Re: ATM 3" F/6 and basics

["Frank Ward" <thewards@mindspring.com>]

Dymicki,

A 6" is not one half of a 12" it is 1/4  of a 12" because we are working
with area, not
just diameter.

Look at it this way.  Glass is a liquid that happens to be sitting still at
room temperatures.


When you  move it it bends from its own weight across the center like a
piece of bread
held by the edge. In our case it only has to bend a millionth of an inch to
be useless to us.

If you have a piece of glass you want to make into a reflecting surface and
make all the
rays reflect to one little place, make it as thick as necessary to achieve
this.

In my opinion, no mirror of mine should be less than 3/4" thick (18mm).
People before
me have taken the trouble to experiment for me and have told me what they
found.  I
believe them.  What they have found is that bigger mirrors do not have to be
as thick
as originally believed, i.e. a 16" doesn't have to be 3" or 75mm thick, 50mm
will do
and even thinner, but that doesn't mean an 8" (200mm) which is 1/4th of a
16"can be 15mm .
An 8" (200mm) Schmidt mirror of f/1.5 will have a central thickness far less
than 15mm
and take months to make.  I'm not willing to take a chance when the cost of
the glass
for a blank is such a small part of the whole scope.

Frank Ward
Atlanta, GA (55degrees and still dark)
-----Original Message-----
From: Dymicki, Krzysztof <Krzysztof.Dymicki@hit.com.pl>
To: atm-digest@shore.net <atm-digest@shore.net>
Date: Thursday, April 01, 1999 1:53 AM
Subject: ATM 3" F/6 and basics


>
>Hi,
>
>I started to play with mirror grinding. Because I didn't know, where to
grab
>a right blank from, I took a plate glass square 10 mm thin,
>and broke from it a 8-sided 'pre-blank'. I have rounded it (mostly by
hand).
>The tool were two 3 mm plate glass squares. Also by hand broken and rounded
>and glued together with epoxy. I know it is only a play and I do this
mostly
>for fun.
>Would be nice to became truely right mirror from this piece of glass. Even
>if not, experience counts also.
>In nearest future I'm going to grind 6" f/8 10 mm thin mirror.
>Here the question. From Mel Bartels homepagehe made succesfully:
>
>" 12" diameter x 3/8" center thickness, f/4, plate glass
>  20" diameter x 1" center thickness, f/4, fine annealed pyrex
>  30" diameter x 1 1/4" center thickness, f/4, fine annealed pyrex"
>
>One could think, if he made succesfully  such mirrors then for 6" is
>adequate (?) 3/16" center thickness - only 2 mm ?
>(1/2 from thickness from 12" mirror ) ? Does mean that, 10mm glass is OK
>for 6" f/8 ?
>Should tool from same glass be OK aslo ? Or will be better if I glue two of
>such pieces together ?
>What with astigmatism during grinding with such (10mm) tool same 10mm
mirror
>?
>
>Thx in advance
>
>
>Krzysztof Dymicki     Dzial programowania i rozwoju
>kdymicki@hit.com.pl,  tel (22) 5330 814, -815
>
>HIT Centrala Polska Sp. z o.o. 01-460 Warszawa , ul.Gorczewska 212/226
>hitcent@ikp.atm.com.pl tel (22) 5330 990, fax (22) 5330 829
>
>