To: tkrajci{at}san.osd.mil
From: ray{at}stabilite.com (Ray Desmarais)
Cc: atm{at}shore.net
Reply-To: ray{at}stabilite.com (Ray Desmarais)


 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

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