[ATM] optimal thickness of Schmidt correctors
vla at copper.net
Sat Jan 14 01:58:54 JST 2012
> 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
negligible, since it is the relative wave retardation at a given zone what
The optimum thickness is the minimum thickness that will provide desired
The effect of spherical sag can be raytraced in OSLO by adding an equal
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
respectively, "f" and F the mirror f.l. and focal ratio, and "z" the neutral
location (0 to 1) - the effective radius for the surface with Schmidt curve
a sum of the radius added and the corrector radius Rc. So if the radius
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
aspherized in OSLO sign convention.
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