Re: ATM Truss vs Composite

[Bruce Swayze <swayze@europa.com>]

At 05:44 PM 7/17/97 -0700, you wrote:
>From: Mel Bartels <mbartels@efn.org>
>
>> Truss tubes can be very lightweight.  For my 20" f/.5, the 8 truss tubes
weigh 6 lbs total. 
>
>Perhaps this is a solution to my problem.  I want to put 16" on my
>eq mount but need to reduce the weight substantially.
>
>Unlike a Dob, the EQ mounted tube needs to rotate in the saddle
>which is not convenient with a truss tube.  However, I have been
>pondering the idea of using two pieces of round tubing, one at
>each end and truss tubes in between.  The bottom has to be long
>enough to reach the saddle but the top only has to be long 
>enough to accommodate the spider and focuser.
>
>I could use Sonotube or composite for the tubes but I am not
>clear on how to determine how long the bottom piece needs to be
>or how the weight distribution would work out.
>
>Anyone done this?  Make sense?

Jack,

My brother Steve built a 24" a number of years ago, and the design is one
that would work very well for your situation.  It consisted of a square box
at the center of gravity of the tube assembly, with a round short tube at
the back end to hold the mirror and a short tube at the front to hold the
spider and focuser.  

Truss tubes were used to hold both tube sections to this box, extending
back a short distance to the mirror, and forward a somewhat longer distance
to the upper tube assembly.  This box held the side bearings of a classic
dobsonian, but could easily hold bearings for a fork mount, or any kind of
equatorial mount.  

Other advantages include:  it breaks down when you remove the tubes from
both sides of the box to 3 very manageable pieces.  The tubes can be
covered with a light-blocking shroud that also serves to limit air currents
through the optical pathway.

Some engineering work could even be used to make the upper tube a rotating
tube assembly.  Some additional engineering would have to be done to
determine the balance point and overall design, or this could be done
through good old fashioned trial and error.  

Hope this helps you find a solution.

Bruce Swayze
http://www.europa.com/~swayze    Swayze Optical home page
http://www.europa.com/~swayze/spider.html  Homemade spider page