Re: [ATM] Calculate Astigmatism

[Woodchuck <djv@bedford.net>]

On Wed, 24 Mar 2004, Stephen C. Koehler wrote:

> > throwing away the ability to make differential measurements to the required
> > accuracy.
>
> I don't understand the need to not disturb the apparatus over a mirror
> rotation.  Couldn't you use the measurement of the center zone on each data
> set as a baseline, or at least make the assumption that your innermost zone
> is not much affected by astigmatism?  This would eliminate the need to
> maintain the test rig distance over a mirror rotation.

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.

A certain precision in construction of the KE apparatus would be needed,
though, otherwise a "re-zero" step with the central zone would be needed.
It would seem easier to construct a rotating knife edge than a rotating
mirror.  Maybe it could be done optically, with a Dove prism?

Dave

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