[ATM] Re: Second Thoughts on Mirror Support

[jgmoreau at cablevision.qc.ca (Jean-Guy Moreau)]

> That assumes that the two surfaces are stiff.  If they are not, one or
> both will sag slightly until more and more points touch.  By using thin
> metal over carpet, we are trying to make it the metal that sags more
> than the mirror and let the carpet balance the forces.


At this point, it might be useful to relate some experimental
results i had while investigating ways to make the fused glass
mirrors i now do.

My first tought was to "glue" the meniscus thin plate to the posts
and backplate using compliant  RTV silicone. The result was
printrough and astigmatism big time.

I had much better luck with a specialty UV cured epoxy called Triolyse,
as i was able to polish and figure a 14" prototype to a sphere
without any problems. For a moment i thought i had nailed it down,
it would hold its figure when knife edge testing outside, in winter
at -20 C ! and even survived repeated cold/warm cycles as i
carried it inside/outside the house, that is the figure remained stable.

However, it started to show problems, again astig and printrough,
when i subjected it to moderate heat cycles, 40 C to 20 C,
it seems the glue, which is supposed to last 20 years, did not like heat   ;-(

Success only came when i completely fused the joints, at high temperature
in a kiln, essentially turning the whole thing into one continuous structure.
Any partial, so called tack fusing of the joints, will led to failure.

These mirrors are nothing more than a very thin meniscus plate, on an
apparently good glass support, can we consider it this way ?
the one i am working on now has a 26" by 9/16 " faceplate, that's a
46/1 aspect ratio ! it has no printrough or astigmatism issues while
polishing/figuring with common thick glass techniques, nor does it display
these problems on a simple test stand, or mounted on a sealed plywood
"cell" with multi point RTV, in other words it behave as a full thickness
mirror.

Does this mean it is possible, with the right choice of materials, if they
exist,
to build a cell that will make practical the use of very thin glass ?

Jean-Guy Moreau