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#1 Alvin435

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Posted 07 April 2020 - 03:39 PM

Hi all, I am going to make a 12" F5  truss tube dob. If I go with 6 poles what diameter should I use to be certain my collmation does not change or move? I am planning on using Aurora UTA and mirror box clamps and 3/4" Baltic birch. Many thanks Alvin 



#2 MitchAlsup

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Posted 07 April 2020 - 04:56 PM

My 13" F/3 uses poles set on a 18" diameter. This is about 2" in diameter wider than needed so the poles to not vignette the entrance aperture; but adds a lot of stiffness to the arrangement.

 

assembly26.JPG


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#3 Alvin435

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Posted 07 April 2020 - 05:11 PM

Thanks,  I asking about  truss tube diameter, thinking 1" or 1 1/8" tube diameter.

.



#4 drneilmb

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Posted 07 April 2020 - 05:46 PM

My 12" f/5 hexapod has six tubes, each 1 inch in diameter. My experience with it is that deflection of the struts is not a problem for shifting collimation.

If I were you, I'd use 1" tubing and worry more about the secondary support and the focuser board.

Just my experience,

-Neil

#5 MitchAlsup

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Posted 08 April 2020 - 01:04 PM

Thanks,  I asking about  truss tube diameter, thinking 1" or 1 1/8" tube diameter.

I used 1.0×0.063 tubing on the 13" F/3.

I will use 1.5×0.063 for the new 20" F/3 :: I used 1.5×0.049 on my 20" F/4 20 years ago

I will use 2.0×0.063 for my 30" F/3.

 

1.0×0.049 will be more than stiff enough. I am not using 0.049 because (of resent) I can get 0.063 at less cost than 0.049. I consider 0.032 too thin and may dent easily, 0.049 is way better, 0.063 essentially eliminates that issue.


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#6 Pierre Lemay

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Posted 09 April 2020 - 07:27 AM

Albert Highe's book provides all the equations to calculate the diameter of the trusses. There are many factors that influence that value: length of trusses, number of them (6 or 8?), triangle base widths, thickness of tubes, shape of tubes (moment of inertia), tube material properties, etc. 

 

As Highe points out, the amount of offset between the tubes at their junction is another very important factor that will determine the minimum size of the truss tube diameter. In other words: how far apart from each other the truss tubes meet at the apex of the triangles. There is an equation for that as well. Depending on how wide apart the attachment of a pair of trusses is, the diameter of the truss tubes may need to be much bigger if that distance is wide than if the tubes meet much closer together.



#7 Alvin435

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Posted 09 April 2020 - 03:31 PM

Thanks everyone, now I think I'll go with 4 so focuser would be more secure. Hopefully


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