Truss Tube Mount Design Goals
This webpage documents construction of an alt-az
telescope mount that could be fitted with Autostar motors
for full GoTo operation. Since the
Autostar motors are designed for smaller telescopes than my 10"
Newtonian, I had to find an alternative to traditional
this is because I felt that the weight of the 10" optics and the added
friction from a Dobsonian type bearing would be too much for the
Autostar motors. I also constructed this mount using aluminum truss
tubes to further reduce the weight.
Many of my earlier telescope designs used traditional Dobsonian type
bearings for the altitude
An example is shown below: a section of PVC pipe rotating
a smooth surface (PVC, Teflon, metal, laminate, etc.)
I began experimenting with drive ideas and the Autostar 492 motor kit,
to use a friction type drive to couple the 492 motors to the drive axis
(this will be elaborated upon in the Autostar
abandoned traditional Dobsonian bearings for a system using rollerblade
wheels. I wish I had come up with this idea on my own, but actually I
first saw it on Stefan
Keller's Roller Bearings for your Dobsonian page.
Since anyone with children has lots of unused rollerblades in the
closet, the bearing materials were readily available and
The ground board is the base that sits directly on the
I found a laminated wood ring, which was the base to an old living room
chair, and mounted 4 hockey pucks to the bottom of the ring to serve as
vibration damening pads. In the traditional roller bearing
small wheels or caster bearings
are mounted on the top of the ground
board and the upper telescope sections rotate on these bearings. I
reversed the design and placed the roller blade wheels on the underside
of the upper telescope section, which is a box made from scrap 8.5 cm x
2.6 cm bunk bed
pine. This design allows all electronics to be mounted in the upper
sections and avoid a motor mounted onto the ground board that could
give cord wrap problems. The original roller blade mounting
brackets were retained because they create a space between the
upper telescope mount section and the rollerblade wheels where a 492
motor can be attached. The rollerblade wheels are set on a 31
relative to the center of the ground board. I installed a wood block in
the center of the laminate ring to support the threaded rod (azimuth axis).
The top mount section is a pine box that supports 4 wood
dowels (2.5 cm square). These dowels carry the telescopes weight and
are inserted into square holes in 4 wood wedges. The wood wedges tilt
the truss tubes
and give extra support. The wood dowels are inserted
into aluminum rod (from an old clothesline holder). This is more
aesthetic than functional, showing yet another use for a discarded
Festivus Pole (for all Seinfeld fans)! I added 4 right angle
shelf supports for added strength and stability.
I leveled the base and dry fit everything in place. I set my
laser level to the height where I wanted the bearing wheels and marked
the drill points. I could have measured down the length of the aluminum
rod, but if any of the wood wedges were at different angles, this would
give an altitude
axis that wasn't perfectly parallel with the
axis-this would introduce error into the Autostar
system. In the end, the laser approach ensured
that all bearings
were at the exact same height above the ground board.
I drilled two holes, with exactly the same separation into 4
pieces of scrap wood, placed one on each side at the top of each dowel
pair, and bolted them in place. This ensured that the
separation between the bearings was exact during the remainder of the
dry fit process. Using a homemade plumb bob, the wedges were adjusted
to be centered and square with all edges, and then secured
with M6 bolts.
A M6 threaded rod was inserted between the bearings on each
side of the telescope. This provided additional stability and
also a way to fine adjust the altitude axis to be
perpendicular to the azimuth
axis. By tightening lock nuts on the
ends of the threaded rods, the bearing separation became
adjustable (tightening forces the bearings closer together and raises
the telescope axis on that side, etc.)
To prevent the OTA
from moving between the two sets of rollerblade
bearings, an OTA
bearing was installed on each side of the
mount. This was a rollerblade bearing mounted perpendicular to the main
rollerblade wheels. The bearing is on a small block that can be slid
perpendicular to the sectors
The prototype Autostar hand controller holder is
holder can be twisted, so the Autostar
still hangs vertical.
This prototype worked fine
simple hand controller holder, but later had to be redesigned to solve
problems with the Autostar cord being too short to reach the computer
The final design (below) incorporated the Autostar
card. Placing the Autostar computer interface on the trusses reduced
tension in the coiled Autostar cord. Like the prototype, this
holder can be rotated so the Autostar hangs vertical.
Finished Mount and Current Status
The finished mount is shown below. The truss tube mount
as designed. It provided a very light weight support for the 10" wood
truss tube telescope. I eventually
decided that I wanted to
equatorially mount my 10" optics for astrophotography. During
of 2011, I began constructing a homemade GoTo GEM (see the Large GoTo GEM Webpage).
The truss tube mount was eventually disassembled and the parts
and hardware used in other projects.
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copywrited © 2007 by the author and all rights are reserved.
not copy or reproduce in any form without prior written consent.