From: "John Sherman" 
To: 
Reply-To: "John Sherman" 




> It seems to me that in the standard 8 tube truss for a
> Dobsonian alti-azimuth mount that only the left and
> right sides need to be very strong as they carry the
> weight of the OTA load against gravity.  The left and
> right pair only help to keep the OTA from rotating and
> aligned left and right  to the mirror when the OTA is
> pushed by a sideways force.  Usually that side force
> is much smaller than the altitude force.  Thus I think
> that the upper and lower trusses can be made out of
> much smaller diameter tubing saving a lot of weight.

> Anybody have any thought about reducing the left and
> right truss diameters?


Hi Dale,

I agree with you (but I think you said it wrong). I call them the altitude
(left&right) and azimuth (upper&lower) poles. The ALT poles hold
the UTA up against gravity, and the AZ poles keep the UTA from pivoting
down, and keep the ALT poles from folding sideways. On my 22" I have
the poles mounted on the inside of the mirror box. The ALT poles go all the
way into the corners, and the AZ poles take up whatever space is left. This
way the triangle formed by the ALT poles has a wider base than the triangle
formed by the AZ poles. The ALT poles are therefore longer, and form a
stronger triangle. To lighten weight I then drilled holes into the AZ
poles, since those poles don't need to be as strong. I did not drill holes
in the ALT poles. This arrangement has served me very well since 1997.

If I was to do it over, I would definitely use smaller diameter poles for
the AZ motion.

Good luck,

John

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