[ATM] Re: Second Thoughts on Mirror Support

[jonah at eecs.berkeley.edu (Jeff Anderson-Lee)]

Jeff Anderson-Lee wrote:

> Here's a scenario that seems promising:
>
> 16 by 1+5/8 inch f/4 pyrex mirror
> 9 RTV pads: 3/4 inch diameter by 3/16 inch thick
> 1/2 inch steel annulus back plate 12.64in diameter with 2in central hole.
>
> The P-V on the thicker back plate is reduced to 1.22e4-4 mm (4.79e-6 
> inches). By monitoring the contour lines drawn by plop and shifting 
> the plate support
> points we can control the relative flexing at the mirror support 
> points to give
> us another factor of 5 or so versus the P-V, bringing us down to 1e-6 
> inches
> of deformation.  The net force for a 1e-6" deflection on the smaller 
> but thicker
> pads  is 0.20 lb, which is safely under our 0.67 lb limit.
>
> It doesn't leave a lot of wiggle room for differential expansion, so 
> I'd probably
> stick to using this for steel with plate glass, but nonetheless, it 
> just might work!
>
> Of course, now you have a huge heat sink sitting 1/8 in behind the 
> mirror, but
> at least with the central hole you could put in a muffin fan to 
> provide a flow of
> ambient air to improve cooling.

I ran the numbers for an 8 inch mirror I have as well.  It also 
seemingly needs a half-inch steel plate underneath to keep the plate's 
flexing within tolerance.  The half inch plate weighs 5 times as much as 
the mirror!  YUCK!

I'm stumped as to why Don's cell works as well as it does, unless those 
turned down/turned up edges are giving it much more rigidity than we are 
crediting it.

Given that this technique seems to require such a thick steel plate I'm 
tempted to shelve the idea, however another thought came up after 
talking off-line with Jean-Guy Moreau <jgmoreau@cablevision.qc.ca> and 
Nils Olaf Carlin <nilsolof.carlin@telia.com>:

Suppose instead of a THICK steel plate you use a very THIN plate--even 
sheet metal.  You sandwhich three layers: the metal, some plush 
carpeting, and an underlying support (e.g 1.5 inches of cured/dried 
plywood sealed against humidity changes).  Use a torque driver and 
screws to make sure all of the connecting bolts have the same tension.  
Put a central hole in the sandwhich so that air can be forced between 
the mirror and the back plate using a small muffin fan. Then, RTV the 
mirror to the metal using multiple, small area pads. 

The hope is that the sandwhich would give similar all-over support as 
the carpet used to grind/polish the mirror on, yet have airspace for 
forced ventilation, while the RTV avoids the need for a sling.

Thoughts?

Jeff Anderson-Lee
Sacramento, CA