From: "Russell Jocoy" 
To: ray{at}stabilite.com, tkrajci{at}san.osd.mil
Cc: atm{at}shore.net
Reply-To: "Russell Jocoy" 






>From: ray{at}stabilite.com (Ray Desmarais)
>Reply-To: ray{at}stabilite.com (Ray Desmarais)
>To: tkrajci{at}san.osd.mil
>CC: atm{at}shore.net
>Subject: Re: ATM supporting thin mirrors
>Date: Sun, 2 Mar 2003 09:25:39 -0600 (CST)
>
>
>  Tom wrote:
>
>I disagree, and recommend that you find a way to get air to the back face
>Of the mirror for better cooling performance.
>
>However, if the mirror is very thin...less than half the thickness of
>other amateur mirrors...you may be able to get away with cooling only the
>front surface of the mirror.
>
>I'd be interested in hearing of tests or field experience from folks on
>how well a very-thin mirror cools/behaves when the back side is not
>exposed to cooling air.
>
> >...A sling would provide the edge
> >support needed, as has been proven.
>
>My concern with slings is friction/stiction.  Some sling users need to
>jostle or bump the mirror cell because of the sling system.  That is a
>sure sign that friction/stiction is at play.  Make sure your design and
>materials don't allow this to happen.  Also, choose a sling that does not
>change length with humidity/temperature.
>
> >Perhaps I don't understand the significance of the point support system,
>and
> >I do use a six point support cell on my 10", but if the spacing is so
> >critical, doesn't that lend to errors in support that would be avoided by
> >thinking outside the box and trying for complete support.
>
>Complete support may work...if you can adress cooling issues, and can show
>that there is not too much stiction/friction problems with the entire
>mirror laying against the cell backing and sling.
>
>Good luck,
>Tom Krajci
>Tashkent, Uzbekistan
>
>
>Hi All,
>
>As usual Tom is on the money with his ideas.
>
>Getting good airflow is absolutely critical; you could remove some of the
>benefits of a thin mirror by restricting airflow to the back.
>
>I found that a sling would severely pinch the mirror and sticks like crazy,
>especially with very large extremely thin mirrors. On the 72" mirror that
>was less than 1" thick, the support I used was cheap and did the job
>required. As a visual telescope it would not work very well. The key is to
>support the mirror from the back with a system that is strong enough to
>hold
>the mirror in the exact position through various pointing angles.
>As the mirror becomes thinner the mount becomes more complex and expensive.
>I've used RTV on donuts on the back. These larger contact points help a
>great deal. The RTV is about 3/8 of an inch thick, this is to hold the
>mirror firmly and has a little give to remove it from the cell expansion. I
>certainly don't have all the answers yet. Although my experiment with 2
>large mirrors has pushed me in the right direction. Recently I made a 67"
>blank that is 4" thick, 1.3" faceplate and a 1"
backplate. I wanted to make
>the mirror stiff enough to be supported with a 27-point cell, which has 7"
>diameter circles to contact. Each of these contact points will have about
>12, 3/8-inch thick small blobs of RTV. On the edge I'll use 8 radius
>contact
>points that, will pivot from 2 points. These pegs will reduce the load from
>two points on the mirror to 8.
>The sling would not be helpful here. The cell will have to be able to
>handle
>the 850-lb. mirror without flexing much. It will flex as everything does;
>this is my current game plan.
>I am certain that this mirror will perform beyond my expectations, as my
>past experience would indicate that it would be stiff enough to help the
>cell do its job.
>Soon I'll post pictures of the blank and update its progress as I go.
>Thin walled steel tubing will be the material of choice for the telescope.
>I'll probably use a 24" flat to fold the scope so the eyepiece will not be
>far from the ground.
>
>RAY
>Ray Desmarais
>479-890-4918
>ray{at}stabilite.com
>
>Education is not something that can be completed.
>It's a way of life.
>
>
("atm{at}shore.net")

  Someone on the list said something about an astro-turf support, using the
little plastic "grass" pieces
to support the glass. This sounds like a backyard support system that is
right down my avenue..!!!
   Going through a computer for support systems is all well and good.
Supporting a thin piece of glass
is something that all of us on this list can experiment with. Many on the
list have completed thin mirrors
and their input on how they have supported their mirrors may help to come
to a happy point that
could become a standard for all of us... I think that  it is the reason of
this list to share all experiments, and not to say I know and you
don"t. This list is for knowledge to be shared, whether you have made
a 4" mirror with a turned down edge, or you have a 7" apo
refractor lens with a flat spot in the middle,
Someone on this list will either have an answer.. or can send you in the
right direction. I love reading
information from people who love this art of creating optical gems that
others can view the cosmos
and learn what a wonderus place in the universe we live. The layman has no
idea, and even most
people,"astronomers" at star parties have no idea how their
optics put that fantastic image to their
eyes. I think that this list has helped me to understand that telescope
optics is a growing entity that
will not stop as long as the imagination of ATMers continue to flood the
list ,or their minds , with new
ideas and absurd notions of a 72"x.9" telescope mirror... Keep
looking for the stars and someone will
be behind you asking questions....      RUSS JOCOY.... may all your spheres
come true..
("atm{at}shore.net")

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