Don Peckham's 12.5" F4.5 String Telescope




This document describes my 12.5" Four Strut String Telescope.


If you are considering building a string telescope, I suggest that you consider a "Tensegrity" string telescope.


My 12.5" F4.5 telescope started as an eight strut truss tube (Dobsonian) telescope.  I later computerized the telescope using the SiTech controller.  The telescope works well either pushed around without the controller, or SiTech controlled, but takes 20-30 minutes each to assemble and disassemble.  I've been intrigued by the string telescope concept since seeing Jane's string telescope.

I've looked at several string telescopes on the Internet.  The three strut designs seem to be prevalent.  However, it seemed intuitive that four struts would better suit my needs for the following reasons:
  • Four struts provide 33.3% greater compression force than three struts simply because there are more struts.
  • The four strut design allows strings to run from the tops to the bottoms of struts which means all the string loading goes into the struts and not the upper ring or mirror box.  That means the upper ring and mirror box do not need to be particularly rigid.
  • My mirror box is square so seemed to be a good fit with four struts that would fit in the rigid corners of the mirror box.
  • The four strut design has about 2 times the lateral force and couple (at the upper ring) of the three strut design.
The more I looked into how string telescopes work the more it convinced me that the four strut design is the most appropriate design for me.  As part of my string telescope study I created the "String Telescope Concepts" webpage.

One drawback of the four strut design is that there are eight strings compared to six strings with the three strut design.  Three points define a plane so the additional strings makes it more difficult to adjust the string lengths... if the upper ring is RIGID.

My solution is to use a flexible upper ring.  I got the idea from Dan Gray's "flex plate" design.  If a "flex plate" can be flexible, why can't the upper ring be flexible?  (See Selectively Flexible Members)

My upper ring (the part that attaches to the strings) is 1/4" thick.  There are no turnbuckles in the eight strings so the flex ring intentionally flexes to compensate for string length tolerances and string anchor location tolerances.  This link describes how the Flex Ring design works.


Design Requirements:

  • Reduce Assembly Time:
My main requirement was that I reduce assembly / disassembly time.  After the altitude bearings are installed, the new string design takes two minutes to assemble / disassemble. 
  • Fewer loose parts:
The strings are permanently attached to the mirror box and upper ring.  My previous design had eight struts and twelve thumb screws to attach the upper ring to the mirror box.  The string design has four struts.
  • Upper Ring Nests in Mirror Box for Transport:
The upper ring nested with the previous design.  It also nests with the string design.
  • One Person Setup:
This design can be assembled by one person in the dark with minimum loose hardware.

Design Details:

Mirror Box:

Here is a view of the mirror box.  I used an 18 point mount calculated by the "PLOP" mirror cell optimization program that is available on the Internet.

Image  Image


Here is a view of the base.  Notice the "pulley" on the bottom.  Notice that Teflon pads are directly above each of the round feet.

Image  Image

Upper Ring (Flex Ring):

My upper ring (the part that attaches to the strings) is 1/4" thick.  There are no turnbuckles in the eight strings so the flex ring intentionally flexes to compensate for string length tolerances and string anchor location tolerances.  The following link describes how the Flex Ring design works.  It actually works quite well and is more rigid than a 3/4" thick upper ring.



The upper ring nests in the mirror box and the mirror box nests in the base as shown below.

Image  Image

Anchor Details:

The following photo shows an enlarged view of a bracket.  The bottom of the strut pilots in the hole in the bracket.  The two string anchors are each a pair of chain links.  The string anchor on the left has a string attached.  The string anchor on the right has both a string and an elastic cord attached.  The elastic cord prevents the shourd (cloth cover) from falling into the light path when the scope is horizontal.


The sketch (below) on the left shows the bracket and the string anchors.  The sketch on the right shows that there is one bracket on the top and a second bracket on the bottom side of the mirror box lower member.  The right sketch also shows the capscrews that go through both brackets and hold the string anchors in place.  The string anchors are two links from a steel chain. 

Bottom Anchors:
  • The holes in the brackets are drilled as a matched pair.  That assures that both capscrews are vertical.
  • Each of the two string anchors is a pair of steel chain links.  When the struts are compressed which results in string tension, the string anchors are VERY rigid so there is no flex in the string anchors.
Image  Image
  • The end of the strut pilots in the large hole, and the strut pushes down against the bracket.  The forces of the strings are very close to the strut.  Therefore, all the string tension forces go into the strut instead of into the mirror box.  Therefore, the mirror box does not have to be very rigid.
  • There are no turnbuckles in the strings because this design has a flexible upper ring that flexes to compensate for tolerances in string lengths and string anchor locations.  If there is a need to make a minor adjustment in string anchor position a couple of washers could be placed beneath the string anchor link as shims.
  • To keep the bow string from falling out the slot in the link a small piece of irrigation tubing is slit and covers each slit.

Top Anchors:

The string and strut mounting at the upper ring is the same as with the lower ring except that the brackets are a different shape.  The following photo shows that strings attach to the two string anchors, and an elastic cord is attached to the left string anchor.  Also, there is a 2-1/2" tall thin plastic piece hanging from the chain anchors.  This plastic piece is used to attach the shroud to the upper ring.

Notice that the brackets are attached to a 1/4" thick upper ring.

