[ATM] Second Thoughts on Mirror Support

[mbyorick at starband.net (mbyorick)]

Yo Steve,

 A very interesting drawing.  Looks a bit like 'double-dipped'
potato-chipping, which is an effect with which I'm not familiar(!).
However, it would seem that that attaching thin mirrors with RTV should
tend to reduce their potato-chipping tendency(?).  All right, then what
would happen to the mirror in your drawing if you did <not> attach (RTV)
the mirror to the cell triangles?  If your depicted fear is warranted,
then it would seem that the mirrors on most DOBs (or other alt-az
scopes) with flotation cells (i.e., all of those that don't have the
mirror attached with an adhesive) should 'sag' even worse than the
RTV-ed ones, (yikes)!?

 In any event (moving right along from science fiction theater to
under-tested theory), the thickness of the RTV layer between each
attachment point should be optimized (read 'minimized') to reduce the
amount of 'stretching' of the RTV.  I've read of RTV thickness figures
in the 2 - 4 mm range working well for mirrors 16" and larger.
Conversely, I have heard a least one complaint that shear-induced
sagging was a problem, where the RTV thickness was perhaps more than a
quarter-inch (~6mm +), so there may be a fairly narrow  thickness range
that provides good results for a given mirror weight, number/size of
attachment pads, etc. 

Cheers,
MikeB 
SW New Mexico


-----Original Message-----
From: atm-bounces@atmlist.net [mailto:atm-bounces@atmlist.net] On Behalf
Of Steve Houlihan
Sent: Monday, February 02, 2004 7:18 PM
To: The Amateur Telescope Makers List
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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