Ross Sackett's amateur telescope making
Ross Sackett's amateur telescope making
|The Moonsilver Series:
Single Pole Travelscopes
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For several years I have been experimenting with an evolving family of highly portable minimalist telescopes that I
have nicknamed the Moonsilver series. With apertures from 8 to 18 inches, these lightweight and compact Newtonian
telescopes feature a single pole supporting a simplified focuser-secondary assembly and a three-point hybrid “Dob-
fork” mounting. The goal of the series was to produce convenient “grab-and-go” visual scopes that could be kept fully
assembled, always ready for spontaneous backyard skywatching sessions. The Moonsilver series has won numerous
awards, including two first place awards at Stellafane and the RTMC’s Astronomer’s Choice award.
The Moonsilver telescopes
Name Aperture, f-ratio Weight Notes
Moonsilver I 8” f/4.5 14.5 lbs Merit award, RTMC 2006
Moonsilver II 12.5” f/5 44 2nd place craftsmanship, 2nd place mechanical design, Stellafane 2006
Moonsilver III 18” f/4.3 75 Merit award, Astronomer’s Choice award, RTMC 2007
Moonsilver IV 18” f/4.3 63 1st place craftsmanship, 1st place mechanical design, Stellafane 2007
RTMC: Riverside Telescope Makers Conference
Why a single-pole telescope?
The evolutionary trend in Dobsonian telescopes has been towards ever lighter weight and greater portability. The
original bulky big-tube “circus cannon” Dobs soon gave way to airy triangulated truss telescopes, and for the last two
decades telescope makers have struggled to build ever lighter and more audacious trusses, creating a new class of
large aperture ultralight telescopes.
But there are limits to how far you can lighten a truss. If you make the truss poles too thin and skinny, they will
tend to bow out from under their load and make the telescope too flexible to maintain collimation and provide steady,
sharp views. To realize its full strength and stiffness a truss must be anchored to a very rigid mirror box and upper
tube ring, both of which impose a weight penalty on the scope. Furthermore, assembled truss scopes enclose a
volume similar to that of the original big-tube Dobs, making them bulky to store and awkward to move in and out of the
house. Thus, most large Dobsonians are stored in several manageable pieces, and must be reassembled before
each observing session. This inconvenience limits the amount of sky-time enjoyed by large Dobsonian telescopes;
not surprisingly big Dob owners often keep a smaller instrument handy for spontaneous weeknight skywatching.
However, we can break through this weight and bulk barrier by replacing the truss with a single pole, yielding a
scope far lighter and more compact than any practical truss telescope. If the pole is sized appropriately, such a
telescope can yield steady, satisfying views of celestial objects and be far more convenient to move and set-up than a
conventional Dobsonian of similar aperture.
Origins and development of the Moonsilver series
Experimental single-pole scopes were built in the 1920s by Russell Porter and the Springfield Telescope Makers,
culminating in Porter’s famous Garden Telescope. Since then a number of amateurs have built compact telescopes
with a single pole (or several parallel poles) supporting the eyepiece, focuser, and secondary assembly. Recent
examples include instruments made my Thane Bopp, Ron Ravneberg, and French amateur Jacques Civetta.
My interest in minimalist telescopes began in 2005 on the eve of my first trip to Stellafane. I wanted to bring a
modest travel telescope using an 8” f/4.5 mirror I had handy. I built a two-pole table top telescope that could be
disassembled and transported in an airline legal carry-on bag. This telescope was received enthusiastically by the
conventioneers, and won an honorable mention award for craftsmanship.
Buoyed by the success of this first attempt, after a little engineering research I realized that a single larger pole
had definite stiffness and torsion advantages over multiple poles, and I redesigned the telescope for one pole.
Nicknamed Moonsilver, this scope won a merit award at RTMC 2006. I took a field-hardened version of this
travelscope to the Ecuadorian Andes that summer, and it performed beautifully under the Southern skies.
Using beam theory and the lessons learned from the 8” Moonsilver, I scaled the design up to 12.5”. At Stellafane
2006 this new Moonsilver II won two awards and was filmed and briefly featured in Timothy Ferris’s video Seeing in the
Dark. The following Spring I built Moonsilver III, a blue 18” f/4.3 version that won the Astronomer’s Choice award at
RTMC 2007. Nicknamed by the judges a “living room telescope” because of its easy portability, compact footprint and
sculptural appearance, this large instrument weighed 76 pounds—light for an 18” telescope, but more than I cared to
carry in and out of the house. To shed some weight I redesigned the instrument using light but stiff carbon-epoxy
components and rigid sandwich construction. Weighing only 63 pounds, this new 18” Moonsilver IV is the culmination
of the series, winning 1st place awards in both craftsmanship and mechanical design at Stellafane 2007.
