Ross Sackett's amateur telescope making
Ross Sackett's amateur telescope making
Dobsonian Evolution:
From Circus Cannon To
   I am Ross Sackett, an anthropologist by profession, amateur skywatcher since the age of 5, and amateur telescope
builder since 1999.

   Today I am going to talk a little about a series of telescopes I have been working on for the last few years.  They
belong to the class of telescopes called Dobsonians, so before our show and tell session I would like to discuss the
origins and evolution of the Dobsonian.

   Dobsonians—“Dobs” for short—are named for John Dobson, a legendary figure among amateur telescope makers
(ATMs).  Dobson was born and raised by American parents in China, and moved to San Francisco with his family in
1927.  He entered UC Berkeley in 1934 to study the physical sciences.  After taking some time out to tour with a
troupe of performers who promoted various progressive causes through music and interpretive dance, Dobson
received his degree in Chemistry and Mathematics in 1943 and briefly worked on the Manhattan Project.  The
following year, however, he quit war work and joined the San Francisco Vedanta monastery as a Ramakrishna monk.

   According to Dobson, Vedanta is a branch of Indian philosophy that seeks an ultimate understanding of the
universe through reconciling ancient Vedic texts with the modern physical sciences.

   As a monk, Dobson decided that if he was to seek an ultimate understanding of the universe, he ought to see it first
hand, for himself.  In the mid-1950s he built a small telescope, and had his first glimpse of the sky.  In interviews he
describes the experience as one might discuss a revelation or a religious conversion—he had to see more, and he
had to share it with others!  So he built bigger and bigger telescopes, and in the evenings secretly went “over the
fence” to share the view with the monastery’s neighbors.

  In 1967 he was expelled from the monastery for reasons that are disputed (but probably related to the fact that he
was more interested in astronomy than monastic contemplation).  To make ends meet he began to teach telescope
making and in the following year co-founded (with two young proteges) the San Francisco Sidewalk Astronomers.

   “Sidewalk astronomy” grew out of his experiences at the monastery.  Dobson felt that seeing the sky deeply and up
close for the first time changes you—it alters the way you perceive your world, and it changes how you view your place
in it.  But you had to see it with your own eyes; photographs just don’t have the same emotional and philosophical
impact.  To Dobson, amateur astronomers have a special role in this.  While it is fine to do “backyard” amateur
science (as promoted in magazines like Scientific American and Sky and Telescope), the amateur has a higher calling:
to bring these views to the public.  These sidewalk astronomers saw it as their duty to bring their telescopes out into
the city streets, to the national parks, and into the schools to show people the universe in which they really live,
beyond their own little terrestrial corner.

Sidewalk astronomy calls for a particular kind of instrument.  The ideal sidewalk telescope should be:
•        Cheap: To have any real impact, there needs to be a lot of them, so they must be cheap and easy to make.  
Dobson imagined a sidewalk telescope on every city street and village green in the world.
•        Portable: You have to be able to bring the telescope to where the people are, and to the clearest darkest skies
where you can see the most of the universe
•        Big: The larger the main light gathering mirror or lens, the bigger, brighter, and sharper the images, and the
better you can see the universe.
The problem, of course, is that in the conventional way of building telescopes big doesn’t go with cheap and portable:
big scopes are usually very expensive, bulky, and stationary.

   The solution is what we today call the Dobsonian (but which Dobson himself still insists should be called a sidewalk
telescope).  Optically, Dobsonians are about as simple a telescope you can build, in the Newtonian pattern (named for
Isaac Newton, who invented it around 1671).  At the base it has a large concave primary mirror that gathers starlight
and focuses it to an image, a flat diagonal mirror that picks off the converging rays and sends them out of the optical
path to the side of the telescope, and an eyepiece that magnifies the image and crams all the light into the ¼” pupil of
your eye.  (Using a typical eyepiece, my 18” diameter scope makes object appear about 200 times larger and 5,000
times brighter: just what you want for seeing the universe!)  Since the telescope is focused on celestial objects that
are an immense distance away, the relatively nearby secondary and its support are so out of focus you don’t see them
at the eyepiece!

   But it is really the mechanical design that makes the Dobsonian interesting and innovative.  The Dob rotates on a
groundboard that sits on three pads or feet, stable on rough terrain.  On the groundboard swivels the rocker, which
carries the pads for the altitude bearing.  Resting on these pads are altitude trunnions, which are firmly attached to
the mirror box, which houses the large primary light-gathering mirror.  And attached to the mirror box is a tube, which
at the top end carries the secondary mirror and its spider-like support, focuser, and eyepiece.  

