that certainly looks like a good worm but I'm not sure I'd buy
it without some details of it's accuracy.
Also, BTW, don't write off plastic or nylon gears as an option
either...
A lot of 'Bartelised' scope users report good results with
nylon, plastic and even wood machined worm gear forms.
Haven't written off the plastic just yet, though my choices are very very limited.
A few weeks ago, I did buy a very cheap plastic worm gear from Jaycar, just on the off chance that I would have a go, seeming I was running into dead ends. It was the best of the plastics.
I have also disassembled (er... destroyed..hehe) a wiper motor, to get to the worm gear in that, as another alternative.
Also had a chat the other day to a guy who was fixing the large automatic gates at work. Mentioned where I can get any gears or anything close to the system they were using (the gate reduction box looked like a commercial worm gearbox), he didn't know, was his reply. They were heavily into mechatronics.
Hmm, can't seem to take a trick.
The hobbystore gear set, yes have thought about the accuracy a bit too.
But at the moment, it's the best candidate.
One thing that was going through my head as a real wild thing, was to try a larger stepper motor to drive it directly.
Yes a heathenous thought it is.
The torque would be well enough to drive the OTA, but the resonance of the motor at low speeds worries me. As Bojan has remarked the dropped steps is an issue, but it's playing with the finite settings that can reduce that over a analog driver.
The test on the smaller motors worked really well, just unsure how the larger motor would do, even though the driver units could accept the larger current draw.
If I was using the old analog drivers, this heathenous thought, would be driven out of my brain by the jitterness of it.
But the digital driver performs way above my expectations, to put this thought out of my mind.
I never did see a large motor drive at very slow speeds with this driver, so it's an educated guess.
No doubt a direct driven OTA with astro gear on it, will show up signs of the jitterness. It's a matter of how much.
So I am trying to think logically, but outside the square as well.
I would strongly suggest at least just having a play with a
'Bartel' type stepper or servo arrangement first before going
too far off on a tangent with ideas that may just end up
frustrating you.
The Bartel setup can be done totally on the cheap...all with
pretty much 2nd hand gear that today is worthless.
You say you have read the Scope-Drive pages tirelessly...
well dive in and have a go.
The best way to get a feel for it's limitations and quirks...along
with it's fantastic range of possibilities...is to do it.
You can get steppers suitable from old photocopier machines, faxes,
printers and even old 5 1/4in floppies....cheap.
You can use an old 386-486/Pent 120 era laptop/desktop....cheap.
You can make the circuit needed on a piece of veroboard in
an afternoon.....cheap.
Finally, you can implement a perfectly sound tracking platform
on a Dob (or GEM if you're adventurous) with just pressure
rollers or toothed belts....cheap.
Finally, I can highly recommend joining the Scope-drive mailing list
if you haven't already....thousands of very experienced Bartel
dabblers at hand to offer advice.
It seems to be some way yet from being viable for amature use but was an interesting read. Mel Bartels, Dan Grey and others are part of a group working to bring large professional telescope technologies to large amature telescopes (1 to 2 meter diameter from memory). I can't find links to articles about the group at the moment but they had some interesting projects under way.
I'd not be racing into trying to use direct drive at this stage but it is an interesting idea.
Bob
Nice link. Bit on the expensive side when you factor in the Aussie dollar shipping and possible GST if directly shipped from them (if it's not marked as a gift).
I like the alum gears. 3 arc seconds seems pretty good. But I think with the cost, you could probably buy 2-3 sets from the hobbystore in QLD.
Still unsure in the accuracy of these gears from QLD.
Bob, yeah the direct drive was just a pure imagination thing, but I knew it was no good as a concept until you got the jitterness under control.
Now the digital driver, I had major smoothness. However it wasn't tested with a large motor.
So it's still a hypothesis that it could work. Because there is so much of a setting to obtain with the currents/steps etc, it change dramtically on how much vibration or jitterness you can have.
The belt attachment has been investigated (belt and pulleys).