One pole is better than two
If one pole is good, aren’t two poles better? Not necessarily. A single pole bends in response to the tug of gravity
and the push-pull of tracking celestial objects. For a given material, length and wall thickness, the bending resistance
of a thin-walled tube is proportional to the cube of the diameter of the pole. Increasing the pole diameter by only 25%
doubles its resistance to bending; doubling the diameter of the pole results in a whopping eight-times increase in
stiffness. In multiple parallel tube designs the overall stiffness is the sum of the stiffnesses of the individual poles. If
the same amount of material was split between two poles with half the diameter, under a given load the paired parallel
tubes would flex about four times as much. If the material was divided among four parallel poles, the structure would
flex over 20 times as much as a single pole under a similar load. Given the choice, go for a single larger-diameter
Experiments early in the Moonsilver series, with the support of a little beam theory, suggested as a rule of thumb
that the minimum diameter of a 1/16” wall aluminum pole should be at least 2.8% of the focal length of the mirror. Any
skinnier than this and the pole is likely to sag enough at low elevations to move the optics out of acceptable
collimation, and subject the scope to annoying levels of flexure and vibration while tracking. Any increase in diameter
above this rough guide should yield substantial stiffness benefits.
The length and shape of the pole matter as well. The flexibility of a pole loaded in bending increases with the cube
of its length, so keep the pole as short as practical given the focal length of the primary mirror. I do this by introducing
a stiff bracket—I call it the “stock”—between the mirror box and the pole that reduces the pole’s unsupported length by
about 25%. The cross-sectional shape of the pole is also important. For a given amount of material a thin-walled tube
is much stiffer than a solid rod, and under a given load a circular tube bends about 15% less than a square-section
tube (and is about 1.9 times as resistant to twisting forces).
The Moonsilver design
Since the details have evolved over time, I’ll focus here on the structure of the most recent version, the 18”
Moonsilver IV. My experience has been that the structural design scales quite well, so that these features should work
in smaller versions of the telescope.
The pole. Earlier versions used 1/16” wall aluminum tubes, sized appropriately to the focal length of the mirror and the
length of the stock. I used a pipe bender to put a shallow S-bend in the tubes to help offset the focuser and
eyepiece, so that at most elevations the eyepiece is viewed at a modest downward angle. This feature is
ergonomically important, since using an eyepiece that juts horizontally from the side of the scope at all elevations is
very difficult on the neck and limits the amount of time I am willing to stand at the scope. It also gave the earlier
scopes an appealing sinuous sculptural profile.
In Moonsilver IV to save a little weight I replaced the aluminum tube with one of carbon fiber-epoxy composite, 2-
1/8” in outside diameter and 58” long. In retrospect, an aluminum tube would probably have been about as stiff, cost
much less, and would not have posed the health risks attendant to cutting and drilling carbon fiber. Since carbon-
epoxy is fiercely stiff and ruptures rather than yields as aluminum does, I could not offset the focuser by bending the
tube. Instead, I made the “stock” a little wider, which shifted the attachment of the pole out of the horizontal plane and
angled the eyepiece sufficiently for comfort.
The pole attaches to the focuser board above and the stock below using rigid rail joints cut from extruded metal C-
sections. The tube is clamped to these using stainless steel machine screws run into threaded inserts.
The stock. I think much of the mechanical success of the Moonsilver design is due to the stout bracket connecting the
pole to the mirror box. I call this the “stock” because of its superficial resemblance to a wooden riflestock. The stock
does double duty in my design. It rigidly connects the pole to the mirror box, and it carries the pivot for one side of the
hybrid altitude bearing. Since it must be both stiff and very strong, the stock should be constructed of thick hardwood,
or as in the case of Moonsilver IV, of a stiff sandwich of rigid carbon-epoxy sheets epoxied to a softwood core.
Focuser board and secondary assembly. One great advantage of a single-pole scope over a conventional truss is
that the upper end can be considerably reduced in size and weight. The focuser board is made of ½” thick plywood.
To save weight I prefer compact 1-1/4” helical focusers (in this case, a “nonrotating” helical from Lumicon). A red-dot
finder on an aluminum bracket also saves some critical top-end weight compared to a conventional finderscope. In
Moonsilver IV the “spider” of a conventional scope is replaced by a two-armed bracket cut from 1/8” aluminum, curved
to reduce diffraction spikes. The 3.1” m.a. elliptical secondary mirror is RTVed to a mirror-carrier bent from 1/8”
aluminum, which is attached to the secondary bracket with a centerbolt and adjustable thumbscrews for collimation.
These thumbscrews fit into dimples drilled in the mirror-carrier to ease precise reassembly and to prevent the
secondary from rotating out of position as the telescope shifts in elevation.
Stray light is a concern in any unshrouded telescope, so I took extra pains to keep it from reaching the eyepiece.
