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Old 08-05-2021, 09:04 PM
Stefan Buda
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New rack and pinion focuser

As part of my CDK250 development, I managed to get precision rack and pinion mechanisms manufactured through Alibaba and then I was able to make my own RP focuser.

I'll update the CDK250 thread, if I can find it, to explain the rationale.
Meanwhile here are some photos I took today.
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Old 08-05-2021, 09:36 PM
AnakChan (Sean)
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That looks really sweet. Have you been able to test it to see that it's rigid? No tilt, etc.? What size is that? 2.5"?
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Old 08-05-2021, 09:42 PM
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Atmos (Colin)
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Oh that’s neat! Curious as I thought your CDK used the secondary mirror for auto focus.
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Old 09-05-2021, 11:33 AM
Stefan Buda
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Quote:
Originally Posted by AnakChan View Post
That looks really sweet. Have you been able to test it to see that it's rigid? No tilt, etc.? What size is that? 2.5"?
I measured a tilt of less than 10 microns over a 60mm diameter and I think it would be unrealistic to expect anything better.
Next I will test backlash and lift capacity.
Rigidity is determined by the thickness of the two sleeves and the spacing of the bearings. To make it any stiffer, I would have to make it longer or to make it out of titanium.
The clear aperture is 50.
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Old 09-05-2021, 11:35 AM
Stefan Buda
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Quote:
Originally Posted by Atmos View Post
Oh thatís neat! Curious as I thought your CDK used the secondary mirror for auto focus.
This requires a long answer and I will do it soon, when I update the CDK250 thread.
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Old 09-05-2021, 03:19 PM
AnakChan (Sean)
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Quote:
Originally Posted by Stefan Buda View Post
I measured a tilt of less than 10 microns over a 60mm diameter and I think it would be unrealistic to expect anything better.
Next I will test backlash and lift capacity.
Rigidity is determined by the thickness of the two sleeves and the spacing of the bearings. To make it any stiffer, I would have to make it longer or to make it out of titanium.
The clear aperture is 50.
Less than 10microns tilt? That's pretty decent.

Would be interested to see your next test, esp the backlash as it seems the EAFs are known to have. I'm guessing the issue is usually inside the EAF. But new AF overshoot algorithms seem to have a way of working around that anyway.

I like the look of that focuser - it's so neat and nicely integrated with the EAF. Especially with the new EAF where it's USB powered only.
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Old 09-05-2021, 07:22 PM
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multiweb (Marc)
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That looks pretty neat Stefan.
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Old 09-05-2021, 08:42 PM
Stefan Buda
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Quote:
Originally Posted by AnakChan View Post
Less than 10microns tilt? That's pretty decent.
Would be interested to see your next test, esp the backlash as it seems the EAFs are known to have. I'm guessing the issue is usually inside the EAF. But new AF overshoot algorithms seem to have a way of working around that anyway.

I like the look of that focuser - it's so neat and nicely integrated with the EAF. Especially with the new EAF where it's USB powered only.
Thanks for the nice comments.
I did the load test today, but I ran out of time to properly determine the backlash.
The EAF had no trouble lifting one of the EQ6 counterweights that is a bit over 5kg.
Under that loading most of the backlash was taken out by the load, but still there was some hysteresis that I was able to control with a backlash compensation of about 4 steps.
The step size measured with the dial indicator is about 4 microns.

Quote:
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That looks pretty neat Stefan.
Thanks Marc.
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Old 10-05-2021, 01:59 PM
Stefan Buda
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I've done the backlash testing this morning and, just as Sean suggested, practically all the backlash is coming from the EAF reduction gearing.
With a small load applied to the focuser and trying to rotate the flexible coupling back and fort, I measured a backlash of about 60 microns, corresponding to about 15 steps.
However, there is so little friction in the actual focuser, that even a small loading gets rid of the backlash. Under a load of about 1kg, I was getting the best result with a backlash setting of 1 step.
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Old 10-05-2021, 03:48 PM
AnakChan (Sean)
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That's good to hear! This project is getting more and more exciting!
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