From: DWilliams1128{at}aol.com
To: atm{at}shore.net
Reply-To: DWilliams1128{at}aol.com


Hello:

I was wondering how you get any accuracy in a Foucalt tester.  I understand
that it has a light source, a knife edge (to cut incoming light or outgoing
light), an eyepiece, and a micrometer.  Also if you put the light source at
2 times the focal length and close to the axis line and the look at the
same length from the mirror, but next to the light source you will see the
to any degree of magnification.  As the distance to the radius of curvature
approaches zero the magnification approaches infinity.  You can use this
setup to measure for sphericity, parobolic curve, hyperbolic, or eliptical
or another words the change in focal length or curvature in different zones
or how far from the light cone axis you are.

  There are a couple of problems.  The fixed or moving light source, whether
the micrometer screw is left handed or right handed.  And to me it seames
that you would have to know the distance from the mirror to the Foucalt
tester.  And how would you do that without using a tape measure and that
seems awfuly inaccurate.

   Do anybody know how it works?  And how to get accuracies of 1/100,000 inch
or wavelengths of light?

   I haven't built one yet, but probably will have to.  After my 20" mirror I
might build a 40" mirror and get it from Newport Glass.  I might get
the curve pre-generated from there.  Either that or I might buy their
8" refractor telescope lens kit with tool.  If I do that I'll need
dial deflector readout guage that machinists use to determine out of round
of parts.  I'd need to use that to make sure there's no wedge of the lens.

  Has anyone here made lenses before?

  Thank you for your help.
  David Williams
  dwilliams1128{at}aol.com

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