Truss Tube to String Telescope Conversion

Home Page:  dbpeckham.com


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Overview:

This webpage shows how to convert your truss tube telescope to a string telescope by bolting on the string assembly with minimum modifications to the mirror box.  Drill mounting holes in your existing mirror box and bolt on the string assembly.

Advantages of String Telescope:

  • Easier setup (easier setup in the dark by one person)
  • Quicker setup
  • Fewer loose parts (Struts are the only loose parts.   Everything else is attached to the mirror box.)
  • Lighter weight (four struts instead of eight)

Description of Images Above:

Design #1 -  is an 8" F6 truss tube telescope with eight 3/4" diameter struts (trusses).  Here is the actual telescope.

  • Loose parts include:
    • Upper ring
    • Struts
    • Strut mounting hardware

Design #2 - is a "traditional" string telescope with four struts and eight strings.

  • Upper ring is attached to mirror box by strings
  • Struts are only loose parts, no loose hardware

Design #3 - is a stacked string telescope.  Here is the actual telescope.

  • Upper ring and middle ring are attached to the mirror box by strings
  • Struts are the only loose parts, no loose hardware

Differences Between Designs #2 and #3:

Design #3 may be more appropriate than Design #2 for large F-number telescopes.  See String Telescope Concepts for more details.

Item Design #2 Design #3

Number of struts

 4 8 (4 two-part struts)
Turnbuckles in Strings yes no
Number of Strings 8 16
Middle Ring no yes
Strut Compression Force
(see telescope concepts)		 2X Design #3 force
more rigid (heavier weight) struts		 1/2X Desgin #2 force
less rigid (lighter weight) struts
Upper ring Standard Self-Adjusting Flex Ring

The choice of Design #2 or #3 is not addressed on this webpage.  

Assumptions:

String Assembly Mounting Detail:

 

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String Attaching Detail:

The string attaching detail is very important to minimize bending moments in the upper ring and mirror box.  Click this link for a string anchor study.

String Attachment At Upper Ring:

The string attachment at the upper ring is similar to the string attachment at the mirror box. 

  • Drill three holes in each corner of the upper ring
    • The two small holes are used to mount aluminum plates above and below the horizontal member
    • The large hole is a clearance hole for a threaded insert

  

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Why The Design Works:

The design works because the strings attach at the tops and bottoms of the struts, which means bending moments in the mirror box and upper ring are essentially zero.  For a description of how string attaching location impacts bending go to the "String Anchor Location Impact on Bending Moments" heading.

 

Some of my astronomy projects:

12.5" F4.5 String Telescope     
Two Cylinder Equatorial Platform with Floating South Mount      
Flex Ring (Flexible upper ring used with four strut string telescope)   
String Telescope Concepts   
8" F6 Stacked String Telescope 
Greg's Right Angle Telrad   


Don Peckham
email:  don@dbpeckham.com