From: "Dave McCarter" 
To: ,
        "ATM" 
Reply-To: "Dave McCarter" 


> The fixed or moving light source,

Not an issue, as that is taken care of in the Faucault formula.

> whether the micrometer screw is left handed or right handed.

Why would that matter?

You are simply measuring the differences between zonal readings. Get a copy
of Texereau' s book "How to make a telescope" and read it. Or Sam
Brown's book "Telescopes". It is quite clear.

>And to me it seames that you would have to know the distance from the
mirror to the Foucalt tester.

Yup. And a taut measuring tape held at one end on the mirror surface (put a
piece of paper medical tape over the end to avoid scratches) and at the
knife edge when at the center of curvature is quite sufficient. Accuracy of
plus or minus a millimeter is excellent and better than usually required.

>    Do anybody know how it works?

Yup. But rather than my trying to rewrite Texereau or Brown, or Berry, or
any of the other sources, seek them out yourself and read!

>    I haven't built one yet, but probably will have to.

Indeed. Actually building and learning to use a Faucault tester is the easy
part. Understanding what to do with the measurements and what polishing
strokes to use next to get the desired result is what takes time. Best
learned on a small mirror, if only to ease the workout of moving the glass
back and forth between the polishing table and the testing stand.

Dave

--- BBBS/NT v4.00 MP