Many of us have seen examples of truss tube telescopes. The first graphic shows a generic representation of the most commonly used truss arrangement in amateur telescopes. Do not get hung up on details of the upper ring/cage/square, or the main mirror and cell at the bottom end...I've intentionally omitted them. Note that this design uses eight truss members. At each end the various truss members are terminated at four points. Note that for an alt-az telescope...only half of the truss elements resist the force of gravity, and the other half of the truss elements resist sideways forces from wind or people. (Perhaps we should call this the 50% truss because of this quality? ;-) Can improvements be made so that more truss members contribute to stiffness in all directions simultaneously?

Clive Milne has pointed out that some professional literature describes a Quad-Tripod truss arrangement. This graphic shows the concept of one (of four total) tripods, supporting the upper end of the telescope.

Mel Bartels made a travel/backpack telescope, and you can see photos here. In some ways his truss arrangement is very similar to the graphic above...1/4 of a Quad-Tripod truss arrangement.

Here is a Quad-Tripod truss with all four tripods in place. You now have 12 truss members, instead of 8 in the above example. Also, at the bottom end of the truss you have eight points at which truss members terminate. Clive emphasizes that if you are sloppy at the bottom end and and do not provide a stiff, well braced termination for all eight points...you defeat the purpose of this design and your truss arrangement may actually end up less stiff than the typical amateur truss arrangement.

Ross Sackett has proposed a slightly different arrangement of truss members in what he calls an X-Brace. Note that in some ways it is very similar to the Quad-Tripod...the main difference is that the width of truss member spacing at the bottom end is double the amount in the Quad-Tripod. You have the same number of truss elements - 12. However at the bottom end you only have four truss termination points.

In this simplified X-Brace graphic I show the truss members as passing through each other. In real life this will present a fabrication or design challenge. I see three options here.

1. Fabricate truss members so that they indeed cross through each other. (How good are your fabrication skills?)

2. Alter spacing and arrangement so that the truss members cross next to each other. (Does this significantly degrade truss stiffness?)

3. Do not alter spacing and arrangement, but allow the truss members to bend slightly so that they cross next to each other. This will be easier for a truss with long, slender truss members. (Does this significantly degrade truss stiffness?)

...but I need to think again about a comment Clive Milne made to me about the Quad-Tripod truss arrangement...specifically the concern of stiff termination of all eight truss members at the bottom end of the truss. Clive mentioned that one approach for stiff termination of all eight members would be to use another, short Quad-Tripod truss to connect the mirror cell to the bottom of the first/larger Quad-Tripod truss arrangement. Here is one example of how this can be done. Note that this looks very much like the X-Brace, and if you remove the center square element, and straighten all the truss members...you end up with an X-Brace.

What truss arrangement will I pursue? It depends on what wind and vibration testing show. If wind-induced vibration is a problem, then I will probably favor thicker truss tubes. Thicker truss tubes will probably provide a mechanical structrure that is plenty stiff enough to hold acceptable collimation, which means I can probably get away with the typical amateur truss. More testing and analysis is needed before I can choose the design options for truss tube thickness and truss arrangement.

If you are thinking about using steel tubing for your truss, this is a good source of weight per foot (and other) information so you can do balance calculations. For example, 1.5 inch OD steel tubing, with 0.049 inch wall has a weight of about 0.76 pounds per foot. (Same OD, but 0.065 inch wall thickness is 1.0 pound per foot. 0.7 inch OD, 0.035 wall is 0.25 pounds per foot,) A sixteen inch mirror, 1.5 inches thick, weighs about 25 pounds. A primary mirror cell weighs about 7 pounds. A spider weights about two pounds. A CCD weighs about 2 pounds. (Other producers of steel tubing: http://www.copperweld.com/ http://www.markintubing.com/ http://www.hofmann.com/

Here is an example of balance point calculation for a steel truss: (using 0.7 inch OD, 0.035 wall) This truss has an overall length of 80 inches, and is 18 inches square. Balance point is approximately 24 inches from the bottom of the trus...and is where I have placed a 'central squre ring.' The main mirror is now shown, as well as the CCD (red), spider (gray), and baffle (dark gray). This initial sketch gives an idea of the scale and proportion of the telescope. Many more details need to be worked out, such as baffle dimensions.

What is a next step in the design? Making sure we have a stiff method of connecting the truss to the mount. Stay tuned....

All feedback is encouraged!

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Last update: 07 Dec 2002