Help, need this part.

[tnott]

I want to use this part (see attachment) to fix an encoder to so my scope can sense directions for a goto system for my Tridob. http://www.iceinspace.com.au/index.php?id=63,324,0,0,1,0

The conventional way of attaching the encoders to the ALT/AZ pivot points would not work for my scope as there is none. So this roller would run along the AZ bearings and ground ring instead.

It is a roller with an O ring on its circumference, a .25" hole in the middle and a grub screw to fix the encoder shaft.

Anyone know where I can find one?

Tim.

[gbeal]

No, but it would be simple (says he that can't use, nor has a lathe) to make one, quite simple.
Why no ability to mount in the pivot point like all us normal folk? Can you shoot a picture of the alt bearing ?
Gary

[Glenhuon]

I'd be looking for a bearing supply shop Tim, the ones I used to deal with could supply small aluminium pulleys and "O" rings for almost any application.

Cheers
Bill

[tnott]

Just click on the link of my original post to see pics of my scope. I could make extension arms to hold the encoders at the ALT/AZ pivot points but they would be rather long and unwieldly, plus I would have to deploy them every time I set up the scope.

It apparently worked well for this other scope that has a similar structure to mine - with horseshoe shaped bearings, a flex rocker and ground ring.

http://www.foothill.net/~sayre/22-in.%20binocular%202.htm

Cheers,
Tim

[gbeal]

Oh boy, now I see why. Find a machinist like Mick Pinner perhaps.

[Glenhuon]

Hi Tim
Just had a look through the shed. I have 3 of these. Plastic wheels with rubber tyres, ex laser printer. The shaft diameter is 8mm (5/16") so would require a bush of some sort to get them down to 1/4". Your welcome to them if they are any use. (Ooops, forgot the attachment :) )


Cheers
Bill

[tnott]

Thanks for your suggestions guys but I have done some further investigation and the roller-coupled-to-encoders option has been tried before and failed because it proved impossible to avoid some slippage.

I'll try another tack.

Tim

[Astroman]

If you had the drawings Tim, I might be able to get one turned up at work for you to try?

[Glenhuon]

One more suggestion Tim. To avoid the slippage problem, what about a sprocket on the encoders and some very small pitch chain on the curved bearings and base. Just a thought :)

Cheers
Bill

[bojan]

I am using roller couplers for encoders and there is no slippage.. at least no significant one that can be noticed. Of course, this will depend on applied pressure as well.
My rollers are spring-loaded and the surface is slightly serrated (in irregular fashion, like a grit-wheel used on large plotters for paper transport), so the slippage is reduced even more.
The alternative coupling method would be a timing belt....

[Tannehill]

Neat.

That is a friction drive wheel.

That and lots of neat stuff can be found at Stock Drive Products.

Go to https://sdp-si.com/eStore/ then click on Miscellaneous, then Friction drive wheels. .25 bore is a standard size. Most have a hub with set screw, to fix the wheel onto the drive shaft (encoder bearing).

Other options: Rollers, with Hub

Or, try the Pulleys, grooved, and then pursue the o-ring as a separate purchase. Least favorable, as fitting the roller (o-ring) will be a hassle.

Don't know but I'd bet McMaster Carr carry something, also.

you can set your system to accept any ratio, then? Or does the drive wheel have to be a specific diameter? Does that friction drive wheel ever slip? Timing belts would be better, I'd think, but that goes back to your issue with no axis line to use....

Just curious...for my education..since I don't (can't) build scopes from the ground up: How can you not have an Az or Alt axis? Is it a Porta-ball like scope? In which case, how can you use encoders...I guess that answers my own question. But, don't you have a central azimuth bolt as the center of rotation, or is your dob base mounted atop a plate of transfer bearings with no central az axis bolt? The pic looks like that's the alt axis there. You can find the precise center of the alt or az axis of rotation of any dob, even those lacking an obvious axis centerline, using some trig. There is this little dobsonian 'trig for dummies' (Hmmmm...I resemble that remark) powerpoint file out there, if anyone is interested...in how to zero onto the exact axis of rotation of your dob....helps when struggling to precisely align the alt encoder (the harder one, usually) to the exact center of the axis of alt rotation.

