Two input mechanical mixer (telescope)

[picclock]

I have an Celestron alt azimuth mount which I would like to fit sidereal correction (earth rotation compensation). The mount has a simple worm drive slow motion control. I would like to use this to apply the correction.

My best thought is to use a motor with clutch and a belt drive so that the mount worm shaft is always connected to the motor drive, but when the slo motion is turned manually the clutch force is overcome and slips allowing manual fine positioning.

But i'm not very happy with that as force needed to turn the control will be quite high, and telescopes tend to wobble quite a lot after being touched (4-5 secs typical).

It seems a thorny problem. One other solution that occurs to me is to sense the hand/rotation on the slow motion control and use that signal to disengage the drive. But this is fairly complex and likely to fail on wet dark nights.

Any thoughts or ideas much appreciated.

Best Regards

picclock

[awemawson]

Stepper motor with a planetary gearbox attached possibly ?

[BillTodd]

differential with lock on the manual input ?

[vtsteam]

+1 Bill

[BillTodd]

Differential could be a planetary gearbox, lock could be a worm drive.

[picclock]

Thanks for interest and the excellent idea, using a worm to one input makes it seem obvious now.
The stepper could drive the sun gear, and the worm the outer with the output drive from the planet ring.

I did have a further thought about using the manual input to control the stepper speed. But this is always fiddly and does not give immediate response the way a planetary gear system would.

Many thanks for your interest.

Best Regards

picclock