Slow motion for dobs

[Sammas]

I was wondering if anyone has made a simple manual slow motion control for a dob? It seems to me it should be pretty easy to construct such a thing, and it would make it a lot easier to keep objects in the view - I find I can't tweak the movement of my 8" dob very well and end up throwing things out of view as often as I keep them in the view.

If nobody's done it, I'll give it a go and post the results.

[Kokatha man]

Go for it sammas - I've been thinking of something along these lines for when my Round Table EQ platform arrives: have an HEQ5 for my refractor but thought slomo controls for the 10" dob could make the outfit suitable for some imaging experiments. I could see it starting out as something along the lines you're proposing; to just manually keep objects in view in a more sophisticated way than "push-pulling" the scope around - and then growing/evolving into a motorized auto-guide controlled affair for imaging for the 40min or so duration of an EQ platform! (factoring in set-up etc)

However, I digress (or fantasize completely over the top) but: I did think about the basic idea and considered looking for some material to create large rack and pinion gear bands - the one for altitude would not be as challenging but a large gear band around the revolving dob azimuth base is a challenge.

I wondered whether long nylon strips (if you could get such) similar to that on cable ties could be utilized as the rack gear: in fact it needn't encompass the entire base but just a portion, as long as it could be re-set for when you manually slew do a different section of the sky.

Anyways, I've thrown my 2 bob's worth in - glad you've kick-started this idea sammas - hope some others add their views/ideas!

Cheers, Darryl.

[Kokatha man]

Haven't waited for anyone else's response (many would say that I've never let that stop me from barfing on before..!) but another of my earlier thoughts was to install a small motor drive with rubber roller that engages with the edge face of the dob's azimuth board under spring tension: perhaps using a friction layer from a strip of a sanding belt adhered to this edge to enhance power transfer.

A bit like the friction driver units on some self-propelled lawnmowers etc. The innate gearing of such a set-up would ensure quite precise discrete motion which could be enhanced further by a speed control unit and hand controller.....

As mentioned before, the altitude control would be far less of a problem, I am sure a ring-gear or somesuch could be created for this function.

Apologies for rabbitting on over the subject but these sort of "seat of pants" type projects seem to stir the ol' blood far more than the more sophisticated ones. I guess it's like getting out of a jam in the desert, when you've broken down and all you have is some pliers, fencing wire etc - which means you're a bit of a clown for driving around like that in the first place!

Cheers, Darryl.

[Blue Skies]

You shouldn't need a slo-mo control for dobs, in my opinion. If you do, there is probably something wrong with your bearings. Are the teflon pads in the right place? (eg, not too far up or apart?) Are you using telfon? Are they clean? No flat spots on your altitude bearings? The mount should move smoothly without sticking in any direction, so if it's not I'd be looking closely at these things and maybe even replacing some parts.

[tbentley]

I've been thinking about this for some time myself. I was looking at it being friction driven on both the alt and az rather than any sort of gearing. Main reason being that you would still want free motion for the larger slewing motions without having to use a clutch. Of course anything adding friction would make it harder to locate the scope by push/pull but then I would have the fine motion controls to do this.:D
I was thinking of something like roller blade wheels for this purpose, either on a worm drive or spun directly by the cable. Now that this thread has started it up I might stop thinking and start doing.
Trav

[Kokatha man]

Beg to differ blueskies: I'm perfectly at home "push-pulling" the scope, although others too have obviously mused about alternative controls - nothing wrong with my bearings or my capacity to slew the scope as is either, but I can definitely see benefits from a controlled slew; be that geared or friction driven (particularly at higher magnification). Very finite adjustments, especially as I'm envisaging; with a one-hand operated contol box (much the same as my motorized focus controller) would be (to me) an obvious boon; as well as satisfying that male urge to "gadgetise" everything!

Regards, Darryl.

[Sammas]

Same here... I like the challenge!

I was thinking at first of something like a sideways barn door type system for the base - a length of 20x1/4 screw running through a rotating nut attached to the top dob base plate and fastened at another rotating thing attached to the bottom plate - so the main screw is at a tangent to the dob base.

The drawback here is that it's hard to slew (pick up the dob and move it :) and you'd have to reach to the ground to turn it.

I've also thought about attaching some kind of teeth to the dob base and turning it with a gear, but that starts to get out of the realm of easy DIY - unless I can find some old piece of rubbish that has some length of flexible teeth in it.

A better idea would be the friction/spring you mentioned Darryl. A rollerblade wheel held against the top plate of the dob base by a strong spring would work (but might be too clumsy; would it be slow enough?) - and you could release the spring tension to slew it. Same deal for the altitude, but with a smaller wheel.

The friction solution is simple, which is why I'm leaning this way - no need for finely machined parts, and wheels of various sizes should be really easy to find in recycling places. Hmm. I found this, which was an interesting idea:

http://www.atmob.org/library/member/JoeR/DobSlowMoControl.html

I guess my main desire here is less about fixing a problem than refining something that already works well. My dob is smooth and easy to push/pull - I just like the idea of being able to keep my view as steady as possible with a few little twiddles of my fingers. It's also about gadgetising everything... XD

[Kokatha man]

Yea Sammas - that's a really dinky idea; proving that often some of the simplest ones are the most effective.

