Don's 8 in. Stacked String Telescope
This document describes my 8" F6 stacked (Dobsonian) string telescope. Note that this telescope has a counterbalance spring (Tom Krajci's design). Here is the presentation I made during the 2010 OSP telescope walkabout.
What makes this telescope different from other string telescopes? Different or unique features.
The stacked string design came to me as I sat in the OMSI auditorium waiting for the monthly Rose City Astronomer's meeting to start. While working on the "String Telescope Concepts" webpage I realized that rigidity of a string telescope depends on the angle of the strings with respect to the light path. The stacked string design has twice the string angle (the same rigidity for half the strut compression) as an equivalent non-stacked string telescope.
Larger F-number telescopes are a good candidate for the stacked string design. I decided to do a full scale mockup the stacked string for my 8" F6 telescope.
My initial plan was to use the same string length for both the upper eight strings and the lower eight strings. Whichever strings have the shallower angle with respect to vertical determine the rigidity of the stacked string telescope so it would not make sense to choose significantly different string lengths for the upper and lower sets of strings.
At the time I made the first eight bow strings I had not determined the final string anchor designs. It turned out that the upper set of strings were a different length than the lower set of strings, but this worked out for the best. I decided to put the longer strings above the middle ring and the shorter strings below the middle ring. This is beneficial because middle ring has spring clamps that capture the lower halves of the struts.
If you look closely at the following photo you'll notice that each strut has an upper half and a lower half. By putting the shorter strings on the bottom, the spring clamps on the middle ring capture the lower halves of the struts. The spring clamps are 3/4" plastic plumbing pipe.
I used Google SketchUp 3D CAD to draw the stacked string design. It is EXCELLENT, easy to learn, and FREE. I drew the middle and upper rings and then printed them out full scale. The left printout is the top upper ring. The middle printout is the bottom upper ring (The "flex ring" design has a top and bottom ring.). The right printout is the middle ring.
The following photo shows the middle ring. Notice that the spring loaded clamps in the corners are facing to the left and the right. I did this because the mirror box is 11"x10 3/4", but the middle ring is 10 5/8" x 10 7/8". This allows the middle ring and the upper ring to nest in the mirror box for transport.
I don't have a table saw so I used a jigsaw to cut the upper and middle rings oversize, and then I used a router to cut all the surfaces of the rings. It is a little time consuming, but you use the tools you have.
Notice that mirror box is resting in the base oriented 90 degrees from final position so that the altitude bearings can be attached.
Notice the (two-part) struts and the altitude bearings.
I built a full scale mockup before building the final version of this telescope. To save effort I started with my spring counterbalance scope and just replaced the 8-strut truss and upper ring with a 4-strut stacked string design. One of my design requirements was to use the existing optics.
The biggest irritation with the 8-strut truss version of the assembly and dis-assembly time. The string design reduces assembly time from about a half hour to about 10 minutes.
The "stacked" string telescope concept is described on the "String Telescope Concepts" webpage.
What makes this telescope unique - different?
Telescopes typically have rigid members. However, Dan Gray's 28" telescope has a "flex plate". The flexible assembly works because it is backed up by a rigid ground board. The flexible member is paired with the rigid member. Dan's flexible-rigid pairing led to the "Flex Ring" idea and caused me to think about the advantages of selectively choosing flexible-rigid pairs.
With this telescope the middle ring is flexible in the vertical direction (rigid in the lateral direction). This flexibility is paired with the rigidity of the mirror box. The lower strings attach the middle ring to the mirror box.
The upper ring BOTTOM ring is flexible in the vertical direction (rigid in the lateral direction). This flexibility is paired with the rigidity of the mirror box, through the middle ring, connected by upper and lower strings.
The struts are two half struts that stack one above the other. The struts are flexible in the lateral direction due to the pin between the top and bottom halves. This flexibility is paired with the rigidity (in the lateral direction) of the middle ring. The middle ring captures the struts laterally.
The "flex ring" (noted above) has a TOP and BOTTOM ring. The strings and struts only contact the flexible BOTTOM ring. The spider, focuser and finder scope only contact the TOP ring. The TOP and BOTTOM rings are connected by three spacers (three points define a plane).
Each of the 24 string anchors is a #4 chain link and a #10 carriage bolt. This design is inexpensive, compact, light weight and works well. The upper and lower eight chain links are long #4 links that are bent in a vice. The middle chain links are short #4 links.
The upper and lower chain links are located to minimize / eliminate bending moments in the upper ring and mirror box. By minimizing the bending moments I prevent the need to have a more rigid (heavier or more expensive) upper ring and mirror box.
This telescope has no turnbuckles or eyebolts to compensate for variations in string lengths and string anchors. I intentionally allow the middle and upper rings to flex in the vertical direction only to compensate for these variations.
Here are some sketches:
Two-part struts work well. They take up less storage space.
Making String Anchors
String Anchor Study
(Note: For both photos the strut compression eyebolts are tightened to give the same strut compression. The upper ring is 5 mm thick plywood.)
What would I do differently next time?:
If I was starting from scratch I'd use larger side bearings and eliminate the counterbalance spring.
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
Truss Tube to String Telescope Conversion
Greg's Right Angle Telrad