Re: [ATM] COG of mirror - was astigmatism as seen via ronchi?

["Nils Olof Carlin" <nilsolof.carlin@telia.com>]

Arjan,

> However, we should not take the CoG of the cap, but that of the remaining
> glass. Location would be around 0.3*s from center thickness (C), and hence
> the CoG of the mirror is at approximately:
>  CoG = (C + 0.3*s)/2, or alternatively CoG = (E - 0.7*s)/2
>
> This answer has been suggested by Dale a.o., however with a small error.


For Richard's test case, there seems to be still another half mm or so 
difference, whereas Jim's expression agrees accurately. Knowing Jim, I'd 
trust his maths without understanding it - Jim, I know you're there, how 
*did* you do it????
Calculating backwards, the factor 0.7 should be rather a bit less than 0.5 
instead, but judging from the denominator in Jim's presumably exact 
expression, converted to E and s:
(E-s+s^2/3E)/(2-s/E), things are not quite so easy.

Half a mm won't make all the difference in practice, though...

Nils Olof 

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