I wasn't too worried about the altitude control, I thought that there would be numerous options for that (but I'll most probably give that one, or a version of it, a try.)

But my idea of a motor that is spring-loaded pressing against the azimuth board (anchored via a plate to the base-board) is something I'll investigate further: because the (rubber?) roller on the motor shaft would be a much smaller diameter relative to the diameter of the edge of the azimuth board's circumference, you would have good gearing for finite control, with the added bonus of the motor providing a degree of friction (braking) control when rapidly hand slewing without having to disengage the motor. This could entail freeing up the azimuth board's current fairly stiff movement to facilitate this type of conversion. There are some cheap, relatively "pokey" little 12 volt motors around.....

[Dooghan]

What about a bike chain?? I'm not quite sure how you would go about attaching it to the base. Where I work they have tumblers to clean the product. What they are is a metal tube with bike chains raped around them on each end. In this case the chains are tack welded to the tube. Then these tubes a sat on top of 4 cogs which turn the tube. Just another idea for you to think about.

[vindictive666]

gday

ive tried this idea on my 12 incher

it works for a while but the ota is too heavy at lower angle and it start to slip
so i gave up on it and it really works well too for those finite movements on high mag :)

http://www.atmob.org/library/member/JoeR/DobSlowMoControl.html


regards john

[bojan]

Guys, go for bartelization :-)
http://www.bbastrodesigns.com/BBAstroDesigns.html#Computer_Operat ed_Telescopes
Plenty of ideas on this website...
You can take gradual approach also: steppers (with simple driving) first, computer control later.
The project will never be finished anyway :-)

[tbentley]

Gee this has advanced since I last stepped in. Now we're talking motorizing the thing! If you were going to that effort wouldn't and EQ platform be what we are talking about? You'd need some pretty sophisticated electronics to alter the speed of movement on alt and az based on the direction you were pointing the scope.
Too advanced for me I'm afraid. I'll stick to my idea on manually operated controls like Sammas was originally talking about.
Trav

[monoxide]

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.
although he doesn't go into detail how it works you could probably pick up pretty easily what he's done. also the way he did it, it doesn't need to be reset as if you kept going with the brake lever it would do a 360 and keep going.

[vindictive666]

dont suppose you can remember the guys name by any chance there is a lot of seeing the dark info out there :)

tia :)

[eqcradle]

What about letting air out of a bicycle tube?:
http://members.cox.net/tfangrow/hissdrive.html

I've wondered whether it might be possible to make a simple hand-operated EQ platform based on the same idea as a Dob friction bearing. Ie., rather than have the EQ platform move on a couple of roller bearings or wheels, just sit it on a couple of teflon pads. To track an object, you'd still need to push it by hand, but you'd only need to worry about one axis rather than two.

Picture a 30cm section of fat PVC tube sitting in two V shaped wooden blocks with teflon pads (and pointed at the SCP). The platform is bolted on top of the PVC tube in such a way that it sits level. The trick would be getting enough friction so it doesn't tip over but it's still movable.

[Kokatha man]

I'm a sucker for these mental gymnastics eqcradle, but you'll have to provide us with a little sketch or a more fullsome description - I've tried to visualize/sketch from your above but am finding it hard going. Thought the "hiss-drive" was great!

As for your post vindictive666 (that name sounds like some sort of ominous biblical omen brother!) - I can envisage the brake lever creating a one-way azimuth control but not bi-directional: if that was the case (uni-directional) it wouldn't be much cop - a bit like me going to Melbourne via Sth. Africa and Sth. America!

Regards -

[eqcradle]

Well, it's not a great sketch, but hopefully it helps you picture it. The blue part is the PVC tube which sits in the 2 'V' parts on the base (they could easily be circular instead of V shaped). You would put some teflon pads in the 'V' so that it works like the side friction bearing on a Dob. Then, you attach the platform somehow to the top of the tube - maybe you could bolt the tube between 2 triangle sections. The Dob then sits on the platform. It's a bit like sitting a Dob on top of another Dob mount that is on an angle. Anyway, the point is the tube can rotate in the base. Naturally, you'd need to aim the tube at the SCP.

You could add a couple of stoppers so that it only rotates 5 degrees either side of vertical (so that the scope doesn't fall off). 10 degrees movement would be enough to follow an object for 40 minutes i think. Just reset it when you want to follow something else.

I have no idea whether it's feasible though. Eg, I don't know how you'd adjust the friction so that you could move it by hand without it trying to tip the scope off.

Yes, i like the hiss drive idea too. I love the simplicity.

[bojan]

This seems to be unstable, prone to topple on either side.
Designs like this should take into account the position of the centre of gravity.. the vertical line from it should never fall outside the area formed by support points. And it should be below the axis (not easy to achieve), or as close to it as possible. That is why conventional platform designs generally have limited tracking range.

[eqcradle]

You're right, it would be inherently unstable. But I wonder whether you could engineer enough friction in it though so it wouldn't move by itself? (But then it might be too much friction to move it by hand.) The stoppers would prevent it from tipping.

Incidentally, you don't always need the centre of gravity below the axis - the hiss drive is a good example of one that is stable despite the COG being above the axis. I've separately posted a similar barn door idea I made that was stable.

[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!

[circumpolar]

[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]

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.