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Telescope focuser clutch idea needed

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I have a telescope with a Crayford focuser (should say will have ~7days), and I'm looking to remote/automate the focusing for observing and astrophotography, using a stepper motor, controller etc.

But I would also like to be able to adjust the focus manually for observing which would mean a clutch of some kind. Because of the accuracy needed any meshed teeth are out. The critical focus zone is around 0.1mm, and focus is achieved by rotating a 6mm steel rod, an angular rotation ~ 0.52 degrees.

The focuser incorporates a 10:1 reduction (black, picture right), but I would like to use that for manual control. The control motor would replace the silver knob on the left of the picture.

To sum up I'm looking for a very simple design which will allow the motor/gearbox to be disengaged and engaged, in the dark likely with cold fingers - or possibly electronically.

Any thoughts or ideas much appreciated.

Best Regards


Given relatively low forces involved, a  flat plate or perhaps a cone clutch, (as typically found on drill/mill quill drives)  would suffice.

A simple alternative would be a belt final drive , with releasable tension (assuming you can control the lateral force)

My Nikon camera lens has something like what you're describing: The auto-focus does its thing electrically, but at any time - even if it's still trying to autofocus - I can grab the focus ring and manually set it. I have another lens that won't let me do that... I have to disengage auto-focus before I can turn the focus ring.

My guess, it's a friction plate of some kind between the motor and the focus ring. As Bill says, the forces involved are quite low, so anything that will allow slip when manually overridden will do. However, that does mean you'll need some kind of sensor/encoder on the actual focus mechanism, as you won't be able to rely on the stepper count to tell you the mechanism's position.

@ Adev, BillTodd
Thanks for the comment. I think a friction clutch is a good way to go.

It occurs to me that I could make an additional 10:1 (or thereabouts) reducer for the driven end, but spring load the outer bearing ring (which is made with a slightly conical interior), such that it would slip on manual override. This would mean I could basically just hang a stepper off the end of the adjusting shaft without any further gear reduction. Any backlash would be as a result of machining errors in the brass ball carrier, hopefully taken care of in software.

Will do a bit more thinking on it, but it seems good now.

Many Thanks


Just found this old post....thought these that I make would fit the bill.

I made a few prototypes by hand and then farmed my first big batch to a CNC shop....kinda cheating I know.


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