Re: [ATM] Supporting thin mirrors for testing

[Mark Cowan <toolontop@yahoo.com>]

--- Ken Lowther <hermit@outofoptions.org> wrote:

> How are the backs of the mirror prepared?  Banchard
> ground?  

Either that or float plate (ie, on the order of .001"
flat over inches).  I did have one WB 12.5" blank with
serious oval back thru here, it had to rely on
structural integrity alone.

>What kind 
> of board
> do you use that it is that flat? 

The absolute contact surface doesn't seem to matter as
much as that the back is prevented from bending back
because the bottom of the mirror can't move forward -
I think I left that point out.  But I am going to make
a more elaborate stand possibly using my favorite
grinding turntable material (Corian countertop), or at
least 3/4-1" machine sanded plywood to get that
variable out of the picture. 

> Or am I reading too much in?  

No.  It surprised me to get such good repeatable
results out of such a simple support, in the
beginning.  Improvements/suggestions/brickbats are
always welcome. ;)

Is there good
> contact between the mirror and board over the entire
> back and how do you
> determine that?

Pretty good - I press it against the board while
sliding it down onto the supports.  Obviously the
board can flex some under the forces, this is why a
heavier setup may work better.  But it's surprising
how little it takes to reduce flexure.

Cheers,
Mark 

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