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Equipment Discussions >> ATM, Optics and DIY Forum

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Starman1
Vendor (EyepiecesEtc.com)
*****

Reged: 06/24/03

Loc: Los Angeles
Re: 16" Ultralight Dob new [Re: B9Robot]
      #6129268 - 10/10/13 02:48 PM

Some good things:
--poles intersect at the ring
--pole pairs are not parallel
--pole attachments are centered on the poles, not offset
--spider is vertically symmetrical
Some bad things:
--3/4" poles won't be stiff enough to hold collimation in the lengths chosen. 1.25" would have been far more rigid.
--upper ring too thin
--focuser board WAY too long and heavy eyepieces will cause a lot of sag. The closer you get the focuser to the ring, the better.
--no primary mirror baffle--not only bad for light scatter, but also for dew.

Since you're going to be reading Albert Highe's book, pay attention to his chapters on stiffness measurements. You'll see that there are a lot of issues with the design in the link that could be addressed in construction.

Also, look at some of the design features used by Dennis Steele on dobstuff.com


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mrcotton73
member
*****

Reged: 10/31/08

Loc: Portland, OR
Re: 16" Ultralight Dob new [Re: Mirzam]
      #6131127 - 10/11/13 01:23 PM

Quote:

Sameer--I hope you will give us more details of your build as it progresses, especially the UTA.

JimC




Hi Jim

Sure - I will post some pics as I make progress with my project (which is fairly slow with all other commitments!). I finished the mirror box recently, and have a mock up of a wire spider.

Thanks
Sameer

Edited by mrcotton73 (10/11/13 01:26 PM)


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Arjan
super member


Reged: 01/21/09

Loc: Netherlands
Re: 16" Ultralight Dob new [Re: Starman1]
      #6131316 - 10/11/13 02:51 PM

Quote:

--3/4" poles won't be stiff enough to hold collimation in the lengths chosen. 1.25" would have been far more rigid.



Why not? Trusses only need to handle push or pull, 3/4" surely won't buckle...


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Starman1
Vendor (EyepiecesEtc.com)
*****

Reged: 06/24/03

Loc: Los Angeles
Re: 16" Ultralight Dob new [Re: Arjan]
      #6131522 - 10/11/13 04:27 PM

Quote:

Quote:

--3/4" poles won't be stiff enough to hold collimation in the lengths chosen. 1.25" would have been far more rigid.



Why not? Trusses only need to handle push or pull, 3/4" surely won't buckle...



When the tubes are that long, you have to be concerned with whether or not they can hold the weight of the UTA without sag. The longer the poles, the larger in diameter and/or the thicker they have to be in order to allow a small enough amount of sag that the scope will hold collimation.
In my experience with other scopes having similarly long poles, even ones in which the poles were farther apart, 1" diameters proved insufficient to prevent sag at the UTA and resulted in too much oscillation in the UTA in a breeze or if the upper end of the scope were tapped.

To avoid that issue, I even went with 1.25" poles on my 12.5" and they're only 31" long. I just barely have sufficient rigidity over the 80 degrees that my scope can move.

Long poles of that diameter will sag and easily twist torsionally (the weight of the focuser and eyepiece are off-center so rotational twist of the UTA is possible--and only resisted by sufficient rigidity in the poles).

I have seen 3/4" poles used in an 8" ultralight truss dob, and they weren't sufficiently rigid in that much-shorter scope. They certainly wouldn't be in the length required for a 16".

It's all about how much sag and collimation change you're willing to allow, of course, and how much you care whether collimation changes with altitude.


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mark cowan
Vendor (Veritas Optics)
*****

Reged: 06/03/05

Loc: salem, OR
Re: 16" Ultralight Dob new [Re: Starman1]
      #6131612 - 10/11/13 05:18 PM

Albert Highe's book goes into these very issues in great depth. Highly recommended.

Best,
Mark


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Starman1
Vendor (EyepiecesEtc.com)
*****

Reged: 06/24/03

Loc: Los Angeles
Re: 16" Ultralight Dob new [Re: mark cowan]
      #6131704 - 10/11/13 06:22 PM

Re: Mark's recommendation of Albert Highe's book, "Engineering, Design and Construction of Portable Newtonian Telescopes" :
+1 !


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acochran
professor emeritus


Reged: 06/19/08

Loc: So. CA
Re: 16" Ultralight Dob new [Re: B9Robot]
      #6131831 - 10/11/13 07:37 PM

Possibly research metal cable string type reinforcing of a truss system? I understand they can stiffen a truss or strut system. See the Dobstuff web site. See also the diameter of tubing that Dobstuff uses for different size scopes.
Andy


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Arjan
super member


Reged: 01/21/09

Loc: Netherlands
Re: 16" Ultralight Dob new [Re: acochran]
      #6132417 - 10/12/13 02:47 AM

Probably a matter of how much reality deviates from theory. Trusses should only be handling axial push and pull forces, so their endpoints could even be pivoting. Bending forces are absent, theoretically at least.

In my 10" Dob, I use 10mm U-profiles. The endpoints can rotate, so that the whole truss structure folds up for storage. I've never noticed any sag, but admittedly, never used a camera on it either...


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salies66
newbie


Reged: 08/25/13

Re: 16" Ultralight Dob new [Re: Arjan]
      #6132429 - 10/12/13 03:21 AM

hello http://www.astrosurf.com/centaurus/dobson-leger/william.htm
ere is mine, made ​​with fully from the recovery, the structure only 8kg for a total weight of 20 kg, a new, more lightweight and compact version is a study, if it can give you ideas


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careysub
Carpal Tunnel
*****

Reged: 02/18/11

Loc: Rancho Cucamonga, CA
Re: 16" Ultralight Dob new [Re: Arjan]
      #6135152 - 10/13/13 02:30 PM

As Albert Highe makes clear any deviation from the ideal force alignments in a truss can increase actual deflection by a factor of two or three (or even more) - this includes the connection points not being exactly on the member centerline, and the truss attachments not pivoting and being at the wrong angle.

In addition it should be remembered that ideal trusses only carry in-plane loads attached to their ends. Any other load will generate bending forces on the members. This includes torsion (mentioned by Don) and the weight of the truss members themselves. They are, after all, just beams on their own and can sag from their own weight if very long and thin


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