A curved glare shield attaches to the bracket immediately behind the secondary mirror. Made of Kydex plastic and
lined with black velvet, the shield is curved to reduce diffraction spikes (since the primary sees it edge-on, little light is
lost). At the base of the focuser I installed a variable iris (a stock part from Edmunds Industrial Optics) to intercept
any stray light leaking past the glare shield.
Mirror box. To save weight and bulk I built the plywood mirror box just large enough to hold the 18” primary mirror, its
6-point flotation cell, and a ventilation fan. To help protect the highly exposed mirror from fumbled filters and falling
flashlights I added a hinged lid. Kept latched shut during transport and telescope assembly, for observing the lid is
held open by a strong rare earth magnet attached to the stock. The hinged lid is so convenient that I frequently flick
the lid down when I change eyepieces or filters, something I probably would not bother to do with fully removable mirror
covers. Both the mirror box deck and the inside of the lid are covered in black velvet to cut reflections.
The 18” f/4.3 primary was made by John Hall of Pegasus Optics. It weighs 35 pounds, more than half the total
weight of the fully-loaded telescope and base combined.
Base. The unique configuration of the Moonsilver telescopes make them ideal for the hybrid “half-Dobsonian, half-
fork” mount described by Francis Milsom (S&T July, 2001). The base carries a conventional Dobsonian rocker on one
side supporting an altitude arc mounted on the mirror box. On the other side, a single fork arm carries the bearing for
a ¼-20 pivot bolt attached to the stock. A plastic knob with locking insert, and brass and teflon washers adjusts the
friction of the pivot. A similar locking knob tightens the conventional Dobsonian azimuth bearing to the groundboard.
Both the altitude arc and the azimuth bearing are lined with high-pressure laminate running on teflon pads.
For transport, a draw latch clamps the altitude arc to the rocker bearing; since the telescope is already attached to
altitude pivot bolt on the other side it is easy to pick up the scope as a single unit and carry it in and out of the house
and shift it around the yard. Padded handles on the mirror box make carrying more secure and reduce the subjective
weight of the instrument.
Finish. I consider the form and finish of a telescope important functional features since they make it more attractive
and thus easier to live with a large scope. Telescopes lead two quite different lives. At night, they are machines that
extend and amplify our vision of the heavens. By day, however, they are reduced to a kind of furniture, patiently
awaiting dusk and their true purpose, too often relegated to dusty garages and back rooms. I felt that the sculptural
appearance of Moonsilver provided an opportunity to build a “living-room telescope” whose visual appeal would make
it more welcome in the public parts of the house, readily at hand for weeknight “grab-and-go” observing sessions in
the backyard. To compliment their sculptural shape, I have wrapped the Moonsilver telescopes in attractive wood
veneers and finished them as I would furniture, in layers of shaded stain and hand-rubbed polyurethane varnish.
Moonsilver IV is sheathed in African mahogany finished in a warm honey-colored satin varnish reminiscent of Danish
Using the telescope
I continue to be delighted by the performance of all the Moonsilver telescopes, especially Moonsilver IV. I had
originally built the larger Moonlight instruments as “weeknight” telescopes, intending to shift their optics into my
conventional ultralight truss scope for weekend DSO observing at a dark sky site. However, I have found that most of
the time I use Moonsilver IV as my primary telescope for both informal stargazing and “serious” observing alike.
The single pole is surprisingly stiff, flexing only about one arcminute while tracking objects. After a sharp rap the
telescope vibrates freely for 3-4 seconds, but dampens immediately when the hand is on the tube in the usual tracking
position. I don’t notice any collimation shift over the usual range of observing elevations. However, on the rare
occasions I observe an object close to the horizon I like to tweak the collimation to obtain the sharpest image I can.
Alternatively, I sometimes put a cardboard aperture mask over the primary, converting the telescope to an
unobstructed 6” f/13 Herschelian. The long focal ratio dramatically broadens the acceptable collimation tolerance and
eliminates the diffuse diffraction cloud around bright objects imposed by the curved secondary holder and glare shield.
I love the convenience of this telescope. Kept fully-assembled close at hand in the living room of my house, it is a
simple matter to latch the mirror box to the rocker and carry it into the backyard for a spontaneous observing session.
The large handles make the telescope seem considerably lighter than its 63 pounds (which is already exceptionally
light for an 18 incher). Its single pole and greatly simplified top end make it compact enough to easily carry out the
door and down the stairs. Brought out at dusk, the 12V fan speeds the mirror’s acclimation while I enjoy dinner, and it
is ready to go to work by the time dessert is finished. Most evenings observing I pick up the scope and move it several
times as I shift attention from object to object, avoiding the trees and street lights in my urban yard. It is truly a grab-
and-go telescope: an exceptionally large grab-and-go telescope, however.
Copyright 2009 Ross Sackett