[Andrew C]

This is an interesting thread. I think a solution would be invaluable at times when hand tracking using high magnification.

Firstly, the essence of a Dobsonian is to keep it simple to use, so I think the starting point is a hand operated design.

For the altitude control, and building on the roller blade wheel idea:

• What about adding an outrigger fitting to the Dob’s OTA cradle on the eyepiece side, to house a bearing mounted short spindle. The spindle protrudes horizontally into a suitable spot in the V gap underneath (or above if necessary) the Dob altitude bearing.
• Our Teflon? roller blade wheel is fixed to the inner end of the spindle, so that it contacts the outer surface of the altitude bearing journal. Teflon has the great advantage that it does not ‘stick on start-up’ like other materials.
• A knob is fixed on the outer end of the spindle to turn it.

Obviously, the key is to come up with a specific design for each Dob type where the diameter and material of the wheel provide the appropriate amount of friction to adjust the altitude smoothly, yet still allow easy movement of the OTA in the normal hand held way. My suspicion is that the wheel diameter will need to be quite small in practice (i.e. less than that of a roller blade wheel) to provide the leverage to move the OTA smoothly in fine increments – but we will have to wait and see when someone has built one. There also needs to be some initial fixing adjustment so that the wheel does not bear too much of the weight of the OTA assembly, since this would affect the balance of the telescope.

For the azimuth control, same idea, but in this case we need a large diameter circular hub fixed to the outside of the Dob base, with the outrigger fitting attached to the side of the cradle body so that the spindle is vertical and facing downwards, and reasonably close to the altitude knob so that both knobs can be accessed at once.

• Using this method, the knob is on the top and reasonably accessible.
• Maybe consider a small bike wheel rim for the hub, equipped with a smooth tread tyre
• Probably remove the spokes and just fix the rim directly to the base. Spindle wheel on inflated tyre might be a forgiving enough combination to cope with any minor misalignment in the mounting of the rim.

Any volunteers with access to the mechanical bits to build one and try the idea out? The altitude seems the easier part, so maybe that is the place to start. Even having that half of the solution would be a useful improvement.

Andrew