Author Topic: String Telescopes  (Read 1068 times)

Robert Bilbruck

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Re: String Telescopes
« Reply #15 on: January 31, 2018, 02:15:40 AM »
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I imagine extremely large telescopes (ELTs) use both tension and compression.


All truss designs of any flavor use tension AND compression. In a "string" scope the tension elements are converted to ones with no compression ability at all. It's not that complicated.

Demetrius Bryan

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Re: String Telescopes
« Reply #16 on: January 31, 2018, 06:45:49 AM »
With all respect Mark I'm disagreeing with your statement that string telescopes are stiffer overall; I'm disagreeing with the statement as a matter of principle from an engineering pov, as obviously execution is going to be the dominant factor, and yes of course, it is a given we are talking about for the same mass.
I'm happy to discuss it and be shown to be wrong. I'd like to think you can do the same without getting cranky.
Granted professional telescopes don't need to be knock down, but minimising mass whilst retaining stiffness is a major consideration in professional telescope design. After optical quality it is probably the single most important constraint for telescope design. If string principles offered more benefits than losses then that benefit would be exploited. But we don't see them, they simply don't exist as a successful design, because for a given mass string telescopes cannot - as a matter of engineering principle - be stiffer overall than a truss telescope.
Which isn't to say they can't be made stiff enough for an ATM who may have other objectives in mind, such as portability. They probably can.

The reasons are simple.
1. A string almost by definition has less mass than a truss tube, and thus will stretch more than a truss tube when in tension.
2. for a string to work as a structural member it must be kept in tension. In practice this means the structure must be preloaded to ensure that adequate tension is maintained at all angles to keep the string working as an effective structural member. As this preload must be taken up by the member in compression, the mass of the member in compression must be increased to compensate aka to resist flexure. As this preload invariably involves forces several times that imposed by gravity (depending on the material selected for the string, for carbon fibre this statement may be wrong), it follows that each structural element must have sufficient mass to resist flexing under the higher forces contained within the structure itself, which are greater than those imposed by gravity on either structure.

In short, a truss tube only needs to resist gravity.
A string tube must resist its own forces as well as gravity.
Therefore for an equivalent stiffness and choice of materials the string tube must have higher mass.
Or, for an equivalent mass, the string tube will be more flexible.

ridafimist

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Re: String Telescopes
« Reply #17 on: January 31, 2018, 07:10:24 AM »
Well hey, you can disagree with me all you like but the physics remains the same.   For the same mass it's demonstrably incorrect (see below).

Andthere are numerous examples of exactly this sort of approach in professional telescopes, the MacDonald 82" comes to mind, although it was completed in 1939. However that also doesn't affect the physics.

I saw Dan Gray's first string scope way back in '98 at the Oregon Star Party and I've followed this subject ever since, with my own string scope under construction for some time as a result. It is, indeed, a common sense approach to the design problem for many sizes and f-ratios. It's not a panacea, it's just another tool in the toolbox. For short fast scopes where truss sections the advantage become almost nonexistent though. At 14.7" f/5 as in my all-aluminum project it's an integral part of the design because, indeed, it offers better performance for the weight. But then you'd probably like that the primary collimation is accomplished at the EP via the three compression tubes (well yes, you only need two for that so ones a dummy) and that's tough to due with tube trusses (outside of some sort of hex platform design I read about somewhere  ).

Recommend you gather up a copy of Albert Highe's book on string scopes and have a read. He makes exactly the same points I've made and he provides the real world testing to support it. What he has to leave out (in the first edition anyway) is the application of tensegrity to string scope design, as first applied by Don Peckham, also here in the pacific northwest, with rather surprising results that pretty much blow everything else away. Perhaps Albert will get to include it in the next go round.

In the meantime: http://dbpeckham.com...StringScope.htm will get you started.

<p class="citation">Quote
If string principles offered more benefits than losses then that benefit would be exploited. But we don't see them, they simply don't exist as a successful design,
[/quote]
No, that's not a valid argument.

NB: I also want to point out for the OP that the tensegrity design works better the larger that central spreader becomes, due to the angles involved. But everything I've heard is that it works extremely well.

Also, **** forum ate part of my post - a fairly important part about tension and materials and tubes vs various string layouts of different materials - maybe it'll reoccur to me but at the moment it's doubtful. Ah ****, just get Albert's book and see for yourself.

erenlinra

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Re: String Telescopes
« Reply #18 on: January 31, 2018, 01:31:59 PM »
Also of interest in Dan Bakken's Hercules, a 41.2" f/3.9 that went through a couple incarnations in the mount design - turning into, yeah you guessed it, string scope the second time around:

http://wonderfuluniv...scope_page1.htm

Click on the 2nd link, or here, for the string version:

http://wonderfuluniv...pe_newherc1.htm

And see it here at OSP: http://bbastrodesign...html#Dan_Bakken

The views through that version are incredible to say the least. The tracking can be set to objects like the ISS, but you have to stay ahead of the EP as it moves quickly.

Adam Rice

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Re: String Telescopes
« Reply #19 on: February 02, 2018, 08:28:46 PM »
Mark

thank you for the link. Very interesting. Its extremely well thought through and superbly illustrated web site so definitely worth a closer look. Don is obviously an engineering purist. But I'm immediately struck by his tensegrity design and his arguments around it; Don directly addresses the issues I raised with respect to the relationship between tension/mass/flexure, so I've no doubt his design would work very effectively. But of course...note the example in this link, it has six members in compression plus the strings. Why not stick with a six member truss?
Here is the real minimalist telescope,although as he concludes the 4 pole design is more practical.
I'm not disputing that these designs will work. I think they are excellent fun.
What I don't see is any evidence or reason why either of these designs are more efficient than a similarly well executed 6 pole truss. Despite the improvements that reduce tension, these designs are still working against greater forces than a simple six pole truss, and they have more components. What I see is added complexity.
I will concede that as professional telescope engineers push the boundaries they have moved to more complex trusses, obviously in an effort to reduce mass, such as Gemini here. So its true that added complexity can reduce mass while maintaining adequate stiffness.
But I still don't see strings.
OK, granted the MacDonald 82 uses thin crossbars in tension so yes OK a string design. So such things exist and on that technicality I stand corrected. But that only amplifies the point; if it worked, if it was a great solution, why don't we see more? Why don't the newer larger lighter telescopes that push the engineering envelope use strings if strings offer a stiffer solution for a given mass? I still think the answer is because they don't, and all the arguments on Don's website only serve to illustrate why.

Scott Bentley

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Re: String Telescopes
« Reply #20 on: February 09, 2018, 11:35:59 AM »
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Several folks here in Portland have built string telescopes - Dan Gray is the original creator; David Nemo built a 20" string telescope and had a build thread here.


Link to my archived buildthread:http://nemoworld.com...ghtsthread.html

Last year I had the mirror reground to around f3.5 - as I got tired of moving and climbing a ladder. Rebuild was a snap. Just shortened the strings and poles, and good to go.

.........David