Slow motion for dobs

[circumpolar (Matt)]

[quote=monoxide;302641]if you haven't already seen the doco 'seeing in the dark' try to chase it down, theres a guy that made a slow mo control for his dob using a bicycle brake lever, squeeze it and it will move the base in azimuth.
quote]

3:45 mins only. Doesn't show the bike brake though.

Youtube link:
SEEING IN THE DARK | Excerpt | PBS
http://www.youtube.com/watch?v=aubezyXXAjc

[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

[Kokatha man]

Originally Posted by astro_nutt

Hi everyone..I've been building motorised slow motion mounts for my 10" dob for a few years now..generally you have to provde near zero friction on both axes firstly to allow the motor/gearboxes to do their work..for the altitude motion you provide an adjustable tension for the friction drive..from the output shaft of the gearbox to the friction drive wheel then to the altitude bearing this will stop any rollback when the gearbox is disengaged from the friction drive wheel.
For the motor/gearboxes..I used the Tamiya type from Dick Smith (6501 : 1 reduction at 3 volts) which I find more than enough..the motor engagement is via cables and levers..
I'll be at the Messier Night ( see www.asv.org.au ) at the Leon Mow dark
Sky Site this Saturday
Cheers!

Hi astro_nut: got any piccies of your set-ups? Would love to be able to eyeball some pics of your gear.

Seems like some of us pipe-dreamers on this thread should get up and at it - I threw my bit in 'cos I thought it could come in handy with or without my EQ platform: my thoughts about motorized Vs manual are part of my pipe-dreaming logic about the ease/suitability of either/any motive drive.

As seems to be a consensus amongst many, friction has to be factored in; perhaps more specifically for azimuth rotation.

With this in mind I'm still leaning towards creating a metal arm extending out from the edge of the (fixed) circular ground-board of my dob, this "arm" carrying the motor with a spindle on its armature shaft. (this could also just be a manually operated spindle)

A spring-tensioner could create pressure between the motor spindle and the edge of the (rotating) azimuth board of the dob. I would propose gluing a long thin strip of sanding belt around the azimuth board's edge and (possibly) on the motor spindle's rubber roller to improve "drive friction." (on the motor roller I'd use the cylindrical sanding "tubes" of drum sanders/spindle sanders)

To minimise azimuth friction I'll cosider a lazy susan adaptation for the az bearings, reasonably confident that the motor would provide sufficient "braking/stick" to stop too free a movement of the az. A manual az operation might need more friction "drag."

For the alt movement I'm still thinking of a sort of "crown wheel gear" affixed to the OTA's plastic bearing "blocks" and gear driven via an armature cog on the motor for this operation - again a manual equivalent would be feasible - perhaps easier here on the alt motion than a motor drive re the friction control considerations for this motion.

Oh well, time to stop waffling and start tinkering around!

Regards, Darryl.