EDIT: Hey, just saw your scope on the articles page. neat. you can fit a conventional alt encoder to that, but it'd need a special bracket...kinda like how the Obsession 18" UC accepts encoders. But best regards however you do it, very nice set-up. I esp like the low profile and ability to avoid ladders with that design...wish I could drill a hole without hemorrhaging and make something like that....

Scott

[tnott]

Wow! That was the exact part I was looking for Scott so thankyou. I may give it a go yet but am trying a different strategy at the moment. The roller-coupled-to-encoder idea was a backup plan for me anyway. Putting long, unwieldly encoder arms on it are a last resort for me, but possible. BTW, I am useless with tools too but still built it - thanks to Dave Kriege's The Dobsonian Telescope book and the internet.

Bojan how accurate is your system? It looks a bit more heavily engineered than what I had in mind. Also the ALT roller would need to ride on the ebony star laminate of the ALT bearings - which is very slippery.

Glenhuon, the timing belt and toothed pulley approach would not slip but is not possible on the ALT bearing at least.

A DSC manufacturer informed me that they tried very hard to engineer a friction roller-coupled-to-encoder approach for dobs but that it always ended up with slippage errors.

Next time I design a scope, I'll factor in Goto from the beginning.:doh:

Tim

[bojan]

It is quite good, I do not have more that 1-2 steps difference when going to and back 90 degrees couple of times.... The sensitivity I have is ~6000 steps per 360 degrees of the telescope axis rotation which is enough to give me reasonable accuracy for visual observations for the night.. If I need to have it more accurate, I can always point to the desired object and reset coordinates (in my Bartelized system) .
As you saw on the images, I am using al wheels as bearings on which the tube is riding, and the small grit-wheel is spring-loaded upwards onto the ALT bearing so I can remove or place the tube into cradle and ALT encoder is automatically decoupled or coupled.
In your case, you can have the slippery stuff covering only part of the thickness of the ALT bearing, and the rest (without ebony star laminate) can be used for riding a wheel... Or you can attach the bigger or smaller sector wheel on one of the bearings for encoder roller in parallel to the bearing.
I think the slippage errors they are talking about are inevitable with rubber wheels. Grit-wheel is different.. on the high pressure it will press-in its own path into softer material of the bearing and this will act as a gear... and no significant slippage.
You can make one by wrapping around the existing wheel the tape cut from sand-paper and gluing it on.
Or, you can apply the araldite on the wheel, then pour on this a sand particles obtained from sand-paper.. When the glue is set, simply remove the sand which is not firmly attached and you have a grit-wheel.

Have you considered chain option? There is no slippage here either... It may be a bit more fussy to assemble the scope for a nigh but worth a try...
Or, toothed rack, wrapped around the bearing (or additional sector). Then, you can have a gear on the encoder axis...

[Glenhuon]

I've only looked at the pic on the link in your first post but wouldn't it be possible to clip the chain to the side of one of the alt bearings and use like a rack and pinion. I'm no scope builder either, but was a fitter for 40 odd years and these sort of problems still get me thinking :)

Cheers
Bill

[Tannehill]

The 18 Obsession UC has alt encoder bracket fittings and tangent arm that are quite compact and easily fit. It fits a scope just like yours, with the "virtual" mirror box design. If you wanted, you could email Dave Kriege (his website) and ask if anyone can send you a pic of the alt encoder bracket for the 18 UC. Or join the obsession_users yahoo group, and post that question, and within 24 hours you'll have someone post a pic on the Photos section.

But, then again, it'd be kinda fun to see if you can out-kink that friction-wheel design concept...I like seeing folks pursue different angles, because we learn by each other's trials and tribulation.

Please post an update to your efforts in the future, so we can all see the result!

Regards

s

[Rod]

Hi Tim and others,

I have the same problem with my scope's alt bearings. I have been thinking about using a timing belt turned inside out - like Dan Gray did here for his azimuth axis:

http://www.siderealtechnology.com/28inch/azimuth.jpg

There must be some way to then connect the timing gear to the encoder. Would this work?

I also thought about whether a platic rack and pinion would work - bend a plastic rack around the alt bearing circumference. However I have not been able to locate a plastic rack large enough. Any suggestions on where to look?

Thanks,

Rod.