Equatorial platform

Images to left and right show general views of the platform. It has been built to take both my ten-inch and six-inch telescopes. Ground board feet locate in the holes in the platforms top board providing a secure position for the telescopes. The platform has three levels: level one (Ground board) has three leveling screws - 10 mm metric coarse threads passing through 'tee nuts' fitted into the underside of the ground board. Level two holds the rear pivot point, ball races for the front sector, drive screw, motor, electronics, level indicator and a central pivot bolt. Level three has the rear pivot pin, front sector and telescope locators.
Here the drive screw and driven sector are shown. The spring has proved a very good way of locating the drive screw in the thread form of the sector when in use. It also acts as a clutch, if the telescope gets a heavy jolt it allows the threads to disengage preventing tooth damage. Photo left shows the ball races which support the sector these run behind the threaded aluminium strip. Aluminium angle has been cut to form the support for the ball races. The upper platform is upside down in this photo to show the threaded aluminium track.
Plan view of the drive rod and its support. The spring pivots the rod upward to engage in the sector. Ball races take all the weight of the scope leaving the drive rod free to drive the platform with just enough pressure applied to keep threads engaged
Drive motor and electronics have been omitted from above photographs to allow clearer views of the drive and platform support.
(insert image of stepper motor drive)
Photographs here show the drive motor (200 step motor) gear box and the stepper motor controller circuit board (Quasar electronics kit). The motor is linked to the drive screw via a short piece of polythene tubing, this was used as a 'smoothing' joint to reduce the effect of the 'steps' from the stepper motor showing at the eyepiece. The motor mounting is secured to the pressure plate via a large piece of double sided foam tile pad - again to reduce the visible effects of the motor steps. In practice the motor is running at a high step rate (approx. 180 steps per second) because it runs through a gear box, this step rate is not visible in the eyepiece unless an extremely high magnification is used (X700+), the 'seeing' except on very, very, very, rare occasions never allows this magnification anyway.
(insert image of stepper drive control)
(insert images of scopes on platform- 6 & 10-inch)

Platforms work with a restricted latitude variation, unlike their EQ mount cousins. Mine is built to work at 51.5° north (or south). To build one you need to know a few basic facts about your telescope: the size of the ground board and principally the height of its centre of gravity (CoG). This is important in that the CoG needs to be placed on the virtual axis of your latitude cone or just below, so that any drive will only need to turn the telescope not try to lift unbalanced weight at the same time! If the CoG is too high the platform will be unstable - safest being slightly below the axis and use a slightly larger motor to drive the platform.

The circle the bearing sector is part of has a radius taken from the virtual axis of the cone, in my case the ball races have been arranged to support this circle diameter with the rear pivot being placed along the virtual axis. It could be placed at the vertex point but this can lead to oversized platforms as latitude angles decrease.