Re: ATM Ribbed blanks , CORRECTION

[jgmoreau@lino.com (Jean-Guy Moreau)]

>You also failed to mention whether your blank is plano-plano or slumped
>to concave-convex configuration which,since this is a section of a sphere,
>would make it stiffer still.A concave frontplate and a plano back side is
>not as stiff but is easier to mount in a conventional cell or add
>acutators to for image manipulation.


The faceplate ia a meniscus, slumped to radius.
The ribs are first groung against the back of the frontplate for a perfect fit.
Then the backplate plus ribs are fused to the frontplate at 740 C.
So the finished blank is a meniscus frontplate of uniform thickness, ribs
in a radial arrangement, although many others are possible,
and a flat backplate with central 2" hole for ventillation.
The process to achieve this took years to develop , and involves
quite an amount of work.....     ;-(

I am also experimenting with meniscus with strips of 1" wide glass stacked and
fused to the back in such a way as to provide support points for 9 and 18
point flotation cells.

Yesterday, i hand polished it for 30 min with a 10" lap and was delighted
when the foucault test revealed no trace of printhrough ! ;-)))
just a very smooth oblate spheroid with TUE (tool on top)
So it seems that a faceplate thickness of 5/8" is enought to
completely avoid this problem, with a maximum span of 2.2"
between ribs, (1/4" thick)   I made one before with 1/2" faceplate
and had a bit of printhrough (sp?)

I look foward to test the mirror in a real observing situation,
and will report,

Good luck to all with the Leonids,

Jean-Guy

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Jean-Guy Moreau         jgmoreau@mail.lino.com
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