Don's Tensegrity String Telescopes

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Overview


What Is A Tensegrity String Telescope?

A "tensegrity string telescope" is a string telescope that uses a tensegrity structure as the basis for the string telescope.

IMPORTANT:
Tensegrity frees the designer to be creative.  A few basic examples are shown.  I encourage the designer to use the examples and concepts on this webpage, and the String Telescope Concepts webpage, as a starting point.  Create your own tensegrity variation.

What Are The Benefits Of Tensegrity String Telescopes vs. Non-Tensegrity String telescopes
There are MANY variations of string telescopes.  However, the most common string telescope design is the 3 strut telescope.
  • String tension and strut compression forces are ~50% to ~90% lower than a non-tensegrity string telescope.
  • Bending moments at the upper ring and mirror box are zero.
  • Small forces and bending moments allow the tubes, upper ring and mirror box to be less rigid and lighter weight. 
  • Smaller string and strut forces lower the natural frequency of the telescope so vibration should be a lower concern.
What Is A Tensegrity Structure?

Tensegrity stands for Tension Intengrity.  Buckminster Fuller came up with the name "tensegrity" but he did not create tensegrity.  A tensegrity structure (also known as "floating compression") has the following:
  • Strings are in pure tension
  • Tubes are in pure compression
  • There are no bending moments
  • Tensegrity structures are ultra lightweight
  • If one string or tube breaks, the structure fails.  Each string and tube depends on every other string and tube.
  • Tubes are limited by column buckling.
How Does A Tensegrity String Telescope Work?
  • The strings and middle ring tubes are fixed length and inflexible. 
  • The struts are variable length.  The strut length effectively changes by tightening screws at the ends of the struts.
  • The location of the upper ring is controled solely by the lengths of the strings and middle ring tubes.  As long as the strings are in tension middle ring tubes are NOT bowed, the tensegrity string telescope maintains collimation, even if the struts are bowed.
  • With both 3 strut and 4 strut telescopes, the middle ring compensates for tolerances in string lengths and string anchor locations.  That means the upper ring remains flat regardless of string length.  Note that the upper ring may not be parallel to the mirror box but that doesn't matter.
Features Common With All String Telescopes
  • Few loose parts
  • Quick setup
  • Maintains collimation between setups
  • Compact (disassembled) size for transportation

3 Things To Observe With String Telescopes:


There are 3 things I look at with string telescopes:
  1. The weight at the upper ring.  This includes the weight of everything attached to the upper ring plus half the weight of the tubes.
  2. The string angle.  This is the angle between the string and the strut axis.
  3. Where the strings are attached.  This determines what bending moments are applied to the upper ring and mirror box by the strings.
Weight At The Upper Ring

With string telescopes the struts are (typically) vertical and are in compression.  When the tube assembly is horizontal, the full weight at the upper ring is supported by the lateral force component of the string tension.  The red arrows represent the upper ring weight.

String Lateral Force Component  =  Upper Ring Weight

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Important:  When a designer is selecting a telescope design for a specific mirror, etc., the lateral force component is a CONSTANT regardless of the number of struts or pairs of strings of a string telescope


String Angle With Respect to Strut Axis

This is a visual way to compare the string tension force [F(string)] and strut compression force [F(axial)] to lateral force component [F(lat.)].

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Note:  For comparison purposes, the following examples will have the same length for F(lat.). 

The image on the left is a "Traditional" 3 Strut String Telescope. 

F(axial) / F(lat.) = 8:1

The image on the right is a Tensegrity 3 Strut String Telescope.

F(axial) / F(lat.) = 2:1

Therefore:  String tension and strut compression forces are approximately 75% lower than the Non-Tensegrity telescope.

Note:  The string and strut foces can be further reduced by making the middle ring bigger.



Where Are The Strings Attached?


When strings are attached at the ends of the tubes the bending moments are zero.  When strings are attached away from the ends of the tiubes, the strings apply bending moments to the part (upper ring and/or mirror box). 


Some Tensegrity String Telescope Possibilities


The tensegrity telescopes described in this webpage have a middle ring(s) with floating tubes.  There is no "standard" tensegrity telescope design.  Tensegrity telescopes can have multiple middle rings with floating tubes.
  • One middle ring (with floating tubes) is required between the rigid upper ring and rigid mirror box.
  • Two consecutive middle rings (with floating tubes) are acceptable between the rigid upper ring and rigid mirror box.
  • The structure becomes unstable when there are three or more consecutive middle rings (with floating tubes).
The more "efficient" designs have vertical struts and horizontal middle ring tubes.  When struts are vertical, strut compression and string tension are lower than when struts are on an angle with respect to vertical.

Strut compression can be reduced by using a larger middle ring.

Here are a few examples:
  • The first example has 3 struts with the minimum size middle ring to clear the struts.
  • The second example has 4 struts and the minimum size middle ring to clear the struts.  The string angle is larger than the the first example which means the string and strut forces are lower than the first example.  In addition, string and strut forces are spread over 4 struts instead of 3, which makes makes string and strut forces even lower than the first example.
  • The third example has a larger middle ring so larger string angle, thus even smaller string and strut forces than the second example.
  • The fourth example has two middle rings and an even larger string angle.  Having more than one middle ring may be a vialble option for large F-Number telescopes.  Note:  I am planning to build a variation of this design during the spring of 2016.

