Strut Telescopes: Flex and Rigidity
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
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.
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.