Installing the DobDriver II

by Ed Stewart <stargazerskymtn.com>

Download Installation and Operating Manuals for the Tech 2000 Dob Driver II

The installation manual for the Tech2000 Dob Driver II calls for some significant modifications to the Dob's mount--did I really want to modify my mounting as much as was going to be required? Here's is what I learned.

Hard Lessons

The first time I had to modify the mount from its original design was to add a digital setting circles (DSC), because the DSC's location and size were unknown at the time of construction. Then when I wanted to add the Dob Driver II, it called for replacing two of the Teflon pads for the azimuth axis on the ground board with ball bearings on short 1/4" diameter shafts. It is necessary to route or chisel a pocket into the rocker base to recess the bearings so that only as much of the bearing protrudes as the thickness of the Teflon pads. In place of the third pad, the azimuth drive motor with a knurled roller has to be mounted on the edge of the rocker base so as to have the knurled roller track along the ground board to produce rotational motion and protrude as much as the two ball bearings.

Again, this was unknown originally so there were problems such as the Teflon pads had been recessed into shallow pockets in the plywood ground board. Now ball bearings and the knurled roller had to roll smoothly over these recesses. Other problems were that there was no place to mount the altitude motor, because I had cut away so much of the side boards, the ball bearings cut deep grooves into the Baltic Birch plywood of the ground board; and there was no place for the 12-volt battery to ride on the mount. You get the idea that the installation took a number of hours and some engineering beyond those described in the instructions.

In fact I reached a point where I decided that the original mount was now a test bed to prove out the drive; and if workable, then I would redesign the mount knowing the specs and incorporate some enhancements that occurred to me. Several of the things learned in the test phase were:

1. that a single little sand grain between the ground board and rocker base could bring the ball bearings to a screeching halt,
2. the azimuth drive motor needed more down force on it for traction plus it needed greater leverage in turning the base, and
3. the belt drive for altitude movement stretched slightty overcoming the friction in the Teflon pads causing a small jerkiness in the movement.

New Design

The problems of preventing the sand getting into the ball bearing track and giving the azimuth motor more traction and leverage were solved by making the ground board and rocker base round rather the classic rectangular shape. To prevent the sand problem, the circular rocker base has a slightly larger radius than the ground board with a skirt of Formica attached to it that completely overhangs the ground board. A piece of fuzzy string was glued around the ground board's perimeter rubbing the inside of the Formica skirt as a gasket to prevent even dust from entering the track of the ball bearings. In the photo Old Drive to the right, the old mount is shown with the azimuth motor/rocker base rotated 45 degrees to the ground board--you can see about 3" of leverage is lost by the motor not being out as far as the foot on the ground board. In the new design (see photos below), a hinged shelf straddles the azimuth motor for the battery pack to ride on and to apply about 6 pounds of cantilevered weight onto the drive wheel--

For those of you wishing to retain your original rectangular mount, the sand problem could be addressed by putting some self-adhesive foam insulation stripping in a circular pattern outside the track of the teflon pads just high enough to barely touch or not quite touch the base board.

Other problems were solved by covering the ground board's top surface with Formica face down to provide a textured surface for the drive wheel to grip and a hard surface for the ball bearings to track on. To spread out the pressure of the 1/4" wide ball bearings on the Formica, they were doubled up side-by-side to make a 1/2" wide bearing surface. I used replacement roller blade bearings--see photo of the Rocker Box Base before gluing below:

 For the altitude axis the Teflon pads were replaced with roller bearings recessed into the side boards. These were too frictionless, requiring critical balancing, so experimenting later lead to placing Teflon pads on the non-driven side board and ball bearings on the side with the drive pulley--see photo Drive On-Edge below.

Other Enhancements

The two computers (DSC and DobDriver) are attached with Velcro to a plywood paddle (seen photo above) that mounts into a eyepiece storage hole during observing, and fits into a storage shelf near the azimuth encoder during transport--they stay with the mount at all times. The side boards are just 3/4" (19mm Baltic Birch actually) plywood with triangular supports, and the rocker base and ground board are three layers of 1/2" Baltic Birch plywood laminated together with the center layer mostly a hollow core--
see photos: Ground Board below & Rocker Box Base before gluing above.

Hint--using a crosshair eyepiece can greatly improve the tracking accuracy when initially centering the object and the subsequent recentering after allowing the object to drift to the edge of the field of view. This sequence establishes the first two vector points for the computer to calculate the tracking. It is especially difficult to get accurate centering and therefore good vector information using wide field eyepieces. (Dob Driver II has other functions besides tracking that are too numerous to mention in this space, but the neatest one is the outward spiraling search pattern.)

Is it worth the price and effort? Absolutely--for visual observing. It's a real joy using the drive motors to maneuver the scope while watching the read-out of the digital setting circles as it guides me to the object, and then having the scope track while I enjoy observing the object. Finally, an unexpected benefit from this somewhat forced redesign is the increased stability and rigidity over the original rectangular mount.

Contact Info:

Mr. Dave Masters, Tech2000, 3349 SR99 S, Monroeville, OH 44847, Phone: 419-465-2997.
      <http://www.tech2000astronomy.com/dobdriver/>

Another DobDriver II related site:
      <www.optcorp.com/pdf/Tech2000/dobdriverII.pdf>