Re: [ATM] Calculate Astigmatism

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

Dave,

> Another alternative would be to use a pinhole, and rotate the direction
> that the KE moves to intersect the image.  Horizontally for horizontal
> zones, vertically for vertical zones.  If pinhole and mirror are not
> moved, then one might measure on-axis astigmatism this way.  I imagine
> a mask with four apertures at the edge at 12, 3, 6 and 9 o'clock.

The problem with this is that one the mirrors you really want to test
for astigmatism (i.e., thin mirrors), there's quite a lot of test stand
astigmatism oriented in the vertical direction.  Often, this will be in
excess of 1/4 wave.  By doing all your tests across a horizontal diameter,
this effect can be minimized.  You are still measuring a deformed mirror
(ROC lengthened for the horizontal diameter), but it should be deformed by
about the same amount for all placements of the mirror.
-- 
Steve Koehler
koehler@securecomputing.com


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