Still looking at concepts but still an alternative.
Well it's been quite a journey to get here, I have to laugh
when I look back at some of the pictures from when I started
this thread.....
Tonight, between lightning and thunder and some
pretty heavy downpours it was clear enough to point the
scope skywards and get this.
This is the first tracking session (unguided) with the new
homemade RA worm gear (317 teeth).
Stepper is belt driven through some temporary reduction using
drive belts to test the periodic error.
I didn't have a clear enough gap between showers to get several
worm revs logged but I am ecstatic at just getting my first shot.
Like I said Simon, I'm just happy to get my first result.
It's only 5sec exposures Newt focus 12" F5 and Meade DSI Pro II
with IR filter.
30 exposures, seeing only average.
My serious test set will probably be something in the order
of 10-30sec shots on a VERY steady night for an hour
or so close to meridian and celestial equator.
A lot of things , including the weather have to co-operate of course
I'd be pretty happy with that Steve. Excellent result first up.
Thunder, lighting wind and rain are all out to test us.
Pretty good shot with the elements against you.
Tonight the seeing looks promising.
A front of showers has passed through and
sky was clear at sunset.
I've just finished a 10sec set at Declination -55
somewhere in ARA.
A few very faint fuzzies here.
This set is the best frames from a set in which the
worm completed several cycles.
With the Bartels Arc sec per step size almost spot on for
this new gear, the PE was minimal.
An early impression is that this 10sec set had more good frames
than a set with the old spur gear drive.
So it's looking very promising.
I still have to log a lot of cycles and put it into an Excel
spreadsheet to visualise the PE but it's looking pretty good!
Hopefully I can continue tonight, weather permitting.
A 15sec set is in progress right now.
Edit: 2nd image added
As per my usual setup: 12" F5 Newt / Meade DSI II/ Baader IR
FL 1500mm.
Steve
Last edited by kinetic; 14-07-2009 at 09:36 PM.
Reason: added the 15sec set image
Excellent Steve, I'm itching to get started on my own wormwheel.
If only the guys at Carbatec would get back to me about cost and availability of a replacement feedscrew for my lathe.
Tonight I had some success quantifying the PE of the 317 Teeth RA worm gear.
Seeing was very good, high cloud occassionally spoiling it.
Firstly, to minimise any confusion, I refined the tracking rate in my Bartels stepper drive
so that any PE would not have a sky rate error superimposed on it.
Once that was done I had a nice cyclic wobble showing with a 4.5 minute period.
This is the period of one complete worm rev. {1440mins/317 teeth = 4.5mins)
To represent this graphically, a picture tells a thousand words:
I captured a set of long exposures again but deliberately dialed in
some sky rate error so that the wobble wasn't drawn over itself
cycle after cycle.
Pic 1 shows the 4.5min wobble in RA (west is left)
Next I deliberately dialed in some Polar alignment error, again to
prevent the wobble drawing over itself (Pic 2)
Now the polar alignment error causes the main PE to show better,
still with an approximate 4.5min cycle. Note the drift now in DEC
Pic 3 is all sky rate and polar alignment errors zeroed again
and PEC tuned to the 4.5min cycle and TURNED ON.
Not bad eh?
An open cluster in Norma....I think it is NGC 6067.
The clearest stars I have ever had in an image!
Is the cyclic wobble from the gear itself, or the shaft (talking about a slightly bent shaft)?
Brett,
it is either from a non concentric worm in the carriage or the
toothed belt pulley on the end being non concentric.
Either way, I'm happy it's cyclic and the only substantial PE.
Seems the worm gear accuracy doesn't even come into it.
I have attached some shots of the jury rigged reduction which
is driving the worm at the moment.
Also attached are two old favourites from earlier tonight.
PEC was turned on for both shots but only sharpening applied to
the NGC5139 result. Seeing was only average, some high thin cloud.
Your ratio, is that 60:1 x 2:1 x 317:1? (equals in total 38040:1??)
Yep, close Brett, not sure where you got the 2:1 from.
(Im assuming you guessed by the sizes of the toothed pulleys?)
Stepper 1.8 deg/step into 60:1 reduction then
Toothed pulleys 1.8:1 then finally
Edit:"sorry: it's actually 1.75:1! (24Tpulley drives a 46T pulley)
Worm driving 317 teeth worm gear 317:1
I get 33285:1 (RA axis only BTW)
That's possibly not a final value either, for the record.
Steppers as you know all have comfort zones.
This setup is close to that, but it's not ideal for fast
half step slews.
How's your options going? Has anything come together yet?
Yep, close Brett, not sure where you got the 2:1 from.
(Im assuming you guessed by the sizes of the toothed pulleys?)
Hahaha, well yeah, it was a guess. 2 isn't far off 1.8
Had a day off last Friday, went and visited a company that distributes some gear products, that I thought were "perfecto" for scopes.
Planetary drives that were perfectly matched to scopes and it's finite movements. Best solution so far, vast devices
The product was made by Faulhaber. German company.
Now considering the sizes, engineering and the precision, the costs weren't as bad as I expected.
The intent, was only to see what products they had, that were good enough for functionality then price would have been an issue.
I'm looking only to see what is out there, rather than the intent to buy (just yet)
I found a size that would have suited the scope, planetary drive, which can cost about $300 and up. (500:1 and rising).
The guy that I was talking to, also had a scope and he motorised it, so atleast finally someone I can talk turkey with. (relief)
But the common problem with all these drives, is "supply".
Had to order from Germany, none kept in Aus.
I've been in a very large business for nearly 20 years, and have grown up on the words of "Can't sell from an empty wheelbarrow" (from the GM).
I find this too common of an occurance in Aus with O/S sourced parts.
Hence local products will get the nod if any.
So the concept is, either have a crack at the "smallparts.com.au" worm gear set (which is the best solution in all aspects ATM from geared POV), or direct drive.
I've also considered the epoxy/plastic threads. But that's more of an alternative.
I'm just working on the actual driveshaft mechanism now.
As it's a Bintel with the newer style brake system, I've been looking at a way to mount it all. http://farm3.static.flickr.com/2446/...a8ab2b5410.jpg
I've figured out the internal hole is perfectly drilled for a 18mm helicoil.
Didn't want to re-drill a machined surface, if I could help it (didn't want an out of round hole)
Then I can use a 18mm bolt, and mill it down somehow to a point where I can make stuff fit to it.
It's just that I am not trying to bugger up the brake system whilst making a driveshaft that works, so I can independantly move the brake cover on the outer thread without an issue.
Once the shaft is mounted, it will be easier to see how to mount a drive.
I've had a rethink about my drive (direct).....I've been in conversations with Mel Bartels.
Had a chat about direct drives and gearing. I didn't fully understand the reason for stepping down the gearing a few times now, but for "sub arc" resolutions. Like using a 360 teeth gear, then step it down to a 30:1 drive (etc).
My thoughts are now back on the local supplied worm and some stepdown system (Brisbane sourced worm).
As my efforts to get a "new age" gearing system in AUS, have failed, on the supply front.
But another alternative is, in late Jan-Early Feb, the wife has organised a trip back to the states.
I'm looking for USA companies who can supply a gearing system and a reasonable cost for it's function and most of all, have supply of them. Sheesh, do I have to physically go overseas for it as well?? Hahaha..
And the stupid thing I'll find, is that 4 companies within a 5k area will be able to supply. The USA version of bunnings "always" has everything in stock! Haha
Visiting NY (Way upstate) - yes lots of snow -yay!, Kansas, maybe New Orleans district as well , and my fav, NASA - Cape Canaveral.
I put my bidding, to the wife, for a shuttle launch visit around Feb 4th. And wanted so desperately to visit.
So the shuttle visit / launch trip is there, just organising it.
If I can get a shuttle launch on the few days we are there, that's my bucket list right there.
Hahaha..
