RE: [ATM] Help needed with foucault

["Jerry" <wa4guu@verizon.net>]

Greetings, Oliver...

	Well, let me chime in and say that I agree with Bob and Richard
but... This turned edge is one that you might do better Tool On Top. The
shape on the mirror is basically an oblate spheroid. This is the shape you
would expect after polishing. If you adjust the knife just slightly out from
the first image (but nowhere near as far out as the second image) and make
the center look flat instead of like a hill you will see the turn up that
starts at the inner circular scratch will look as if it turns up even more.
And then that last 3/8" or so of turns down. So you have a turn up and then
a turn down.

	If you work Mirror On Top and lengthen the stroke to remove the
central hill you are left with all of the turned edge because this method
basically speaking extends the curve of the outer zones into the center
zones. If you work TOT and take the "rim of the crater" down, meaning the
peak out at the edge between the turn up and the turn down you will reduce
the turn down as you work the shape closer towards the sphere and parabola.
To put it another way, look at the center as being right (not a hill) and do
your figuring to extend that curve to the edge. 

	The circle in the center area and circular marks farther out and a
few other marks look like maybe polishing agent in abrasions on the surface.
Looks as if it was a "machine incident". Are you doing this on a machine?
And what size lap?

	Some reading on Foucault shadow interpretation and on Ronchi testing
would help you to understand how it is that you can take different paths to
slightly different parabolas.

	It's true that it is quite likely that the diffraction ring will be
brighter on the left than on the right when you are finished but that would
be because you are almost certainly not going to have a perfect edge. On an
f5.6 with a good edge you should be able to see the diffraction ring all the
way around with the knife set for the 70 % zone. It will be darker on the
right but it should be there. If it is not there at all on the right your
edge is not so good.  

Jerry


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