Re: [ATM] RE: Berry's 10-inch telescope plans

[Andy Saulietis <iss@pvtnetworks.net>]

Tom K writes,


>One idea I've tried is to walk through a big place like Home Depot...pick up
>pieces of various grade of 3/4 plywood that are the same size (such as 4ft x
>4ft)...hold them in the air by one corner and give them a rap with my fist.
>The piece of plywood will 'ring' at its resonant frequency...at least for a
>short time.  The brand of plywood with the best stiffness to weight ratio
>will have the highest oscillation frequency...or will it?  Am I making
>errors in thinking here?

Weeel..if you get a 4 x 4 sheet of 1" styrofoam and give it
the 'rap' test it'll ring similar to plywood..but not be strong
enough for structural use.

Maybe some sort of dial gage gadget that measures deflection
between 2 known points under a fixed load would indicate
which plywood (or whatever) was the stiffest?

I've used West System epoxy to both weatherproof plywood and
increase its stiffness..the stuff penetrates wood to some
extent and makes an external skin, so the plywood becomes
a sort of monococque structure.

One of the stiffest lightweight sheet materials I've seen
is thin aluminum sheets sandwiched and bonded to hex cell
expanded metal..like a honeycomb. At NASA, we had a 1' x 8' x 1" thick
board of this stuff supported at each end. A 200 lb guy (myself, back 
then..:( )
standing in the center deflected that thing by maybe 1/16".
The cheap version of this is interior hollow core doors..thse
have cardboard honeycomb in them. Back to styrofoam, putting
a thin skin on it can also make a very stiff lightweight (but bulky)
structure. One example I saw was a 24" eq fork mounted scope
mostly made pf styrofoam covered with a fiberglass skin, the
whole thing including the base and optics was <200 lbs.

Also, Tom discusses edge supports at the 'neutral'
points of the mirror..not sure what that means or how one
would calculate that location..I assume there would be
no tangential force at the support point? Of course
the trivial example would be a single point directly under the CG
of the mirror.
Andy Saulietis
ISS Enterprises
10 Milky Way
Mayhill NM 88339


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