From: "William Cook" 
To: atm{at}shore.net
Reply-To: "William Cook" 


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


_________________________________________________________________
MSN 8 helps eliminate e-mail viruses. Get 2 months FREE*
http://join.msn.com/?page=features/virus

--- BBBS/NT v4.00 MP