Re: ATM 3/4" mirrors, multipoint cells, machine generated optics

["Peter" <peter@kitgear.com>]

Excellent and courageous post!!

I am working on a 25" scope using a 1" blank. Currently I am polishing and I
must say that all of the advice given to me on this ATM list have made a
tremendous difference. The mirror is polished out to a 14" central diameter
and the optical tests look very good so far. Some people on this list
probably think I am nuts to attempt such a mirror but I still feel it will
work fine and I think it is possible to make mirrors larger than mine using
the same glass. I did not slump the glass due to not having access to the
equipment so that would only help the situation.

I have a good source for either 3/4" or 1" thick blanks up to 48" in
diameter so please contact me privately if you are interested.

Cheers,

Peter

----- Original Message -----
From: Kreig McBride <astroguy@nas.com>
To: <atm@shore.net>
Sent: Friday, January 03, 2003 10:32 PM
Subject: ATM 3/4" mirrors, multipoint cells, machine generated optics


>
> 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 <billcook50@hotmail.com>
> To: atm@shore.net <atm@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
> >
> >
> >_________________________________________________________________
> >MSN 8 helps eliminate e-mail viruses. Get 2 months FREE*
> >http://join.msn.com/?page=features/virus
> >
> >
>
>