, Testing, and coating site)

--- BBBS/NT v4.00 MP