From: "James Lerch" 
To: "COLJA, Mike" , "ATM
List" 
Reply-To: "James Lerch" 


Hi Mike,

Well, I know for certain that I have SEEN several of my Silver coats that
were 'LUMPY'..  Especially visible when viewing a bright light source thru
the coating...

When I visited the WSP in '02 one mirror was 'smooth' and the other was
'lumpy looking' (whatever that means) however I could not see any difference
between the two in the eyepieces.

As far as I could tell, the 'lumpy' ness was small in nature, and looked
evenly 'lumpy' across the surface, so I doubt it effected the overall
figure..

However, I imagine the lumpy mirror would not have been able to hold the
power if I had had any power to try out that week ;)

In theory a Chemically deposited silver coat 'should' exactly replicate the
surface, with even thickness through out. (in theory!)


Take Care,
James Lerch
http://lerch.no-ip.com/atm (My telescope construction,testing, and coating site)
----- Original Message -----
From: "COLJA, Mike" 
To: "'James Lerch'" ; "ATM
List" 
Sent: Thursday, March 27, 2003 4:59 PM Subject: RE: ATM coating affects figure?


> What about chemically deposited silver?
> Mike Colja
>
> -----Original Message-----
> From: James Lerch [mailto:jlerch1{at}tampabay.rr.com]
> Sent: Friday, 28 March 2003 04:37
> To: ATM List
> Subject: Re: ATM coating affects figure?
>
>
>
> Hi Michael,
>
> The simple answer to your questing is YES!  In practice I would HOPE that
> professional coating shops like Clausing and Spectrum have setup their
> chambers so that the effect is minimized.  (BTW, I'd love a tour of their
> chambers! :)
>
> As far as empirical data,  I've read that a nominal single layer metallic
> coating is ~100nm thick
>
> In playing with my home made coating system here's some experiences I've
> had:
>
> #1 A single on-axis evaporator 10" from mirror surface (on a 10" F/5)
should
> have caused a coating thickness change of ~19%, with the center being
high.
> Assuming a reference wavelength of 550nm, and a center coating thickness
of
> 100nm, that would mean the surface figure was altered (19/550 = 1/29 p-v
> surface = 1/14.5 p-v wave-front)  Visually and on the Robo-Foucault this
> pretty much appeared to be what happened..
>
> #2 I now use a 12" diameter ring of 6 filaments, about 5" above the
mirror.
> This results in ~1.5% thickness deviation IF I keep the mirror surface 5"
> away from the plane of the ring.  If I move the mirror closer to the ring,
I
> add additional material to towards the 70% radius of the mirror.  Moving
the
> mirror further away adds more material towards the center of the mirror.
I
> have been known to use this trick to 'bully' a figure around a little (a
> very little!)
>
> #3 My next chamber will have two rings of filaments:
>     A) Ring 1 will have a diameter of 12" and 5 points
>     B) Ring 2 will have  a diameter of 24" and 10 points
>
>     The plane of both rings will be ~4 inches away from the mirror
surface,
> thus affording the option of only needing a 24*24*8" rectangular chamber
> that will have a ~3% coating thickness change (if all my math is
correct...)
> The up side to this arrangement is keeping the chamber small, shortening
the
> mean free path between filaments and surface, at a cost of having to load
> and fire 15 filaments for each coating!
>
>
> If your really bored, I have written a 'Near Field Evaporative Simulator'
> that I use to help me figure all the above data out.  Your welcomed to
play
> with it by downloading it from here:
> http://lerch.no-ip.com/atm/nearfieldsim2.zip (10kb) This version will do
up
> to 3 Evaporative rings, and models up to a 150mm radius mirror.  I have
> another version that will do up to 600mm radius mirrors, but its a CPU/MEM
> hog from hell!
>
>
> Take Care,
> James Lerch
> http://lerch.no-ip.com/atm (My telescope construction,testing, and coating
> site)
> ----- Original Message -----
> From: "Michael Burr" 
> To: "ATM list" 
> Sent: Thursday, March 27, 2003 10:01 AM
> Subject: ATM coating affects figure?
>
>
> >
> > Greetings:
> >
> > I've searched the archives and found little information about this, so I
> > thought I'd ask the list.
> >
> > Does anyone know of empirical information regarding the thickness of
> > aluminum coatings, and how variances in thickness from the center to the
> > edge of a mirror might affect the mirror's figure?
> >
> > I've seen oblique references to aluminum coating being 1/2-wave thick.
If
> > that's the case, and there's a 15 percent difference in coating
thickness
> > from center to edge on a 16" mirror, does that mean the coating
> effectively
> > undercorrects the figure by 7.5% (i.e., 1/2 times 15%)?
> >
> > If so, would I be completely nuts to try and "tune" the
figure to the
> > coater? In other words, I could try to overcorrect the figure by 7.5%.
It
> > seems to me a better solution would be to use a coater that distributes
> > aluminum more uniformly, but it's an interesting theoretical question
> > anyway.
> >
> > Thanks
> > MTB
> >
>

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