[ATM] Help with Strut Flexure in my 3 Truss DOB

[RP Sutton <rp@suttonsonline.us>]

A friend of mine who lives several states away from me made a telescope for me 
based on a 3 strut design. When I received it and put it together it became 
evident that the trusses were flexing/bending. It is very hard for him to help 
me since he is not here and I have no mechanical engineering abilities 
whatsoever. I was told by a gentlemen on cloudynights forum who also owns a long 
focal length 3 truss DOB that I just need to get stronger tubes and all will be 
well. Is that true? and if so, how do I know what to get? The current trusses 
are 1.25 OD with a wall thickness (I THINK) of .058. They are aluminum tubes and 
are a 56.75 inches long. The trusses have to be "forced" a little bit to line up 
with their bolts on the upper ring and it was my friend's thoughts that this 
tension force would keep the flexure from happening, but it did not work out 
that way. My upper case assy w/o any eyepieces (it does include the telrad 
finder and focuser) weighs 6.5 lbs and I cannot imagine any current of future 
ep/barlow combination would add any more than another 2.5 lbs. for a max total 
weight of 9 lbs. The scope is a 10" F/7 and I was wondering if you would please 
figure out what tubes I would need to keep this from flexing. I am not able to 
increase the OD of the tube so I would need to know what wall thickness will 
work and if I would need to go to a stronger tube like steel instead of 
aluminum.
 
I would greatly appreciate it if someone could find the time to help me.
 
Sincerely,
 
Patrick Sutton
 
 

 
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