Re: [ATM] The 15.5 inch Ultrathin, more pictures

[vorblesnak@peak.org]

> Very nice,but when you test the mirror given the
> thinness,exactly how are you supporting the mirror on
> the test stand?
>
> Ric
>

I have not really had a need to test on this mirror so far. Other than a
basic spit test for focal length.  However, my test stand is very simple
one. A vertical board with hole pairs drilled in it and short lengths of
dowel. I can  move the dowels between holes to adjust for different
mirrors.  It is like the bazillion other test stands built by ATM's over
the years.

The 16 inch showed no deformation when standing on edge on the stand. 
That will be one of the bench marks on this mirror, will it stand and be
tested, or will it fold and distort.  I think it will test.  The warping
is a combination of shape and weight.  The 'shield' shape of the glass
fights distortion.  The weight of the glass is very low, (I should weigh
it, shouldn't I), I think it will test fine.

The one concern I have is shaping the pitch lap.  For polishing I will
make a full sized lap.  That, and the polishing, both require pressure to
be successful.  I am thinking of a simple stiffener of plaster for shaping
the lap.  Or maybe a plaster copy of the mirror to shape the lap.  I have
had success setting a garbage bag of lead shot on the back of the mirror
while polishing.  This allows the weight to spread out uniformly over the
back but not pinch the glass.

I am toying with the idea of a workshop on untrathin mirrors in Oregon. 
Maybe in the spring.  Anybody here be interested in getting together and
making a few 18 inch diameter by 1/4 inch thick mirrors?

David Davis
Toledo, OR 97391


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