Re: [ATM] Calculate Astigmatism

["Stephen C. Koehler" <koehler@securecomputing.com>]

Mike,

> OK, I've posted some pictures that I hope will show why this doesn't work.

Wow!  I find this very unintuitive.  I tried this myself, comparing knife
edge readings for 1/2 wave P-V of astigmatism, 1/4 wave P-V of spherical
aberration, and a perfect mirror.  The astigmatism readings lie right on
top of the readings for the perfect mirror.  See

    http://www.visi.com/~mkoehler/ke_readings.png

I guess what's happening is that on its primary axes, astigmatism acts like
a defocusing error (i.e., a change in the radius of curvature).  The change
in the ROC shifts the knife edge readings, but does not change their 
relative spacing.  Therefore, when you normalize to the inner reading, most
of what you are trying to detect thrown out.  This is why you need to do
both sets of measurements using the same setup, so the change in ROC can
be detected.

-- 
Steve Koehler
koehler@securecomputing.com


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