   Dobsonians can be built very cheaply, out of ordinary plywood and cardboard tube (often inexpensive concrete
form tube) using ordinary hand tools.  Matched for aperture, compared to other types of telescope Dobs are highly
portable since the bulk of the scope is in the optical assembly rather than in the base, and Dobsonians don’t require
heavy counterweights to balance on their mountings.  Even large Dobs break down to components that can be carried
by just one or two people, and packed into a car or van for a trip to a popular street corner or dark national park.

   And Dobs can be big.  Before the Dobsonian, large amateur scopes generally had apertures of 10 or 12”; Dobson
and his proteges in the Sidewalk Astronomers regularly built 16-22” scopes.  Today the largest Dobsonian has a
mirror 42” in diameter, and there are rumors of a 60” instrument in the works.

   Dobsonians aren’t perfect, though.  They are alt-azimuth telescopes (they rotate in horizontal and vertical planes)
and on their own cannot smoothly track the ever-rotating sky, making them inappropriate for most astrophotography.  
(Recently, tracking platforms and dual-axis drives have alleviated this liability, but at considerably greater expense and

   During the early 1970s Dobsonians began to appear at the big western star parties like the Riverside Telescope
Makers Conference (RTMC) and slowly spread eastward.  In the late 1970s and early 1980s detailed construction
articles appeared in Telescope Making and Astronomy magazines, and soon Dobs were being built by ATMs around
the world.   Today, more Dobs are built by amateurs than all other kinds of telescopes combined, and Dobsonians
regularly take top awards in mechanical design, craftsmanship, and optical quality at big amateur astronomy
gatherings like the RTMC and Stellafane.  But you don’t have to build your own; today a number of commercial firms
would be delighted to sell you a Dobsonian.  However, the great innovations in Dobsonian design have always been—
and continue to be—made by amateur telescope makers.
   As Dobsonians went mainstream, their design evolved in a number of directions, adapting the basic Dob pattern to
the diversity of styles and purposes of contemporary amateur skywatching.  One important trend is towards ever-
greater portability and lighter weight for their aperture.  

   Portability is easy—just build the scope to break down into manageable units that can be more compactly
rearranged (like a 3 dimensional jigsaw puzzle) into a vehicle or trailer.

   Reducing weight is tougher.  If you just cut away material at willy-nilly the scope can become so flexible that it can’t
hold the optics in critical alignment (“collimation”), robbing images of sharpness and contrast.  The challenge is to take
off the pounds without significantly reducing rigidity.  

   The secret to putting your Dob on a diet is to reduce the weight of the superstructure (everything skyward of the
primary mirror).  A lighter superstructure means you can build a lighter and more compact mirror box, which in turn
means a smaller rocker.  And when you add it all up, a lighter telescope that is more compact, to boot.

   In the 1980s a number of amateurs experimented with replacing the relatively heavy and bulky cardboard tube with
a light and airy triangulated truss made of hollow aluminum poles.  This allowed them to shed considerable weight
while maintaining critical rigidity.  In 1988 David Kriege published his design for a truss Dob called Obsession I, which
has been widely imitated ever since in both homebuilt and commercial scopes.

   Most big Dobs today are made on this 8-pole triangulated truss pattern.  But since the early 90’s a small group of
ATMs has experimented with ultralight designs considerably lighter and compact even than conventional truss
scopes.  There have been several lines of development, and so far no one ultralight design has come out as the
frontrunner.  This makes ultralight Dob building a risky and exciting endeavor for ATMs like me.

  One approach is to put the truss on a severe diet, reducing the poles to their minimum size and number (3/4”
diameter, and 6, respectively).  The top end is often reduced to a single light-weight ring, and with such a lightened
superstructure the mirror box and rocker can be reduced considerably.

  Another approach to ultralight design has been to abandon the truss for a framework of poles and wires, the so-
called stringscopes.  In an ordinary truss some members are in compression while others are in tension.  If sized
correctly, an aluminum tube can be a very efficient way to handle compressive loads, but it is overkill when it comes to
tension: steel cables or cords made of high-tech polymer can carry the same loads at much reduced weight.  Like the
bracing on a WWI biplane, stringscopes use stiff posts to take the compressive loads, and diagonal cables to resist
the tensile loads.

