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NHRob
Post Laureate
Reged: 08/27/04
Posts: 3155
Loc: New Hampshire
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Scott,
Can you post pics? I'd love to see how your scope.
Thanks,
Rob
Quote:
I've had good luck with the following 6-tube arrangement. First, with 6 truss tubes, you get a slightly higher angle per tube than you do with 8. 6 is not only lighter, but has higher truss stiffness. 8 is overconstrained and MUST have some tortional loading. 6 is not and does not. I also don't have true truss mounts at all. All 6 tubes are hard mounted (glued) to the secondary cage and to a 6-sided mounting plate that attaches directly to the primary box with pinned and registered holes. I almost never have to collimate. When I do, it us usually the secondary mirror, not the trusses that is out of whack.
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Rob
**********************
Canon 12x36 IS binos
TEC-140: #433 on CGEM
Faworski orthos (7,10,16.7mm)
stuff ...
no free time ... lots of clouds !
" Life is what happens to you while you're busy making other plans" !!
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RossSackett
professor emeritus
Reged: 08/17/07
Posts: 692
Loc: Memphis, TN
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Yes, I have seen several examples of a counterweighted secondary assembly. However, I am skeptical that it would be lighter. While it would be less likely to rotate with changes in elevation, it will definitely put more stress on the vanes and these might need to be beefed up anyway.
Definitely worth experimenting with, though.
Ross
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"A craftsman relies on science when the state of knowledge allows it, tradition and experience when it does not, and makes art whenever he can."
12 scopes from 4.25 to 18" and a 24" in progress. 12 ATM awards. Webpage: http://stardazed.com/ Some more scope pix at http://www.flickr.com/photos/8315630@N04/
Anagrams: Amateur astronomer = A mature moon-starer; Dobsonian maker = Debonair as monk
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llanitedave
Humble Megalomaniac
Reged: 09/26/05
Posts: 12939
Loc: Amargosa Valley, NV, USA
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As for torsion on the truss poles, my design made that hard to avoid, since I wasn't able to line everything up completely parallel between the upper and lower truss connections.
I have to admit, the scope is pretty stiff.
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"Since the process of science generates more mysteries than it solves, I predict that we'll never learn everything: and we'll continue to generate new ignorance at the speed of knowledge."
"S.O.E." (Sauron's Other Eye), with 16" Royce conical mirror: A permanent work in progress.
10" Homebuilt dob, old Coulter mirror
Under Construction: The "Eye of Sauron" Observatory!
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jg3
sage
Reged: 05/27/07
Posts: 329
Loc: near Auburn, CA
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Having an engineering background, I can't come up with anything out of structural theory why pre-torsioning the poles improves rigidity of a truss. But experience with building and fixing stuff tells me it's a way to mitigate free-play slop that exists in many kinds of attachment joints.
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Scott Watson
sage
Reged: 05/26/06
Posts: 282
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Let me see if I have something on photobucket
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Starman1
Vendor - Scope City
Reged: 06/24/03
Posts: 12223
Loc: Los Angeles
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Quote:
Has anyone tried a balanced secondary assembly? There would be a counterweight on the opposite side of the spider from the secondary. This should eliminate the need for extreme tightness of the spider vanes. The amount of counterweight might be less than the weight of structure needed to support tight vanes. The counterweight could consist of batteries to power an anti-dew heater for the secondary.
My scope came this way, but the weight that balanced the secondary made the UTA way too heavy, so I removed the weight.
Paradoxically, with my new, rebuilt, lighter UTA, the UTA is too light when the scope points high, and I end up hanging a weight on the UTA exterior. Maybe I'll see if I can find that weight again......
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Don Pensack
12.5" Truss Dob, 5" Maksutov, Fujinon Binos
Sustaining Lifetime IDA member
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Nathan F
sage
Reged: 10/10/08
Posts: 221
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Thanks everyone for the great advice, especially Don. I'll keep those points in mind. Incidentally, do you have any pics of your scope? I'd love to see some close ups of the various design features you discussed in your post. And also, please write an in depth article on the subject. ;-)
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12 inch f/5 Deep Space Observer Dob
31mm Nagler, 13mm Ethos, 1.6x Antares 2 inch Barlow
Celestron SkyMaster 15x70 Binoculars
Homemade Maple Parallelogram Binocular Mount
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sixela
Postmaster
Reged: 12/23/04
Posts: 10861
Loc: Boechout, Belgium
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Quote:
2) the pole pairs should not be parallel to one another.
Come to think of it, this has another manifestation (and probably more important): you want the angle between the poles *in* a pole pair to be as large as possible.
But, of course, "possible" is constrained by other design criteria (the angles on my new scope aren't as large as on my old scope, but I'm rebuilding it because I want a small mirror box and a small UTA...) - but it's important to make the truss poles meet as close together as possible at the UTA and at the mirror box to squeeze out extra angle out of the room you have.
That's one reason David Kriege puts his clamps outside of the mirror box, even though it has ugly side effects as well (in particular where the clamps can interfere with the Alt bearings). But it's one of the reasons my mirror box truss clamps are *on* the mirror box and not *in* it (and damn the aesthetics; without consideration for aesthetics, it makes little sense to have the clamps inside, as you can just as well make the mirror box shorter and move them slightly out to put them on top).
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400mm f/4.46 self made Dobsonian on Tom Osypowski equatorial platform
Orion Starblast (114mm f/4 reflector, Alt/Az)
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474747
newbie
Reged: 04/19/08
Posts: 4
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"Has anyone tried a balanced secondary assembly? There would be a counterweight on the opposite side of the spider from the secondary."
It was suggested in Allyn Thompson's book. (1950's?)
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