[ATM] Re: Second Thoughts on Mirror Support (Art Bianconi)

[tkrajci at san.osd.mil (tkrajci@san.osd.mil)]

>From: artbianconi@blast.net

>...Has 
>anyone considered the effects of time and exposure on all those balance 
>beams on their ability to maintain equilibrium?

A whiffletree/flotation system...by design is meant to handle 
changes/warps/etc by action through the pivots.  Look at the windshield 
washer arm on your car...it also uses a system of pivots as a "1-
dimensional" example of a whiffletree...it is able to change/adapt to the 
shape of your windshield...as long as you minimize the stiction of the 
pivots.

>The degree of torsional stiffness possible today, by the proper 
application
of 
>advanced composites, exceeds the dreams of even the most imaginative of 
>engineers 25 years ago.... 

>The geometry of the cell is too complex to describe with words alone. 
Suffice
>it to say that it will resemble a thick wheel with one row of spokes, 
each 
>having an "I" beam cross section.

That does not suffice for me.  Because "words alone" won't describe 
it...I'm looking forward to seeing draings/diagrams/photos.

>A thin (0.020" film of Teflon will sit between the mirror and the cell. 
>This 
>relatively friction free interface should allow the mirror to seek it's 
>own 
>equilibrium.

Now you have "thermally isolated" the entire back face of the mirror.  You 
will probably need aggressive use of fans on the font face to let this 
mirror track the ever-chaning air temperature.  Good luck.

>The perimeter of the mirror is attached to an outer ring on the cell 
whose OD
>is about 2 inches larger than the mirror (17"). That's done with an 
advanced 
>space age polymer developed for NASA (by Dow Chemical?) Perhaps six or 
>eight webs of that material , around the perimeter of the mirror will alow
the 
>glass to expand, contract, and achieve some degree of geometric 
equilibrium. 

Again, looking forward to diagrams and drawings of this scheme.

>No moving parts, a minimum of constraints and an optically flat,slippery 
>surface. 

How do you measure and guarantee that the surface of the teflon 
is "optically flat?"...I don't recall seeing any catalog entries for 
optically flat teflon.  ;-)

>Will it work? I think so. If it doesn't  . . . . . I'll try something 
>else.

If you are going to design a "new kind of" mirror cell, then I recommend 
starting mirror cell desing from sound engineering concepts and 
assumptions.  If you don't do that...you'll be making many more physical 
prototypes instead of many sketches on paper.  I'd prefer to waste a few 
sheets of paper through sketching, rather thay try to make prototype cells 
from carbon fiber, optically flat teflon, space age polymers, etc.  Good 
luck.

Tom Krajci
Tashkent, Uzbekistasn