Don's Tensegrity String Telescopes
What Is A Tensegrity String
A "tensegrity string
telescope" is a string telescope that uses a tensegrity
structure as the basis for the string telescope.
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
There are MANY
of string telescopes
. However, the most common
string telescope design is the 3 strut telescope.
and strut compression forces are ~50%
to ~90% lower than a non-tensegrity string
at the upper ring and mirror box are zero.
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
What Is A Tensegrity
How Does A Tensegrity
String Telescope Work?
Tensegrity stands for Tens
Fuller came up with the name "tensegrity" but he did not
create tensegrity. A tensegrity structure (also
known as "floating
) 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.
Features Common With All
- The strings and middle ring tubes are fixed length
- The struts are variable length. The strut
length effectively changes by tightening screws at the
ends of the struts.
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.
- 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:
Weight At The Upper Ring
- The weight
at the upper ring. This includes the
weight of everything attached to the upper ring plus
half the weight of the tubes.
- The string
angle. This is the angle between the
string and the strut axis.
- Where the
strings are attached. This determines
what bending moments are applied to the upper ring and
mirror box by the strings.
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
String Lateral Force
= Upper Ring Weight
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
[F(string)] and strut
[F(axial)] to lateral
comparison purposes, the following examples will have the
same length for F(lat.).
on the left is a "Traditional" 3 Strut String
F(axial) / F(lat.) =
|The image on the
right is a Tensegrity 3 Strut String Telescope.
F(axial) / F(lat.) =
and strut compression forces are approximately 75%
lower than the Non-Tensegrity telescope.
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
- 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
- The structure becomes unstable when there are three or
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
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
- 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
I am planning to build a variation of this design
during the spring of 2016.
Middle Ring String Anchors
Here is a description of my middle
ring string anchors:
I started with 5 inch stainless steel
I cut off the heads, put the safety
pins in a vice and pried open the coils.
I inserted the string anchors into the
middle ring tubes. I inserted a Kevlar cord into the
middle ring tubes to hold them together when the telescope
is disassembled. I slipped the string ends onto the
It is OK to use an elastic (shock) cord
as previously suggested as long you also
use a Kevlar cord to keep the middle ring tubes together if
the elastic cord breaks.
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
- 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.
- 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
- 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
- I suggest using larger diameter tubes than
needed. A small increase in weight will give a
LARGE increase in rigidity. I prefer to overdesign
My Tensegrity Telescopes
3 Strut Telescope
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.
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 plane. This
design applies zero lateral forces at the upper ring
and mirror box.
is something to keep in mind when designing tensegrity
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
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.
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
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"
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
- 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
- The middle ring is kept smaller for better packaging
This photo shows the
telescope disassembled with the SiTech
Servo 1 controller installed. The altitude
bearings are attached with thumbscrews and are removed
- 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: