Dobsonian Truss Mount

Truss Tube Dobsonian Telescope Mount

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 Dobsonian type friction bearings; 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.

Bearings

Many of my earlier telescope designs used traditional Dobsonian type friction bearings for the altitude axis. An example is shown below: a section of PVC pipe rotating against a smooth surface (PVC, Teflon, metal, laminate, etc.)

As I began experimenting with drive ideas and the Autostar 492 motor kit, I decided to use a friction type drive to couple the 492 motors to the drive axis (this will be elaborated upon in the Autostar GoTo Friction Drive Page). I 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 free.

Ground Board

The ground board is the base that sits directly on the ground. 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 design, 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 cm radius 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).

Top Section

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 azimuth 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.

Support Struts

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 perfectly 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.)

OTA Centering Bearings

To prevent the OTA from moving between the two sets of rollerblade bearings, an OTA centering 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 for adjustment.

Autostar Holder

The prototype Autostar hand controller holder is shown below. This holder can be twisted, so the Autostar still hangs vertical. This prototype worked fine as a simple hand controller holder, but later had to be redesigned to solve problems with the Autostar cord being too short to reach the computer interface.

The final design (below) incorporated the Autostar computer interface 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.

The Finished Mount and Current Status

The finished mount is shown below. The truss tube mount performed 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 the spring 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.