Re: [ATM] Figuring help for Lutfu

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

Hello Lütfü

Well... there have been a few suggestions on how to get that sphere and good
edge. I hope there will be more.

And more mention of a Foucault tester. I'm sure you wouldn't need to cross a
continent and swim the ocean to get find a micrometer or dial indicator. I'm
sure you have some source for such things in your country. Even so, it does
not have to be that complicated.

You can use a 40 thread per inch screw for a micrometer. About an inch of
travel is enough for the mirrors you have mentioned you will be making. If
you make a slitless tester, the total movement from center zone to edge zone
in Foucault testing the mirror will be about 3 turns of the screw. 6 turns
if the light source is fixed and only the knife moves. One turn of the screw
will be .025" of movement. You can put a knob on the screw and calibrate it
into 25 divisions around the circumference to represent .001" per division.
Plenty accurate for your purposes and cheaper.

The important thing in making a good tester is a smooth straight movement
and solid enough that turning the screw to adjust the knife does not move
the tester except for the movement made by turning the screw. You will want
a stable stand for the tester and the mirror. Then you want a good still
place to set it up and test. Your Ronchi images indicate to me that you have
a suitable place to test. 

Make it so you can get your good eye up to the knife without bumping your
nose into the tester. Maybe they have updated the design since, but the last
time I looked at it the Stellafane tester was not designed for use by a
person with a face. It has been a few years, I'm sure someone would have
mentioned that to them by now.

The knife does not have to be razor sharp. If you use a razor blade, it can
be dulled by grinding it several strokes on a piece of flat window glass
with some very fine grit. Hold the blade nearly at a 90 degree angle to the
glass.

Jerry  




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