   But there is a limit to how light you can make the poles, either in truss or string telescopes.  If the poles resisting
compression are too thin, they easily buckle and the telescope becomes so floppy it cannot keep the optics in critical
alignment (and won’t stay where you point it).

   This has led a few to experiment with cantilever designs, where poles are to resist bending loads, rather than
compressive loads that could buckle them.  In cantilever scopes the poles are parallel to one another, without any
bracing.  I have seen cantilever scopes with 6 tubes, but most have only 4, 3, 2, or very rarely just 1 pole.  So far,
most successful cantilever scopes have been small, in apertures of 8” or less—it is a great way to make a travel
telescope since they are very compact and there are fewer poles to pack.  But larger cantilever scopes had not been
very successful, because (so far) they hadn’t been as rigid as the heavier truss and stringscopes.

   Which led me to think a little about ultralight design.  I realized that cantilever scopes act like, well, cantilevered
beams: that is, they are anchored at the mirror box end and free to sag at the sky end.  The forces at the end of the
pole can be considerable, from the weight of the pole itself, but also from the secondary, focuser, eyepiece, finder,
light baffling stuck on its end.  And from the forces of aiming and steering the scope.

   These forces are worse the larger and longer the telescope.  Many of these forces scale nonlinearly—you can’t just
scale up a 6” travelscope and expect it to work well as 12” or 18” instrument.  Beam theory can help us out here.

   In cantilever scopes poles must be very rigidly attached to the mirror box—any flexibility there compromises the
stiffness of the whole structure.  Look at the designs of others I noticed that many cantilever scopes seemed to have
rather undersized attachments.

  Wider poles are better.  According to beam theory, the bending stiffness of a thin-walled tube increases with the
cube of its diameter.  A slightly wider tube makes a much stiffer pole.  Yet many cantilever scopes are made with 1” or
even ¾” tubing, far too flexible for a scope larger than 8” aperture.

   Perhaps contrary to expectations, one big pole is much better than many small poles.  In cantilever scopes, the
stiffness of the individual poles is additive—the total rigidity of the superstructure is just the sum of the stiffness of the
individual poles.  Since the stiffness of a pole increases exponentially with its diameter, one wide pole can be much
stiffer than a bundle of smaller poles.  In fact, calculations suggest that all else equal, matched for the same total
weight of material one pole is 4X as stiff as two smaller poles, and a whopping 16X as stiff as four skinny poles.  (Here
“all else equal” means the poles are of the same material, same length and wall thickness, and carry the same loads.)

   I had built several telescopes before I became interested in cantilever ultralights, with apertures from 4 to 13
inches.  I knew enough to not just jump in and try for a big ultralight first time out of the gate.  My experiences had
taught me the value of evolutionary design—start with an examination of the precedents made by others, and build up
incrementally through a succession of sketches, models, prototypes, and built versions.

   I started modestly.  I knew that cantilever scopes were proven to work fine in small sizes, so I began with a
commercial 8” f/4.5 mirror I had lying around waiting for a scope.  In 2005 I had a month of time before the big late-
summer star party at Stellafane in Vermont, so I put together a travelscope to take to the meet.  With hindsight I see
that I was pretty timid—though I knew that one pole was probably better, I also knew that others had made workable 2
pole scopes at that size, so I made my first Stellafane scope with two poles.  During the telescope competition the
instrument was received very enthusiastically, and the modest award in craftsmanship gave me the boost to try more
radical designs.  During the Fall and Spring I used the scope frequently, getting a better feel for what worked well and
what didn’t.

   Early the following summer I took my experiences with this prototype and rebuilt it to a more audacious plan I
nicknamed Moonsilver.  Moonsilver I used the same 8” mirror, but was a single-pole design.  For ergonomic reasons I
bent the pole so the eyepiece could be viewed at an angle, from above, which is much more comfortable than the
horizontal position of the Stellafane prototype (especially in such a short telescope, but really true for any instrument).  
I added a latch to the altitude trunnion so it could be attached to the mirror box, making it one-hand portable.  The
whole scope breaks down and fits into carry-on luggage.  To make it pretty and presentable to the judges I covered
the scope in mahogany veneer and put an inlaid design in the hinged mirror cover.  In May of 2006 I took it to the
RTMC, the main west coast start party, where it won one of only 5 Merit Awards given that year.

