Re: ATM CeO, Foucault, and grinding newbie questions

[Robert Duvall <forcefive@earthlink.net>]

OK, I'll answer since it appears to be quiet out there and I've got 1 min
(am I getting all the mail, I winder?):

At 9:27 AM -0400 6/30/97, Michael P Lindner wrote:
>
>1. I've been wondering about the toxicity of grit and polishing compounds.
We've never worried about it at the workshop. But we don't eat off the
spoons afterwards either ;)

>
>2. Why does the foucault tester have a "tower"?  Why not make the slit and
>   knife as low as possible, to make it more stable?  Also, why the "tilt"
>   adjustment screw on one side of the tester?  To change height?  I can't
>   imagine the tilt matters, when the mirror is round.

My web page should help with some of these. The tower needs to be high
enough so your chin doesn't hit the tester when your eye is there. In the
photos of my tester, I have it at the front; I have since made it so I move
it to the back when eye testing, and use the front position when using the
video camera to capture. The camera sits on the tester and doesn't move as
I adjust position from focus. The tilt screw on the side is to gently easy
the knife edge into the light cone. The title doesn't matter, but touching
the edge of the light cone from the mirror with the razor does!

>3. Texereau recommends the "W" stroke from the end of rough grinding on.
>   Berry recommends the "W" stroke only during the end of rough
>   grinding, then the "center-over-center" for the rest.  Is this purely
>   a matter of taste?  What effect does length of stroke have on shape
>   in the center-over-center stroke?  In the "W" stroke?

A W stroke tends to parabolize and also blends to avoid zones. I assume
that's why Tex recommends it. A longer stroke also parabolizes but can
easily turn the edge. Then you have to resort to short strokes for a long
time to get rid of the edge.


>   On a related note, I've never seen a grinding machine close-up.  Do they

I've made several mirrors up to 12.5 and never used a grinding machine. I
will for the next size up, though (18" or larger)

>
>4. Last one, I promise.  In Berry's book, he has the base board made from
>   3/4" plywood attached to 3/4" particle board with formica on the bottom.
>   Why 2 layers?  I don't think it makes it any stiffer, given that the
>   groundboard is a single 3/4" plywood piece.  To avoid making holes in the
>   formica for fasteners?

Partical board is not very strong. I hate it (there, I feel better now ;)
The partical is 3/4" probably cause that's what they normally mount the
formica to at shops. It needs a good 1/2" ply to stay solid and stable;
Berry suggests 3/4". If you can get a sheet of black ebony star, just mount
it to the 3/4" base. I know Crazy Ed Optical sells small sheets.


Good luck.


Robert Duvall
ForceFive Telescope Making Resources

Software Engineer, Amateur Astronomer (TN),
and General Scientist (according to my son)
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