Attaching focus motor to FeatherTouch

[ChrisLX200]

I had this job to do for a mate, I thought it would be simple but of course it wasn't . He has bought a focusser upgrade and already had a SharpSky autofocus kit and needed to mate the motor to the focuser somehow. Problem with this focuser is there are no flat bits on it anywhere - no place to attach an external bracket, and using a belt drive with the motor on a remote bracket was equally problematic. I first tried a direct attachment to the 1:1 drive knob shaft but it seemed to me the movement was too coarse and it struggled to move the focuser, which meant instead it would have to fit onto the 10:1 fine adjustment shaft. The big question was How? After some head-scratching this was my solution:

The focuser sans knobs - this is the fine drive which the motor needs to attach to:

The motor itself is a geared stepper mounted inside a subtantial casing which also contained a temperature probe. I removed the front plate and found room for two screws to pass through so that a flanged tube could be securely attached to it. The tube has a bore large enough to accommodate the flexible drive coupling, and had some slots machined in it to allow access to the clamp screws. At the other end a recess was machined to fit over the 10:1 gear-reduction bearing housing on the focuser, the clamp screws are at an angle so they bear on the rear side of this thus pulling it firmly into its seated position:

Assembly is then quite straightforward, attach the tube and the drive coupler to the motor, then slip it in place onto the focuser and nip up the retaining screws. Finally, tighten the clamp screws of the drive coupler onto the 10:1 drive shaft using a hex driver through the access slots.

Not the simplest of adapters to make but it does the job :-)

ChrisH

[Horwig]

Nice bit of machining.

Huw

[r3i]

Superb engineering Chris, I wish I had your skills.

[PaulB]

Nice one Chris,

That focuser looks very nice doesn't it.

As soon as the other "M" arrives. I'll be in touch.

[Mick J]

That's really nice work, and good photo's to boot. 

[steppenwolf]

I admire your machining skills, Chris, you've done a lovely job - I'd love to be able to do that!

My own solution, using the same focuser and SharpSky stepper motor, is somewhat more 'clunky' and two dimensional!

[ChrisLX200]

I admire your machining skills, Chris, you've done a lovely job - I'd love to be able to do that!

My own solution, using the same focuser and SharpSky stepper motor, is somewhat more 'clunky' and two dimensional!

Nice one ;-) I had thought about that solution but figured there might be some flexure in having such a long bracket held only by 2 screws, but anyway I had the lathe so why not. I always seem to make hard for myself!

I've come across a couple of those motors that have failed which came a surprise really - not the motor part but the integrated gearbox. One is the very motor shown above which is currently awaiting a replacement. It seems there is a huge amount of torque available thanks to the high gearing, and whilst I would expect a stepper to simply stall under load it seems there is enough torque in this one that the gears will shed teeth instead. Be very careful you don't repeatedly run it into a hard stop (like end of travel in a R&P focuser). I've not personally had one fail but that was perhaps more luck than judgment because once set up it never goes near the end of travel, but I have two here that have and are toast...

ChrisH

[steppenwolf]

whilst I would expect a stepper to simply stall under load it seems there is enough torque in this one that the gears will shed teeth instead. Be very careful you don't repeatedly run it into a hard stop (like end of travel in a R&P focuser).

Thanks for the warning, Chris, I will watch out for that! I have to say that I have been delighted with my complete unit - beautifully made. I did worry about the potential for flex but in use, it doesn't appear to be an issue - time will tell but your system will certainly obviate that risk.

[ChrisLX200]

Well I have talked to Dave about this and my analysis is that the danger only applies to short-travel R&P focusers where the motor is directly attached to the focuser pinion (like yours in other words :-) )  In all other cases - where the focuser is a Crayford type for e.g., it will likely slip without causing damage. My own NP127is setup uses an intermediate belt-drive which adds some cushioning. What does the damage is a sudden stop where the R&P focuser reaches the end of its travel - you have to remember that the motor itself is spinning rather fast and there is a heck of a lot of reduction gearing between it and the final drive, that motor is NOT going to just stop instantly due to inertia and so the massive torque generated at the final gear does the damage. In my case and my firend's it was running the focuser to each end of travel in order to calculate the total number of steps from one end to the other (a figure that SGPro needs). This is no longer a recommended procedure with a R&P ;-)

I stripped the gearbox down to examine the damage done:

So this is not a criticism of Dave's design, I love my SharpSky and have motors attached to 3 scopes (I have just one controler). You just need to be aware not to run it into a sudden stop at the end of travel using a R&P focuser where there is no flexibility. Crayfords are fine, belt drives are fine. And if the worst comes to the worst Dave is happy to help out (as he has done with me).

ChrisH

[rpineau]

I ran into a similar problem with my Feather Touch and a RoboFocus and went the 3D printer route for this :

http://www.rti-zone.org/astro_diy.php#focuser_bracket

I since have replaced the RoboFocus with a QuickSync FT motor.

[catman161]

This absolutely awesome! Such brilliant skills. I have a feather touch focuser and I have really struggled to motorise it too. The Jmi motofocus didn't work. And I'm at a loss what to do. I have just procured a cheap second hand skywatcher Autofocuser and will have a go at trying to fit it to the feather touch. I may be messaging for advice in the new year Chris! Have a good Christmas

[redtail]

Fitting a motor to the fine focus is not a good idea and is a quick way to ruin it..well documented on Web.

[Tinker1947]

While its not a Feathertouch the Moonlite CR2 motor fixes to the shaft with a bracket held by 2 screws on the base plate, then 2 more on the motor casing, the silver knob it a slip clutch so the focuser can be used while the motor is attached, then tightened for motor operation.....Nice piece of work there Chris.....Merry Xmas all...

[catman161]

While its not a Feathertouch the Moonlite CR2 motor fixes to the shaft with a bracket held by 2 screws on the base plate, then 2 more on the motor casing, the silver knob it a slip clutch so the focuser can be used while the motor is attached, then tightened for motor operation.....Nice piece of work there Chris.....Merry Xmas all...

I had this setup on a previous OTA with a CR2. I loved it. I wish there was a similar quality/prices equivalent for the Feathertouch line of focusers

Sent from my iPhone using Tapatalk

[ChrisLX200]

Fitting a motor to the fine focus is not a good idea and is a quick way to ruin it..well documented on Web.

Tried it on the coarse knob first - the motor didn't have the grunt to move it and the movement itself was far too coarse for autofocus. I don't know what the argument is about attaching to the fine focus knob, other than the obvious caution not to run it into the hard stop at either end. If you used the coarse knob you would have to gear it down which would then not be much different from using the fine knob (in terms of torque being applied to the rack).

ChrisH

[ollypenrice]

I'm another admirer of Chris's workshop skills.

As an alternative, Per 3D printed a bracket to stop his focus motor turning once clamped to the focuser shaft. That's also pretty cute.

I'd think of attaching my focus drive to the scope permanently but I need my fingers during the day... 

Olly