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Middle Ring String Anchors

Here is a description of my middle ring string anchors:

I started with 5 inch stainless steel safety pins.
temp

I cut off the heads, put the safety pins in a vice and pried open the coils.
Tens

I inserted the string anchors into the middle ring tubes.  An elastic chord (shock chord) keeps the middle ring tubes together when the telescope is disassembled.  I slipped the string ends onto the string anchors.

Dos and Don'ts

  • The middle ring tubes should not be constrained by strings or the struts.  Otherwise, collimation may not be reliable.
    However, it is acceptable for the middle ring to constrain the struts.
  • The strings should not be constrained by the middle ring tubes or the struts.  Otherwise, collimation may not be reliable.
  • However, it is acceptable for the middle ring to constrain the struts.
  • Strings should attach directly to the string anchor.  However, a chain link is OK if only one string attaches to each chain link.

OK
 
  NOT OK
  • The string centerlines should intersect the centerlines of the tubes where the tubes attach to the upper ring/mirror box.  Otherwise, bending moments will apply bending moments to the upper ring or mirror box.
  • I suggest using soft steel (rebar) wire to mock up string lengths before building your strings.
  • Strings MUST be inflexible.  I used BCY 450 Plus bow string.  I used 5 loops with my 3 strut telescope strings and 4 loops with my 4 strut telescope.  See the bottom of this website for string making.  I snaked the bow string loop through a parachord 550 chord after removing the inner strands from the parachord.
  • Use the same string angle (with respect to vertical) above and below the middle ring.
  • I suggest using aluminum tent poles.  Aluminum tent poles are available in many sizes.  I got my tent poles from Tent Pole Technologies in Vancouver, WA.
  • I suggest that carbon fiber tent poles NOT be used.  If carbon fiber tent poles are scored, they can snap when bent.
  • For my middle ring string anchors I used 5" stainless steel safety pins that I modified.
  • Think about how you will transport your telescope when deciding between 3 strut and 4 strut designs.  The 4 strut design can have a smaller middle ring, in addition to having lower string tension and strut compression.
  • I suggest using larger diameter tubes than needed.  A small increase in weight will give a LARGE increase in rigidity.  I prefer to overdesign telescopes.

My Tensegrity Telescopes


3 Strut Telescope

I built this telescope without knowing what "tensegrity" was.  My telescopes had many itterations.  Do NOT use the key rings shown at the ends of the middle ring tubes.  In this itteration of the design I used those rings to add length to the upper strings for the sake of collimation.

Note:  Typically with string telescopes the strings apply lateral forces at the upper ring and mirror box.  However, this design is peculiar in that all strings and tubes are in the same planeThis design applies zero lateral forces at the upper ring and mirror box.  This is something to keep in mind when designing tensegrity structures.

 

This is my "proof of concept" full scale structure for the 3 strut telescope.  Struts were 0.490" x 0.026" tent poles, and middle ring tubes were 0.340" x 0.025" tent poles.  I hung 26 pounds of tools from the upper ring.  The struts and middle ring tubes all bent but did not collapse with the weight.  This means collimation is compromised.  The actual final weight at the upper ring was 3.7 pounds so this structure is way overdesigned.



The finished 3 strut telescope supports a 45.8 pound case of bottled water.  The strings are starting to go limp which means collimation is compromised.  Another observation is that ratio of axial to lateral force is 2:1 (as described above).  The ratio of 45.8 pounds to 26 pounds is approximately 2:1. 



This telescope was used as a tent pole at OSP 2015.



I presented my telescopes at the 2015 Oregon Star Party "Telescope Walkabout".

I presented tensegrity telescopes at the September 21, 2015 general meeting of the Rose City Astronomers.  Here is the PDF file from that presentation.

I decided AGAINST using the 3 strut design because:
  • The middle ring was an awkward shape and did not pack well to travel, and
  • 3 struts are not a good match with a rectangular mirror box.



4 Strut Telescope, My "Final" Design


Having VERY LOW string tension and strut compression, and zero bending moments frees the designer to be creative with tensegrity.  My 4 strut telescope (shown below) deviates from "optimum" design practice.

Things To Notice:
  • The upper ring is rotated 45 degrees with respect to the mirror box for better eyepiece position.
  • The string pattern above the middle ring is different than the string pattern below the middle ring.
  • Struts on an angle means higher string tension and strut compression than with vertical struts.
  • Different string angles for strings with respect to the strut axis means different strings have different string tensions.
  • The middle ring is kept smaller for better packaging during transportation.


This photo shows the telescope disassembled with the SiTech Servo 1 controller installed.  The altitude bearings are attached with thumbscrews and are removed for transportation.





Hardware

I am using an HC-1 helical focuser (2.2 oz.)  by KineOptics of Sequim, WA.  http://www.kineoptics.com/HC-1.html

I am using a Rigel Systems QuickFinder reflex sight.

Conclusions
  • Tensegrity string telescopes have significantly lower string and strut forces than non-tensegrity string telescopes.  I'd recommend tensegrity string telescopes over non-tensegrity string telescopes.
  • I consider the tensegrity 4 strut design to be superior to the 3 strut design.  Not only are string and strut forces lower than the 3 strut design, but the envelope of the telescope is smaller.



Some of my astronomy projects:



Don Peckham
email:  don@dbpeckham.com