From: "Kreig McBride" 
To: 
Reply-To: "Kreig McBride" 


If thin glass can be properly supported then it would seem that thin
mirrors would outperform thick (massive) ones simply from being able to
equilibrate to ambient temp at higher rates.    Have been looking at a
3/4" x 38" mirror
blank supported on a 54 point cell.  The distance between points is about
the same as found in other flotation cells. Further exploration without any
mathematics convinces me that a 162 point cell would easily and properly
support such a system.  162 points?  Yes and actually quite easy to
construct. You just make up a whole bunch of identical parts and assemble
them. Easy.

With the tremendous amount of knowledge in technology along with modern
materials, and the numbers of people involved in ATM, I am rather surprised
there is not more work being done on very thin mirrors, slumping, and
supports.   The increased use of machines to make optics by amatures has
also proven to increase productivity and optical quality.  This technology
can and should be used to explore thin mirrors and related topics of
support during curve generation, figuring, as well as in use in the final
instrument.

This exploration into thin mirrors is not an exercise in cheap glass but an
attempt to look at ways to make very large mirrors for portable scopes.
There are several 40+" mirrors out there made of 2" glass so
thinking that 3/4" glass for a 16" is thin, changes our
perspective.

Just my 2 cents worth.  Flame suit on.

Kreig McBride
Telescope Optics Workshop
www.wwu.edu/~skywise/optics.html






-----Original Message-----
From: William Cook 
To: atm{at}shore.net 
Date: Friday, January 03, 2003 1:57 PM
Subject: ATM Re: 3/4-inch x 16-inch mirror


>
>Our list has more than its fair share of sweeping generalizations. I just
>can't avoid this one.
>
>3/4-inch plate will make an unbeatable 4.5-inch mirror.
>3/4-inch plate will make a fantastic 6-inch mirror.
>3/4-inch plate will make an incredibly good 8-inch mirror.
>
>However,
>
>3/4-inch plate will make a marginal (at best) 16-inch mirror--unless:
>
>1) You have a NASA budget to put into an incredibly intricate support
>system, OR
>2) You get your thrill by having people offer you kudos because you were
>able to make such a large mirror with such a thin piece of glass.
>Our list has more than its fair share of sweeping generalizations. I just
>can't avoid this one.
>
>3/4-inch plate will make an unbeatable 4.5-inch mirror.
>3/4-inch plate will make a fantastic 6-inch mirror.
>3/4-inch plate will make an incredibly good 8-inch mirror.
>
>However,
>
>3/4-inch plate will make a marginal (at best) 16-inch mirror--unless:
>
>1) You have a NASA budget to put into an incredibly intricate support
>system, OR
>2) You get your thrill by having people offer you kudos because you were
>able to make such a large mirror with such a thin piece of glass.
>
>I am sometimes accused of trying to discourage some telescope making
>projects. If interjecting truth into the equation makes it seem so, I will
>gladly plead “guilty.”
>
>First, the plate glass may have stresses. Most porthole glass feels no
>social obligation to perform like double-pass BK7 or even Pyres.
>
>Secondly, grinding such a thin piece without introducing astigmatism is a
>bear and, without special care and a LOT of time, is almost an
>impossibility.
>
>Next, figuring will be something from the twilight zone. You will need an
>environment in which the temperature stays very stable and, because of the
>heat generated in polishing  (and from your hands) the mirror will require
>an inordinate amount of time to stabilize between each testing session.
>
>I like to work with plate glass; it takes a great polish. However, if you
>consider your time worth even as much as minimum wage, you might want to
>make a smaller plate glass mirror while saving your money to purchase a
more
>robust piece of 16-inch Pyrex.
>
>Just some thoughts.
>
>Bill Cook
>
>
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--- BBBS/NT v4.00 MP