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Pinbout
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Reged: 02/22/10

Loc: nj
Re: How many trusses? new [Re: Dick Jacobson]
      #5568850 - 12/13/12 09:11 AM Attachment (2 downloads)

Quote:

Don, are you saying that the end-on view should look like a trapezoid instead of a square?




Mr. Jacobson,

I believe his statement is similair to what you said about tipping the tops outwards/inwards.

I've read that before also but I can't remember where.

Edited by Pinbout (12/13/12 09:16 AM)


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Starman1
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Reged: 06/24/03

Loc: Los Angeles
Re: How many trusses? new [Re: Pinbout]
      #5568979 - 12/13/12 10:41 AM

When 8 truss tubes are used with pole pairs parallel, the weight of the UTA is basically borne by the top two poles on the side. The bottom poles have little tension and they mostly sag.
The top and bottom pairs have almost no tension at all and only prevent the UTA from rotating around the attachment points of the side poles.
When the poles are non-parallel (yes, looks like a trapezoid from the side), now the upper and lower poles carry some of the weight from the UTA because the downward force vector now has some horizontal direction as well as vertical, applying tension and compression to the upper and lower pole pairs.
Having the weight of the UTA at least partially borne by the upper and lower pairs of poles results in less sag of the UTA, greater collimational stability, and less "wiggle" in a breeze.

It's no accident that professional observatory telescopes with truss structures (whether Serrurier or not) are widest at the top, or bottom, or middle but do not have pairs of poles parallel.
Here is an example of a commercial truss scope that takes non-parallelism of opposing truss pairs seriously:
Heavy Truss scope


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Dick Jacobson
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Reged: 12/22/06

Loc: Plymouth, Minnesota, USA
Re: How many trusses? new [Re: Starman1]
      #5569310 - 12/13/12 01:47 PM

Quote:

When 8 truss tubes are used with pole pairs parallel, the weight of the UTA is basically borne by the top two poles on the side. The bottom poles have little tension and they mostly sag.



In an 8-pole truss, when horizontal, the two upper side poles are purely in tension and the two lower side poles are purely in compression. This is a classical truss structure where all the members are either in tension or compression, with no bending force on any of them. The top and bottom pairs, as you say, do not bear any weight but merely prevent the UTA from rotating or moving sideways.

The beautiful Officina Stellare telescope that you link to has exactly the octagonal truss structure that I mentioned above. The Officina Stellare scope is a double truss (balance point at the middle, I assume) and my 20-inch was a single truss with a wooden mirror box, but the geometry is the same.


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