[Author Prev][Author Next][Thread Prev][Thread Next][Author Index][Thread Index]
[ATM] Second Thoughts on Mirror Support
Steve,
That is a beautifully built cell. It shows what can be with simple tools
and ingenuity!
I am not the expert, but your intuition about the coupling forces and the
resulting shape are correct. The question is "how much deflection?" In
your drawing, assume that each triangle of the cell supports about 1/3 the
weight of the mirror. The bending moment M (say in inch-lbs) is this weight
x distance from mirror back to the pivot points.
This moment causes forces perpendicular to the mirror surface at the glue
spots. So you can estimate these forces easily. Top force F pushes to the
right. Lower force F pulls to left. Separation "H" between them. Then set
that equal to the bending moment and solve for the forces.
M = F x H. Not knowing your dimensions, I'm guessing 1 to 3 lbs. That is
significant.
This is simplified because you actually have three glue pads. You can solve
for all three forces if you wish. And you will not really use your scope to
look at the horizon. The weight force parallel to the mirror will be less,
but even at 45 degrees altitude it is 71%.
The idea here is to get a rough analysis in PLOP by adding the extra
forces. Again, I am no expert since I've only used PLOP once, but it
calculates the actual distortion of the mirror due to cell supports. It is
freeware from David Lewis
http://pong.telerama.com/~mdholm/atm/cells/index.html
There is more discussion by Mark Holm
http://www.eecg.toronto.edu/~lewis/plop.html . Similar analysis was done
recently by Nils Carlin for the sling supported mirror common in Dobs.
http://w1.411.telia.com/~u41105032/sling/sling.htm
Since you working to
create a quality instrument, the numerical analysis of distortion is the
only way to predict if there is going to be a problem that you can see in
the eyepiece.
Check the ATM archives. Since I changed computers, I've lost some of the
references I collected on the subject. Maybe some others can help out.
Regards,
Stuart Hutchins
----- Original Message -----
From: "Steve Houlihan" <sho@surfnetUSA.com>
To: "The Amateur Telescope Makers List" <atm@atmlist.net>
Sent: Monday, February 02, 2004 7:18 PM
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
>
>
> _______________________________________________
> ATM mailing list http://www.atmlist.net/
>