[ATM] Thin mirrors

[artbianconi@blast.net]

Right Church David, wrong pew

The only material I know of that has high tensile strength and 
relatively low mass is carbon fiber. The ply structure must be 
laid up in such a way as to be in tension when any bending loads 
occur, regardless of where the device is fixtured or where the 
bending loads are applied. The shear webs of a spoke I-Beam in a 
radial pattern help provide that.

If you want zero flexure in a mirror cell under normal 
conditions, you'll want a cell that has a deflection ratio of at 
less than 1/16th inch deflection at say 10,000 PSI. That will 
still likely to be overkill. There's not a telescope on the 
planet, in or out of equilibrium, that subjects the primary cell 
to such forces, so it should be plenty adequate.

40 years ago, we built or repaired racing cars built up from 
welded small chromolly tubes. They looked like "birdcages" and 
one of them, the Type 61 Maserati, was called just that. It 
killed the competition because of it's exceptional torsional and 
beam strength and low weight. There wasn't a load bearing tube 
over a half inch in diameter in any of the many trusses! The 
frame weighed about 120 pounds as I recall.

We tested one on a special fixture, applied bending loads and 
measured deflection at the suspension points. We got an amazing 
1/4 inch deflection at 14,000 psi with steel! A remarkable 
achievement for 40 years ago. Carbon fiber is many times better 
and Formula One cars are proof positive of the stiffness of 
chassis that are being twisted and pounded at speeds in excess of 
200 mph. So are modern fighter planes.
 
Composites will provide that strength and more provided you know 
your load paths and can place the layups so that the loads are 
always or mostly co-axial to the weave. The nice thing about it 
is that the low mass of smaller telescopes and the static nature 
of most loads are such that a stiff cell can be made with less 
exotic and less costly fiberglass and still provide more than 
ample support for a super thin mirror.

Date sent:      	Mon, 6 Sep 2004 21:04:45 -0700 (PDT)
Subject:        	Re: [ATM] Winspot
From:           	vorblesnak@peak.org
To:             	artbianconi@blast.net

I did / do pie plates.  Yeah, I'm weird.  Anyway, on the first 
couple of plates I backed the mirror with various stiffeners.  A 
lead plate. Concrete, plaster.  I was not pleased with the 
results.  Zoning problems, sticktion between the stiffener and 
the glass.  Astigmatism. Of course that is just my experience 
with my muscles and my strokes.  I have read of others who have 
had very good results using a stiffener.

I am nearly done, more or less.  I have found that the heat of 
the wet will cause the mirror to look over parabolized, but after 

an hour of cooling it is a bit oblate.  Not what I expected, but 
close to what I found in the pie plates.  I have about an hour 
into the figuring and suspect there will be at least one more.

I have appended a lousy ronchigram, but it shows the curve is 
regular. 

David Davis
Toledo, OR

> Wow! one quarter inch thick. You may want to bolster that 
> skinny  with some composite backing! The flexing that will take
> place during figuring will make you nuts!
>
> Keep me posted
>
> Art



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