Strut Telescopes: Flex and Rigidity

DISCUSSION:  The topic of flex in strut telescopes continues to come up from time to time as the question begs to be answered -- "are these telescopes rigid enough?". The absolute standard in the construction of any telescope is a completely rigid optical system. You can attain this by using a solid "telescope tube". Many vendors use aluminum or cardboard. But the disadvantages are many: weight, difficult to transport, setup and storage of the telescope -- especially in the larger apertures. If you've ever struggled with a 12" you know what I mean. By the time one gets to the 15"-18" telescopes, the disadvantages are multiplied many-fold!

The weight, setup, storage and transportation issue has been addressed by a variety of construction techniques. For a long time now, the conventional "truss" design has become the new "standard" in telescope construction. By definition, the truss in an inherently rigid configuration. A "triangle" cannot shift or collapse. The only exception is if the poles themselves (the sides of the triangle) change in length. Again, however, these telescopes can be heavy and difficult to manage.

STRUTS: The strut design addresses these issues directly. By comparison, it is a light, transportable, easy to setup and easy to store telescope. The mirror box, with the mirror installed, is usually the heaviest component of the telescope. Yet it is still considerably lighter than the traditional "mirror box". In the DobSTUFF design, even with the dew shield in place, primary mirrors cool more quickly and the assembly is  just as stable and rigid as the more traditional mirror box.

It's the use of parallel aluminum tubing, struts, along the length of the optical tube assembly (OTA) that generates the discussion of flex in these telescopes (the flex of aluminum tubing is proportional to the cube of its length). Not to get too technical in this discussion, suffice it to say that -- given any diameter and wall thickness of an aluminum strut, longer tubes exhibit flex more than shorter tubes.  Finally, you can see whether your OTA exhibits any flex with a laser collimator. Simply set the OTA at 45-degrees and collimate your telescope. Then, with the collimator still in place, move the telescope to the vertical (zenith) and then the horizontal. Watch the dot. Any movement of the dot is the result of flex of the strut.  It's normal to see some dot-shift in these kinds of telescopes. To a great extent, flex isn't much of an issue. But if "perception" is reality, many perceive flex as highly negative and unacceptable. But ones perception can be changed once the facts are explored a bit more fully!

REALITY:  As stated above, longer struts exhibit a bit more flex than shorter struts. One way to "shorten" the strut is a technique of putting stress on the aluminum tubing in the OTA. In a 10" F4.5 telescope, for example, the strut length is about 38". I use the technique of off-setting the mounting holes in the top ring  toward the center of OTA. Considering the mirror box is 10" in height and the aluminum tubing passes through the top ring of the mirror box, introducing stress causes the tubing to stress a bit "inward", toward the optical axis. The tubing bumps up against the top ring, effectively shortening its length from 38" to 28". Since a shorter strut exhibits less flex, the overall flex of the OTA is reduced. I use the same technique on all the telescopes I build, regardless of size.

I've found that for 6"-10" telescope, these telescopes are light enough, and the struts are short enough, that there is virtually no flex at all. I use 1.25" aluminum tubing in these telescopes.  For 14" and 15" telescopes, I tend to use 1.75" tubing. Larger telescopes, 16"-18", I like to use 2" O.D. tubing.  Generally, wall thicknesses vary from .049" to .065". I make the mirror boxes for these telescopes 12" deep, so the "virtual" length of the strut is reduced by that amount. Obviously, F4 and F4.5 telescopes have a bit shorter strut length, so that helps mitigate the issue. Since the design of the DobSTUFF ultra-lite telescopes uses a single top ring (as opposed to an Upper Tube Assembly), the weight at the end of the tubing is minimal. But, eyepieces can vary in weight from a few ounces to a pound or more.

And, while I'm on the subject of eyepieces: What is it that the observer actually sees? First, a well collimated telescope will return the best images -- no doubt about that. But what does one see if there is flex in the aluminum tubing and a slight shift in collimation. The answer is -- it depends!  Shorter focus telescopes are less effected than longer focus telescopes (shorter struts, less flex). Lower powers are less effected than higher powers.  Deep sky objects are less effected than planetary objects. It's also a matter of trade-offs. Weight, ease of setup, ease of transportation and storage versus the alternatives. Given all the variables and for all practical purposes,  the observer will usually not see any degradation in image quality at the eyepiece.

String Telescope Variant: Here's an interesting solution from Jon in San Diego. After assembling one of my kits, Jon found that adding threaded rod, in sort of a truss configuration, addressed this issue for him.


A way to look at this implementation is that of a "string telescope". A string telescope uses parallel struts with the addition of complementary tension "strings" which changes the geometry to a "virtual" truss. The truss is a triangle, the triangle is inherently rigid and the result is completely stable optical tube assembly -- but with all the benefits of the ultra-lite design. In this design, Jon uses 1/4" threaded rod, a "eye bolt" in the top of the mirror box and a turn buckle. He uses an "eye" bolt through the top ring.  It looks like Jon is using a "string" on three sides.

The parallel strut telescope is an elegant alternative to the traditional truss telescope. It's lighter, easier to manage, easier to set-up and store. Most of these telescope will fit into the trunk of a small car and will be ready to use in a matter of just a few minutes. And, yes, they hold collimation well.

Finally, in reply to the question posed at the beginning of this discussion: "Are these telescopes rigid enough?"

The answer, given the issues discussed, is absolutely yes.


Click here for a more complete discussion of string telescopes.

Here's a couple of websites I found that address the subject of "string telescopes":

1) A 28" String Telescope.
2) An 8" String Telescope.
3) A 12.5" String Telescope.
4) The Study of the Dobsonian String Telescope.

As always, I invite your comments, additions and corrections. If you've found this discussion valuable, please feel free to let me know.

Thanks, Dennis   

For more information about your requirements:
--Dennis Steele
--Tel: 650-315-6578 (anytime)
--Email: densteele@dobstuff.com


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