The Concept
Dual speed focuser design borrows the planetary gearing idea used in automobile gearboxs. However, in order to eliminate backslash, balls and axles are used instead of gears. All the transmission inside dual speed focuser is achieved via static friction.
The cylinder case of dual speed focuser is fixed on the telescope tube. 3 metal balls are placed and separated 120° between each other inside the case by a holder shelf. An input axle penetrates through the hole on the center of holder shelf and is pushed and tightened by 3 surrounding balls. The fine focusing knob connects to the input axle while the fast focusing knob connects to the holder shelf. On the other side, the axle from Crayford focuser is connected to the holder shelf.
When you rotate the fine focusing knob, the 3 balls are driven by input axle and rotate along the inner circumference of the case at slower speed. The metal balls then push the holder shelf and drive the output axle. As a result, rotating either of the focusing knobs will cause the other to rotate in the same direction.
Read more about this topic: Dual Speed Focuser
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