[ATM] optimal thickness of Schmidt correctors

[vladimir sacek]

> I was after equation defining thickness (like which thickness I
> would purchase to make a Schmidt corrector) that gives least sag
> for a given diameter.
>

There is no such thing, because the sag will asymptotically approach zero
as the thickness increases to infinity. As already mentioned, the effect of 
sag is
negligible, since it is the relative wave retardation at a given zone what 
matters.
The optimum thickness is the minimum thickness that will provide desired
mechanical stability.

The effect of spherical sag can be raytraced in OSLO by adding an equal 
radius
to both sides of the Schmidt corrector. Since the side with aspheric curve
usually already have a radius value - Rc=32(n'-n)f(F/z)^2, where n' and n
are the refractive index of the medium following and preceding Schmidt 
surface,
respectively, "f" and F the mirror f.l. and focal ratio, and "z" the neutral 
zone
location (0 to 1) - the effective radius for the surface with Schmidt curve 
is
a sum of the radius added and  the corrector radius Rc. So if the radius 
added
is Ra, the radius for the aspherized surface is (1/R)=(1/Rc)+(1/Ra).

Since the corrector is either flat, or convex at its center, where the 
radius is measured,
Rc is numerically positive with front surface aspherized, and negative for 
rear surface
aspherized in OSLO sign convention.

Vla