A year ago I built a Cylindrical Bearing Equatorial Platform as described by Chuck Shaw. Recently Chuck suggested I should change the dc motor to a stepper motor for more accuracy and control of the tracking rate.

While I was rebuilding the platform I changed the linkage from the motor to the table to reduce a slight error caused by being a tangent arm drive. The previous tangent arm drive drove the conical section a little faster at the beginning and end of travel when compared to the middle portion. The new drive is via a section of tape measure so the circumference of the conical bearing plates of the table are driven at a constant rate. A somewhat similar tape drive was discussed in Sky and Telescope September 1996.

I found the stepper motor created some vibration as it stepped. At higher rotation speeds this vibration disappears, so I built a gear system from model car gears to reduce the drive speed with the higher motor speed. This gear system drives a threaded rod at a 20 to 1 reduction. The rod, in turn, drives a plastic block that was treaded to accept the rod. The plastic block has an aluminum plate screwed to it that travel in a slot to keep the plastic block and metal piece from rotating. The plate is attached to a section of tape measure that rides on three plastic idlers. The idlers do not rotate. If they were slightly out of round and rotated an periodic error would be introduced. Two idlers turn the tape through 180 degrees. A third idler keeps the tape clear of the cylendrica bearing plates and tensions the tape.

The base section of the Tracking platform Three leveling screws are seen in left, right and top of the photo. A small bubble level is mounted in the middle of the platform to level the base. A Boy Scout compass serves to roughly polar align the table. Power for the table is two 6 volt batteries. On the left is the stepper motor and gear reduction system. A threaded rod can be seen traversing the front of the base. This rod is connected via a small aluminum plate to a section of tape measure that drives the top section can be seen riding on the three idlers. The control box for speed control is in the foreground.

The table is seen here ready to accept the base of my 10 inch dobsonian. The base of the telescope fits snugly in the top. An aluminum cylindrical bearing piece can be seen resting on the tape that is guided by three idler bearings. A similar aluminum cylendrical bearing piece rides on 4 ball bearings on the back of the top piece

The top piece of the table has two aluminum bearing plates that are positioned and were fabricated to be sections of a cone that corresponds to latitude of Seattle where I live and so the axis of the cone corresponds to the center of gravity of my scope.

On the left is a stepper motor that drives the 4 gears from model cars this turns the threaded rod. These reduce the engine rpm by a factor of 20 to 1. The section of tape measure used moves the cylindrical bearing pieces can be seen around the idler bearing.

In this view the tape is shown between the aluminum cylindrical bearing and the plastic idler. As the tape moves it takes the bearing move. The tape measure is very smooth and it was necessary to add pieces of making tape to the tape measure and the aluminum bearing to prevent the aluminum from slipping on the tape.

The tape measure rides on the tensioning idler and on the second 180 degree idler. The threaded rod is seen in the foreground with the aluminum plate that is attached to the threaded plastic block on the rod and to the tape. As the plastic block moves down the rod the tape is moved the same amount. On the right behind the rod is an end of travel switch that shuts down the motor to prevent the aluminum bearing plate from moving off idler bearing causing the scope to fall over. This switch also activates a small horn to signal that tracking has stopped.

The control box has a variable speed control to go from stopped to about 100 rpm. It has switches to cut the speed in half, reverse direction, step one step at a time and other controls to fine tune the speed. It has a series of LEDs that indicate which coil of the motor is energized giving a rough indication of speed. In addition they are fun.

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