Re: [ATM] other Schmidt equations

["vladimir sacek" <vla@toast.net>]

Richard F.L.R. Snashall wrote:

> In the same area, it mentions the desire to use the 86.6% zone
> to minimize color error.  However, there is reason to believe
> that this does not minimize RMS wavefront error; and that it
> will be minimized by using the 70.7% neutral zone -- as much
> contrary to intuition as that may sound.

Nothing to do with intuition, just plain facts. When they say 
"minimizing color error" they actually mean minimizing the blur size. 
Hard to understand why, since even little birds know for some 
time now that blur size and magnitude of optical error are two 
entirely different animals both, in general, and with spherical 
aberration in particular. 

Position of the neutral zone determines a common focus for all 
wavelengths. If it is 0.866 radius, common focus is at the circle 
of least confusion. This is not the best focus location for spherical 
aberration, having more than double the RMS error of the best 
focus for all wavelengths. It is the RMS wavefront error that 
determines the magnitude of energy spread, hence also the "color error", 
not the geometric blur size.

On the other hand, neutral zone at 0.707 radius brings best foci 
for each wavelengths to a common focus. This truly minimizes the error. 
It is also, as mentioned, the easiest to make corrector shape. 

More details at:
 http://www.telescope-optics.net/Schmidt-camera.htm

Vlad
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