Stacked String Telescope Presentation
(2010 OSP Telescope Walkabout)
This is the presentation I gave during the 2010 OSP Telescope Walkabout. stacked string telescope
Over the past year I decided to convert my 8" F6 truss tube scope to a string scope in order to reduce setup time. Setup time was about 25 minutes but is reduced to about 12 minutes with the string design, including mounting the outrigger legs and the counterbalance spring..
With string telescopes there is an interrelationship with
The main advantage of the stacked string design is the large string angle. Strut force is proportional to the arctangent of the string angle divided by the number of struts. This equation works for all string telescopes. Doubling the string angle cuts the strut force in half.
Strut diameter is limited by buckling. Smaller strut force results in lower buckling, and lower buckling allows smaller diameter, lighter weight struts.
This design captures the struts at the middle. That reduces buckling and allows lighter weight struts. It also allows the two-piece struts that take up less space in the trunk.
Bending Moments -
The way to reduce bending moments is to attach the strings at the tops and bottoms of the struts. This depends on the string anchors.
My stacked string scope has 16 strings and 24 string anchors. Each string anchor is a chain link and a carriage bolt. In addition to being compact, light weight, rigid (very important) and inexpensive, this design allows very good control of the string attaching point. For this scope, the bending moments at the upper ring are essentially zero. That means the upper ring is less rigid and lighter weight.
The same string attaching detail is used at the mirror box so bending moments at the mirror box are essentially zero. That means I did not have to reinforce the mirror box to convert to the string design. I just drilled holes and bolted the string assembly to the existing mirror box.
Assembly Weight -
Flexible parts weigh less than rigid parts. The strings, struts, middle ring and upper ring are flexible along one axis but inflexible along another axis. The strings are inflexible in tension but flexible along other axes. The struts are inflexible in compression but flexible along other axes. The middle and upper rings are inflexible in the lateral direction but flexible in the vertical direction.
This scope has no turnbuckles in the strings. The upper ring flexes in the vertical direction to adapt to the tolerances in the string lengths.
I call the upper ring a "FLEX RING". I got the idea from Dan Gray's "flex plate" design.
In addition to the "flex ring" I call the top ring the "optics platform". Strings and struts attach to the "flex ring" but the optics attach to the "optics platform". The optics platform is attached to the "flex ring" by three spacers. Three points define a plane, so the flex in the flex ring is not transmitted to the optics platform.
Virtual Counterweight (Spring Counterbalance) -
The last thing I want to point out is the spring counterbalance. This is Tom Krajci's idea. The difference with my scope is that my scope goes about 20 degrees past vertical, a Jim Girard idea. The scope is top heavy. The counterbalance spring works on both sides of vertical.