   Now it was time to scale up the design to bigger apertures.  I commissioned a 12.5” mirror from Ed Stevens (a
respected amateur-turned commercial mirror-maker), and using a little beam theory scaled up the 8” design for the
substantially larger and heavier mirror.  With my earlier experience the design and building process progressed
rapidly, until a table saw accident temporarily robbed me of the use of my right hand, and I barely finished before the
Stellafane meet in early August.  Too big for the carry-on luggage, the scope disassembled and packed flat in
checked baggage.  At the scope competition there was a lot of friendly interest, and some frankly bewildered stares.  
The judges, though, liked it enough to award it second place prizes in both craftsmanship and mechanical design,
losing out to a spectacular binocular-telescope chair.  During the Stellafane competition astronomy writer Timothy
Ferris was filming a documentary on amateur astronomy, and included a bit of me explaining the optics of the scope in
the released version that was shown on PBS last Fall.

   Now I thought I was ready to go really big.  I had a very nice 18” f/4.3 mirror made by John Hall at Pegasus Optics.  
For the May 2007 RTMC I planned an 18” version of Moonsilver, working out the design on paper and foam-core
models during the Spring.  Scaling up the mechanical design was easy, but I wanted something even more eye-
catching than my 8” and 12” scopes.  My wife commented that these earlier scopes had lines resembling electric
guitars, so I took that as my theme for the 18” Moonsilver III.  I exaggerated the curves, and cloaked the whole scope
in curly maple veneer finished with a transparent cobalt blue “starburst” finish.  Way too large for baggage even when
disassembled and packed flat, I boxed it up and sent it UPS to my mom’s house in L.A.  When I arrived and unpacked
it, the heat had badly damaged the still-hardening finish so I spent a panicked day making it presentable.  At the
RTMC, it was a huge hit: it took a Merit Award from the judges, and the assembled attendees voted it the Astronomer’s
Choice Award, the “best in show” award at RTMC.  Photos of the scope taken at RTMC were published in Amateur
Astronomy several months later, and Gary Seronik at Sky and Telescope wants a construction article, which I am
working on.
   So what to make for Stellafane coming up at the end of summer?  The big blue scope weighed 76 lbs, much less
than the 175 lbs of a conventional “circus cannon” Dob, or the 125 lbs of a truss Dobsonian.  I could carry it fully
assembled out of the house and around the yard, but 76 lbs is a pretty big lift.  For most, the practical limit for a
weeknight “grab and go” scope is 60-65 lbs.  This gave me my goal for Stellafane: redesign the 18” down to that
weight.  It would take some tricks: cut away as much material as I could, use weight-efficient sandwich construction
where possible, and replace some key parts (like the pole) with light but stiff carbon-epoxy composite.  Carbon-epoxy
cannot be bent, so it would have a straight pole.  Removing all the material I dared simplified the lines considerably,
giving it a minimalist appearance.  As I neared completion, I decided it should have a Jetson’s-meets-Danish-modern-
furniture look, which led me to African mahogany veneer with a satin finish, leaving the carbon tube and panels in their
natural high-tech state.  Overall, it came in at 63 lbs, just where I hoped it would.

  This time the competition judging was nerve-wracking.  The usual pattern at Stellafane is for the team of judges to
make two passes through the competition field, the first to decide which of the 30 or so scopes will get an award, and
the second pass to pick 1st, 2nd , and 3rd places and identify any honorable mentions.  The first time through, the
judges (who knew me pretty well from the previous competitions) didn’t say much, listening politely to my patter about
the design features of the scope, and only reacted (guardedly) when I latched it together and carried it around to
show how portable it was.  They left to visit other scopes, and I waited for them to make their second pass.  But they
never did.  I got a lot of sympathetic looks from others who knew what this meant, and I reconciled myself to going
without an award this time—at least the other conventioneers seemed to like my telescope.  Later at lunch one of the
10 or so judges came over and mentioned that while they all knew immediately who would get the top prize, there was
a lot of disagreement over the 2nd and 3rd places and whether to give any honorable mentions this year.  I thought he
was being kind and giving my ego a little insulation from the disappointment to come.

   That night at the ceremony Moonsilver IV was awarded both first place awards, in craftsmanship and mechanical

Copyright 2009 Ross Sackett
Talk presented to Memphis
Astronomical Society, 2007