[ATM] Second Thoughts on Mirror Support

[djd521 at comcast.net (Don D'Egidio)]

Steve,

I have a Discovery 17.5" TD dob that has the mirror secured with RTV. The mirror is 2" thick and what Discovery did was to mount the mirror on one large plate rather than 3 plates. They use 9 points to support the mirror, simulating three separate triangles with three support points each, just like your pictures show. The thickness of the RTV pads, after being squished by the mirror, is 1/4" thick and about 1 1/2" diameter. When I first received the scope I had the mirror tested in the cell, in a vertical position, and it showed no signs of any stress caused by the RTV bonding.

I think if you replace your 3 triangles with one plate, and use 9 pads of RTV to support the mirror, you should have any problems. RTV was also used on the edge of the mirror at 120? spacing to prevent any sideways strain. So in effect you have 9 supports from below, and 3 on the side.

One more point, the RTV very securely held the mirror in place. When I had to remove the mirror, the RTV needed to be cut through with a knife and then the rest of the RTV had to be scraped off the back of the mirror and the mirror mount, once the mirror was removed.

Don

----- Original Message ----- 
From: "Steve Houlihan" <sho@surfnetUSA.com>
To: "The Amateur Telescope Makers List" <atm@atmlist.net>
Sent: Monday, February 02, 2004 21:18
Subject: [ATM] Second Thoughts on Mirror Support


> After studying many websites and discussing this with many people, I built a
> 9 point cell for my 12.5 inch mirror.  I based it a lot on information I got
> from Bruce Sayre's website and designed a cell with the intention of
> securing my mirror to the cell with RTV (Silicone).  Once I had the cell
> completed and could actually look at it and hold it in my hand I am having
> second thoughts.  It seems to me that this design will put large bending
> forces into the mirror at each of the three support trees.  I have made a
> sketch (exagerated a great deal, of course) of what I think may happen to
> the mirror when the scope approaches the horizontal position if I glue the
> mirror to my cell.  Scroll down past the pictures of the cell to see the
> sketch.
> 
> OK all you experts!  Am I going down the wrong path?
> 
> http://www.surfnetusa.com/sho/Astronomy/Telescope.html
> 
> As a reference, here is the link to Bruce's site..
> http://www.foothill.net/~sayre/
> 
> Any help or opinions are welcome.
> 
> Steve Houlihan
> Santa Cruz, California
> 
> sho@SurfnetUSA.com
> http://www.surfnetusa.com/sho
> 
> 
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