From: "Mel Bartels" 
To: "ATM" 
Reply-To: "Mel Bartels" 


Stathis,

I look at the issue of thin glass mirrors a little differently.  To me, it
is most important to look at the engineering aspects of the integration of
the separate parts.  The parts are the coating, the glass substrate, the
mirror mount, and the tube assembly.  It is the sum of the difficulties
that interest me most.

Ultra thin glass does not necessarily have compromised optical quality.  It
may be so difficult to figure accurately that the sum of the difficulties
becomes too great.  The sum of the difficulties can reach a high number too
if the mirror mount becomes too complicated because the glass is ultra
thin. So one ends up with a handful of frustration instead of a scope.

When 12-16 inch mirrors reach 1/2 inch thickness, the difficulties of
fighting their bending during figuring (bending causing zones) can be
frustrating on top of the challenge in figuring large fast amateur optics.
Of course if one wishes to merely experiment, where personal time and
difficulty are not part of the sum of the difficulties, then mirrors can be
quite thin and perform very nicely.  I love my ultra thin mirrors when I
look through the eyepiece partly because I savor the satisfaction of
creating something so challenging with my own hands and brains.

Otherwise, I see little reason to drop below 3/4 - 1 inch thickness in this
range of aperture.  The glass is hardly more expensive, the glass being a
rather small part of the cost of the scope anyhow.

Mel Bartels

--- BBBS/NT v4.00 MP