Flex Ring

(Self-Adjusting* Upper Ring Used With 4 Strut String Telescope)
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This document describes a flexible ring design used with a four strut string telescope. 

*This design is "self-adjusting" because the strings in the string telescope with a "flex ring" do not require turnbuckles to compensate for variations in string lengths and variations in string anchor locations.  The upper ring flexes to account for variations.


The 4 strut string telescope design has some advantages over a 3 strut design (see comparison). However, the 4 strut design presents a problem in that a three point mount is required for the upper ring. With the 4 strut design the strings at three of the struts define a plane. It is theoretically not possible to adjust the strings at the fourth strut to align the tops of all four struts to be exactly in the same plane.

The Flex Ring design provides a solution. The Flex Ring assembly includes a flexible lower ring that conforms to the height of the four struts, and a rigid upper ring that is attached to the lower ring with a three point mount.


The Flex Ring design reduces or eliminates the need for turnbuckles on the strings of the 4 strut string telescope.  When the string telescope is initially setup the links that attach the strings should be adjusted to equalize the tension and keep the lower ring close to flat.  However, distortion in the lower ring does not result in distortion of the upper ring that is the mounting platform for the focuser, spider, etc.

Flexible Lower Ring:

The lower ring shown is 0.25 inch (6 mm) thick and with an outside diameter of 16.5 inches (420 mm).  The ring is rigid in the lateral direction and flexible in the vertical direction.

Bow strings apply both vertical and lateral forces at the lower ring.  The lateral forces put the lower ring into compression in the lateral direction since the lower ring is rigid in the lateral direction.  The vertical forces put the struts into vertical compression.  The lower ring is flexible in the vertical direction so the lower ring distorts to conform with the lengths of the bow strings.

Note that all contact with the strings and struts is applied at the lower ring.  These forces are not transmitted to the upper ring.
There are three spacers on the top side of the lower ring.  These spacers define a plane and act as a three-point mounting location for the upper ring.
Notice the long slot in the lower ring.  The focuser mounting bracket is attached to the upper ring but passes through this slot in the lower ring.  Neither the focuser mounting bracket nor any part of the upper ring, spider or secondary mirror have any contact with the lower ring.  The ONLY contact between the upper and lower rings is the three spacers.

Rigid Upper Ring:

The upper ring shown is 0.5 inch (12 mm) thick and the same diameter as the lower ring.  It is important that the upper ring is rigid in the vertical direction.  The upper ring is a rigid platform that mounts the spider and focuser.  The stiffness of the upper ring is increased when it is attached to the lower ring (see description*** below).  The focuser and spider mount to the upper ring and do not contact the lower ring directly. 
Here is another view of the upper ring with the spider and focuser mounted.

Ring Assembly:

***When the upper and lower rings are attached together the resulting stiffness (moment of inertia) is greater than the stiffness (moment of inertia) of the upper ring by itself.  The three spacers (circled below) are the only contact between the upper and lower rings. 
Here are additional views of the ring assembly.
Here are views of the flex ring on my 8" string telescope.
Image  Image

Lower Ring Flex:

When the struts are compressed, the resulting vertical string force causes the lower ring to flex (distort).  The photo below shows a ring assembly where there is exaggerated distortion** in the shape of the lower ring.  The photo is taken sighting across the bottom surface of the upper ring.  The spacers between upper and lower rings are circled.

**Note:  The string mounting links were intentionally adjusted to different lengths to demonstrate that the upper ring is flat when the lower ring is distorted.  On an actual telescope the string mounting links would be adjusted to minimize the distortion in the lower ring.

SketchUp Model:

The Google SketchUp model below can be downloaded with the "Google 3D Warehouse" link below .

Some of my astronomy projects:

Don Peckham
email:  don@dbpeckham.com