Re: [ATM] Astigmatism and Ronchi questions

["Mel Bartels" <mbartels@bbastrodesigns.com>]

> 1.)  Why does the ronchi reveal good lines, smooth curve with no
> orientation flicker?

The ronchi is insensitive to 'stig unless you are at the radius of curvature
for that band.  Then you pay attention to rotation of the bands, as opposed
to the expected expansion/contraction.

> 2.)  Empirically,  how severe is the astigmatism?

In the magnitidue of 1 wave.

> 3.)  The correction for astigmatism is to drop back to fine grind and fix
> the curve, then repolish.  Could I polish out the  astigmatism if I could
> locate the zone of error?

Yes, don't go back to fine grinding just yet.  See if you can figure out the
high spot, and work that only.

> 4.)  Would a Foucault shadow reveal the zone / zones in error and give me
> some idea of the severity of the deformation?

About as accurately as the Ronchi.  You can try placing the 'stig right on a
zonal mask, but it's hard.

> 5.)  Could this just be a sag / mounting issue?

Unfortunately, I doubt it, since the error is well away from the edge.  Of
course, the 'stig could be caused by how it's mounted during polishing...

> 6.)  Why is the error only seen with finer grids?

The coarser ronchi screenings just are that - too coarse.  Eventually
inteference and diffraction effects limit you, but you can try 200 lines per
inch.

Any time you wish to bring it down to my home, I'll be happy to talk about
it with you.

Mel Bartels

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