Re: [ATM] Mirror cell ramblings re: thin plate glass3/4-inchx24-inch

["Bob May" <rmay@nethere.com>]

The overwhelming in making a suport is more in the monototy of
making all of the parts to the right size than about anything
else and a good mill will take care of that problem with the
exception of the time it takes to do.  Since the actual
dimensions of the holes isn't that critical (it is nice to do
something perfectly but being off by a few thousandths of an inch
won't hurt the quality of the support that much) even a decent
job of layout and driling the parts on a drill press in a lump
group will do just fine.
The problem with the RTV is that it is basically a spring
material and will apply forces in all directions (it is adhered
to both surfaces, isn't it?) and the only force you want on the
glass is the gravational support force.  The failure to support
the aluminum properly (go to town with PLOP again with the
rigidity properties of aluminum and you'll see the fun in that!)
will just be transmitted through the RTV to the glass and you'll
be in trouble again.
Bob May

rmay at nethere.com
http: slash /nav.to slash bobmay
http: slash /bobmay dot astronomy.net

----- Original Message -----
From: Ted Cohen <tcohen@blakeglobal.com>
To: <atm@atmlist.net>
Sent: Tuesday, May 27, 2008 8:34 PM
Subject: Re: [ATM] Mirror cell ramblings re: thin plate
glass3/4-inchx24-inch


> All -
>
> Thank you for that background information and guidance. The
reason, I'm
> being somewhat obstinate, is that I've already put about one
year into this
> mirror and all of the related setup, so I want to be fairly
certain that
> what I was intending isn't feasible. At the risk of sounding
obstinate, I
> would just like to clarify a few points that seem to address
several of the
> important concerns raised.
>
> 1)  The glass/aluminum interface - Several have pointed out the
inefficacy
> of mounting the glass on aluminum - I agree 100% - however,
....  In my
> earlier email describing the setup, I mentioned a thin
membrane, which will
> come between the glass and aluminum. I am/was intending to
create a 2mm
> layer of (some variant of  RTV) directly onto the aluminum
cell. After the
> RTV sets, I will lap this against the back of the mirror to
remove gross
> non-conformities. I may even use the RTV to "glue" the glass to
the
> aluminum, thus providing a perfectly continuous joining of the
two peices.
> The RTV (or some other elastomer) is used to create conformity
of the two
> surfaces, and make a good, continuous contact with the glass.
Any remaining
> print through would be removed during polishing and grinding.
None of the
> comments take this into account. Would not a layer of RTV
and/or optionally
> a second layer of some other cement/resin/polymer/elastomer
(depending upon
> the desired properties) address the "contact" issue?  It seems
that a good
> optical surface quality contact between glass and aluminum is
possible using
> a combination of steps including lapping and using the proper
> adhesive/sealant. After all, mounting lenses on aluminum frames
is done for
> refractors, microscopes, and wave guides?  I think the ATM
community has
> gone overboard on the use of finite support points, and I think
the reason
> for this has been the valid need to provide good air-flow over
thick pieces
> of glass.  However, my glass is not thick, and I plan to apply
a version of
> RTV(or similar elastomer) which has a low coefficient of
thermal
> conductivity to thermally isolate the glass from the cell
(again due to the
> advantage of  having a thin piece of glass).
>
> 2)  Flexure in the aluminum cell. -- My plan was to account for
the
> flexibility in the aluminum plate by carefully designing a
mount for the
> cell. I was intending to mount the aluminum cell using an
hexagonal ring
> support at the 70% zone of the aluminum. The hexagonal ring
will attach to
> the aluminum cell via a series of horizontal and vertical
struts bolted to
> the back of the aluminum and will attach the cell to the tube
assembly. This
> should, it seems, give me at least the strength and rigidity of
some of the
> wood materials used typically in Dobsonian designs, and even
the stiffness
> of the tailgate designs utilizing aluminum tube, No?
>
> 3)  Thermal Considerations - In my original email I addressed
all of the
> relevant thermal issues point by point, so I won't reiterate
them. It looks
> like there are techniques available to attach an optical
component to a
> mechanical device (using the appropriate
cement/resin/elastomer) even if the
> rates of thermal expansion of the two pieces are different. It
requires
> calculation of the different rates of thermal expansion, etc...
It doesn't
> seem to be a show-stopper. For example, an elastomer will
absorb a good deal
> of the differential expansion of the two materials. It is noted
that RTV is
> sometimes used precisely for this purpose.
>
> 4) Finite Element Analysis - I have no doubt that this is an
excellent tool
> for analyzing mirror cell design parameters, particularly those
of large
> diameter mirrors; however, due to the thinness of my glass, I
do not believe
> that I want a discreet set of support points. The mechanical
design of 27 to
> 36 supports is too overwhelming. I believe that while,
theoretically, PLOP
> will give me the most accurate answer, in practice my
implementation of
> PLOP's suggestions will be worse than a more simplistic design
would give
> me. I don't want to sound too disparaging (it might already be
too late
> though) because I know how important PLOP has been in the
evolution of
> ATM'ing.
>
> Please feel free to shoot me down to size as necessary.
>
> Ted.
>
> _______________________________________________
> ATM mailing list http://www.atmlist.net/
>

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