astronomyasylum.com    Homebuilt Astronomical Equipment and Observatory

Several prototype designs can be seen in the Alternative Designs Page. One design was a fork mount on top of an office chair base. I liked this design for its simplicity and it was very portable, however the fork wobbled as it rotated in the office chair base (azimuth axis). I had to abandon this design for something more stabile. I decided to construct a basic Dobsonian type mount, but build it from truss tubes.  

Bearings 

The prototype design used traditional Dobsonian type friction bearings for the altitude axis. I found a PVC pipe connector with a plug. I removed the plug and discarded the rubber gasket. I cut the PVC pipe in half, cut a semi-circle into the top of the forks matching the pipe's outer diameter, and glued the pipe segment into the semi-circular depression. The PVC plug attached to the telescope trusses and rotated in the PVC pipe (below photo).  This traditional Dobsonian style bearing worked very well (other options are Teflon sliding against metal, plastic, or flooring tile).  


As I began experimenting with drive ideas and the 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 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).

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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.
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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.)

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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.


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The Finished Mount

 Many of the construction photos show all 4 of the wood dowels as being equal in height. I later replaced two of the dowels with longer sections of 2.5 cm x 2.5 cm oak and drilled a 3rd set of bearing holes. These upper bearings (not shown) will keep the OTA from falling backwards off of the mount when it is wedge mounted for astrophotography.

Note that the computer interface card is mounted low on the mount assembly. During functional testing, the Autostar cord proved too short and tension in the cord sometimes disrupted the electrical connections to the computer interface. The computer interface was eventually incorporated into the Autostar hand controller holder (see the Autostar Holder section) to allow higher placement on the truss tubes.

Autostar Holder

The prototype Autostar hand controller holder is shown below. This holder can be twisted, so the Autostar still hangs vertical when the mount is inclined on a wedge.  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 when the mount is inclined on a wedge.