[ATM] Ultrathin update discussion

[vorblesnak@peak.org]

Good discussion.

I have also tried to stabilize the thin meniscus shape of the mirror,  I
tried concrete, plaster, dental plaster, spray foam.  What I found was the
glass never sat on the substrate the same way twice so flexing occurred
and zones appeared.  Even when I indexed the two pieces, they never quite
nested correctly.

When I attached the substrate to the glass the differences in CoE warped
the glass and induced astigmatism.  I tried pitch, RTV Silicone, spray
foam, double stick tape.  Of these the RTV Silicone has given me the best
results.  It seems to buffer the unequal expansion.  The trade off is
zones form where the glass contacts the RTV.

The idea of the particle bed intrigues me.  It would support well, mold to
any  back surface, have virtually no CoE.  However, managing it could be a
nightmare.  One odd piece of glass or sand up on the surface and there
would be major scratching.  Perhaps a thin plastic or rubber membrane that
seals the particle bed but allow the glass to nest into the substrate. 
Hmmmm.

The current attack is going back to mirror on top of a full sized lap.  I
have attached a pie plate to the back of the mirror with RTV.  The zone,
should one form, should be hidden by the secondary.  I can add water to
the pie plate for extra weight and also to create a slop gauge to measure
the smoothness of my stroke.

David Davis
Toledo, OR 97391



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