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red dwalf

which focus knob to fit stepper motor to

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red dwalf

Been given a stepper motor, thanks Steve, to fit to my William optics gt 81, currently thinking about which focus knob to fit it too, either the big, normal focus knob or the smaller 10/1 focus knob.

What are people's view point on this ? 

My thinking is if I fit it to the smaller 10/1 knob it will give me even finer adjustmentm

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Brantuk

I agree with your thinking Rob - the smaller wheel will give finer focus. The focuser kit I have replaces the small knob with a specially  manufactured gear for the shaft - and the motor has a cog which drives it. :)

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red dwalf

that sounds interesting Kim, any chance of a link to the manufacture or name of kit please, i`ve taken the small knob off and the shaft is only 3mm thick, shouldn`t be a problem but would have liked it to be a bit fatter so giving any worm screws something to grab onto.  

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Graham

Definitely the small one. 

I have a belt drive on mine giving me even more of a fine adjustment. 

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red dwalf

Thanks guys, smaller 10/1 knob it is, any chance of a picture of your belt drive mod Graham please, looking to buy the parts needed soon, will give me something to do in the summer months. 

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Graham
4 hours ago, red dwalf said:

Thanks guys, smaller 10/1 knob it is, any chance of a picture of your belt drive mod Graham please, looking to buy the parts needed soon, will give me something to do in the summer months. 

 

Rig is imaging at the moment but I will take a pic and post it here when its done 

It's work for the night. 

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Graham

Here you go Rob.

Sky watcher focus motor driving a toothed belt that fits the grooves in the focus knob.

 

DSC_0050.JPG?raw=1

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red dwalf

that is a good idea Graham, this is the motor i`m using at the moment but on the other side of the focuser.

got me thinking a bit now.

thanks for the photo.

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Graham

It also has the advantage of saving you dismantling the focuser and it is interchangeable between all my scopes 

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red dwalf

Does seem a good idea, can I ask where you got the belt from ? Or what size teeth is it, I think I'll go down that route as you say it would be easier.

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Graham

Got the belt from motion Co. 

They do all sorts of wonderful robotics stuff. 

I will measure up the belt when I get home from work tonight. 

Edited by Graham

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red dwalf

been on the motion co website, i`m confused again, they have two types of pulley and wheels, one is a 3M HTD type and the other is a MXL type, not sure which i need, and also should i go for a small wheel or one a bit bigger like an inch in diameter around the same size as the focuser knob ?

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Graham

I use the mxl type.

If you are going to use a drive pulley then I would use one with a smaller diameter than the knob that way you increase the gearing thus giving you a finer focus control. 

Eg

If the drive pulley is half the diameter of the knob it will take two turns of the motor for one turn of the knob.

This in turn will increase the focuser from a ten to one ratio up to a twenty to one ratio.

 

Edited by Graham

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red dwalf

What was the size of the belt Graham ? Are they measured diagonally or circumference ?

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Graham

If you measure up where abouts you want the motor to go and then get what the distance is between the centre of the motor shaft and the centre of the focuser shaft.

On the website there is a calculator. 

Put in the distance, the diameter of the focuser knob, the size of your drive gear you are going to use.

It will then tell you what belt you need. 

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Ibbo

I have just found this info

 

Should I Connect the Stepper Motor to the Fine Focus Knob?
NO. There is a misconception that driving via the 10-speed reduction is safe. This is not good idea as the mechanics and manufacture of the mechanisms employed mean that they are not robust enough to have that level of force (from stepper motors) consistently applied to them. There are small gears (normally plastic or cheap metal) involved that do not have the strength to handle the force a stepper motor can apply. Over time this results in increased wear and tear in the small gears, leading to increased backlash, or if the focuser limits are exceeded, the complete breakdown of the fine focus mechanism by catastrophic failure (breakage) of the gears - in other words, not a good idea.
So why do some users try connecting to the fine focus knob? The answer is that it already provides a 10:1 reduction so this increases the resolution without any cost. The downside is the cost in replacing the focuser mechanism when the 10:1 reduction is damaged by the stepper.
If you need to drive the fine focus mechanism (FFM) then it would be better using a belt drive to prevent damage in the event of exceeding the focuser limits. It is not recommended.
Remember NOT to use the Focus Lock Screw on your focuser; leave the screw loose or remove it. The Stepper motor will hold the focuser in place. If you leave the screw in and accidently tighten the focus lock screw, then serious damage can occur to the stepper motor or focuser.

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