Re: [ATM] sonotube for telescope

[Mark Holm <mdholm@telerama.com>]

I made an octagonal plywood tube for an 8 inch about 10 years ago and it 
still is in fine shape.

1/4 inch ply  I think it was fairly ordinary AB exterior.  Good birch or 
hard wood cabinet grade would be better..

The part that keeps it together is about 6 octagonal bulkheads, one at 
each end, and the others distributed throughout.  You can space a couple 
of them to help carry the load from the altitude bearings, or other 
attachment, and laminate a thicker ply layer on the inside, where the 
bearings or other attachment will go.  The bulkheads have to be cut 
pretty precisely.  I cut them all at once in a stack.  Made some sort of 
jig plate to go on the crosscut fence to make the 45 degree angles.  
Showed that my Dad's table saw blade was not quite at 90 Degrees.  We 
had to true that up to get accurate pieces.  I don't think screws will 
hold an octagonal tube together by themselves.  I used only Titebond 
wood glue, No mechanical fasteners.  (Former model airplane builder.  I 
have crashed enough planes to know that a good glue joint is stronger 
than the wood it holds together.  That is even true when the parts are 
high grade aircraft plywood.

I did make the bulkheads from 1/4 inch aircraft grade birch ply.  Sturdy 
stuff and cuts smoothly.

Of course you have to cut holes in the bulkheads to let the light 
through.  The result is quite effective baffles,.

Mark Holm
mdholm@telerama.com
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