Image  Image

Additional String Anchor Details:

Here are some additional anchor details that I am using on my stacked string telescope.  They are simply a welded link from #4 chain and a 3/16" (#10) carriage bolt.  The link on the left is bent in a vice with a hammer.
Image   Image

Four Strut Angle Struts:

I used a four strut design where strings attach at the tops and bottoms of struts, with angled struts.  I chose the "four strut" design because four struts result in twice the lateral force and couple of three struts (see String Telescope Concepts).  I had to go with angled struts because I wanted the bottoms of the struts to be in the corners of the mirror box (where there is a stiff box section) and my upper ring diameter had to be small enough to nest in the mirror box.

I used 3/4" diameter wood struts because I know how to work with wood.  The four strut design allowed me to use smaller diameter (lighter weight) struts than some of the other design concepts.  In this photo notice that I can lift the telescope assembly with one hand on the mirror box and the other hand under a strut, without buckling the strut.


Strut Design and Tensioning:

The struts are 3/4" hardwood dowels with a hex-socket capscrew and washer at the top and bottom (same detail both ends).  The diameter of the capscrew pilots in the hole in the bottom bracket. 


At the upper ring eyebolts are used to tension the struts.  The end of the eyebolt is inserted into the hex of the hex-socket capscrew at the top of the strut.  The four eyebolts are tightened until all are snug and the strings are tensioned.


Bow Strings:

I followed David Nemo's excellent instructions to make my bow strings.  The photo below show six loops (twelve strands) of bow string made per David's instructions.

The photo also shows what I call parachute cord and fishermen apparently call "Slinky Cord".  This cord was suggested by David Nemo.


This photo shows the bow string inserted into the parachute cord and tied at the end so the cord will remain in place.



The shroud is attached to the upper ring when the telescope is assembled.  The cloth cover remains attached to the upper ring when the telescope is disassembled and the upper ring is nested in the mirror box.  In the photo below notice Velcro strips at the right side (top edge) of the cloth.  There are also Velcro strips at the bottom (vertical edge) of the cloth.


The Velcro at the top of the cloth attaches to Velcro tabs hanging down from the 2-1/2" tall thin plastic strip.

Image   Image


The shape of the shroud is defined by the eight strings and four elastic cords.  In the following sketch the strings are black and the cords are red.  See the Anchor Details for photos of how the strings and cords attach to the string anchors


Dew Shield:

The dew shield is closed cell foam that extends above and below the upper ring.  There is a large hole the size of the focuser.  The hole in the dew shield rests on the focuser.  There is Velcro on both ends of the dew shield.  The dew shield wraps around the upper ring.

Note that adhesive does not stick very well to the closed cell foam.  For both the Telrad mounting surface and the Velcro mounting surfaces at both ends I cut out pieces of black plastic and hand-sewed the plastic to the foam.  The plastic is a surface to which the adhesive-backed Velcro will adhere.

The dew shield serves the following purposes:
  • Prevents dew from forming on the secondary mirror.  The idea for the dew shield came from Gary Wolanski.
  • Acts as a light shield behind the secondary (when looking through the eyepiece).
  • Is the mounting surface for the Telrad bracket.

Assembly Procedure:

It is easy for one person to assemble this telescope in the dark without assistance.  Not counting the time to mount the altitude bearings, it takes two minutes to assemble or disassemble this telescope.  Here are the steps:

Note:  The shroud us typically attached to the upper ring when nested and when assembled.  The cover was removed to make the following photos more clear.
  • Attach the altitude bearings to the mirror box and place the mirror box on the base with the mirror box in the vertical position.
  • Put two of the struts into string anchor brackets in opposite corners of the mirror box .  The struts lean out as shown below.
  • Lift the upper ring out of the mirror box.  As I lower the upper ring to nest in the mirror box I rotate it 360 degrees.  As I lift the upper ring I "un rotate" it.
  • Insert the ends of the eyebolts into the hexes at the ends of the struts.  Do not tighten the eyebolts yet.
  • Then, put the other two struts into the mirror box string anchor brackets.  The struts lean out as shown below.
Image   Image
  • Screw the eyebolts into the remaining two struts that are shown in the photo above.  Tighten all four eyebolts evenly until the strings are tight.
Image    Image
  • Wrap the close cell foam dew shield around the upper ring with the hole in the foam around the focuser.  I have never had a problem with dew forming on the secondary mirror with this dew shield. 
  • The Telrad bracket is "permanently" attached to the dew shield with Velcro.

Disassembly With Shroud Attached:

The shroud is attached "at all times".  The following photos show the scope being disassembled with the shroud attached.  The upper ring is rotated 720 degrees as it is lowered into the mirror box.  The rotation prevents the strings from tangling as the upper ring is lowered.

Image  Image  Image  Image  Image  Image  Image

Google SketchUp:

I used Google SketchUp to design the struts and upper ring for this telescope.

What would I do differently next time?:

I would revise and simplify the string anchors to match the string anchors on my stacked string scope.  See the string anchors on my stacked string scope.

Some of my astronomy projects:

8" F6 Stacked String Telescope  
8" F6 String Counterbalance Telescope  
12.5" F4.5 Computerized Telescope (SiTech Controller)   
Two Cylinder Equatorial Platform with Floating South Mount  
Truss Tube to String Telescope Conversion  
Greg's Right Angle Telrad  

Don Peckham