13" Truss dobsonian
This telescope was built c.2004 around a Coulter 13.1" f/4.3 primary mirror.
At the time I was planning an 18" telescope and I wanted to try out some
ideas before committing myself to the larger project. Thus, I always
considered this telescope as more of a prototype than a useful scope.
I wasn't confident in the focal length of the mirror, so once the mirror box and
upper tube assembly (UTA) were complete I mounted them temporarily to a
board and tried out various spacings, looking for the one that gave me the
best focus for all my eyepieces and barlows.
Once I got the spacing right (not easy to do under the stars) I could design
the truss triangles.
The spider design was something I had been playing with for a couple of
years. The T-shaped hub was made of birch plywood. The vanes were
bent from 20 gauge galvanized steel (easy to cut and bend, but prone to
kinking). Note that they are offset from the central axis of the hub--this
makes them much stiffer in rotation than conventional spiders whose
vanes intersect along the central axis.
Hexagonal coupling nuts were riveted and soldered to the ends of the
The truss poles were arranged into four assembled triangles to make the
scope easier to transport and set up--no loose tubes. They attach to the
UTA with quick release clamps. The base of the triangles attach to the
mirror box with knobs. The scope can be fully assembled in just a few
I added a plywood mirror cover after I found my cat sleeping on the
primary--she had removed the temporary cardboard cover.
Most of the construction was birch plywood, with aluminum tubes and
maple for the truss triangles. I used light but stiff spruce for the UTA struts.
One idea that worked well was a cart to
move the scope. Two steel straps
attach the modified trash can cart to
the rocker, making it quick to connect
the cart and move the scope out of the
house, down the deck steps, and
around the yard.
The telescope is usually stored
attached to the cart.
This arrangement worked well, but I
came up with a more elegant mobility
solution for the 12" Nemo steampunk
Ross Sackett's amateur telescope making
My telescopes and ATM Projects