[ATM] other Schmidt equations

[Vladimir Galogaza]

Dominic, Richard,

>   4)  86.6% vs 70.7% neutral zone


Rutten & Van Venrooij  (Chapter 21.11)
"Often the makers of Schmidt correctors wishes to minimize
 the volume of glass that must be removed. This is achieved  
when the edge thickness of the corrector is the same as
the center thickness. For this case.........the position of the 
neutral zone follows"

                                  NZ=sqrt(2*(Dc/2)^2 /Dc^2) = 0.707

So this is the meaning of 0.707 versus 0.866 choice. 

Regards
Vladimir.