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Dobson Goto Project

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#1 ivazquez

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Posted 14 October 2021 - 03:28 AM

Hi,

 

I'm thinking about converting my Dobson to GoTo using a Raspberry and stepper motors, but the main problen I'm facing is how to get enough steps per revolution. I'm thinking about using a worm gear to drive the axises but I cannot find one big enough. Will a 3D printed gear work? or, where can I find a big enough gear?

 

Thanks in advance.


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#2 ngc7319_20

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Posted 14 October 2021 - 06:09 AM

Commercial Dobs sometimes use a small metal pinion running on a large wooden disk -- a spring provides pressure and keeps the pinion pressed into the wood disk.  Another version has a small diameter steel cable wrapped around a small metal spool and large wood disk / Dob bearing.  In both cases the control is "closed loop" where a separate optical shaft encoder detects the scopes' position.  That way the drive system can slip a little bit (be a little sloppy) and it does not effect the goto accuracy.  Accurate drive gearing is not needed as the encoders provide accurate position information.


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#3 astrokeith

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Posted 14 October 2021 - 06:41 AM

I've done a few Dob drives and currently like to use toothed belts. (standard 5mm pitch) Typically a 12 or 14 tooth pinion and a 300 - 500 mm diameter circle on the base board or altitude. There is enough friction not to need teeth on the larger circle.

 

I use micro steppers with integral 27:1 gearbox and then the tooth belt gives typically 30:1 reduction. This can then give arcsecond resolution plus slew speeds up to at least 10 deg per second.



#4 ivazquez

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Posted 14 October 2021 - 03:28 PM

Thanks. I was thinking about using a toothed belt wrapped over a circle with the teeth facing outside, but I don't know if it would work.

What kind of encoders you suggest? The ones I find are incremental, not absolute.

#5 astrokeith

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Posted 14 October 2021 - 06:06 PM

Thanks. I was thinking about using a toothed belt wrapped over a circle with the teeth facing outside, but I don't know if it would work.

What kind of encoders you suggest? The ones I find are incremental, not absolute.

I have also tried it with a pinion engaged against a big circle wrapped in a tooth belt as you suggest. It was OK but not good engagement between teeth (the exterior curve produces the wrong tooth profile) and so performance was poor.

 

Absolute encoders will cost you more than your whole telescope. I use 40000 tick incremental.



#6 ngc7319_20

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Posted 14 October 2021 - 09:47 PM

What kind of encoders you suggest? The ones I find are incremental, not absolute.

 

 

Incremental.  It is easy to just count the ticks.  Various encoders provide 4000, 10000, 32000, etc. ticks per revolution.  Ideally the encoders would mount to the scope axes, and be mechanically independent of the drives.  That way if the drive momentarily jumps, stalls, etc., the computer still knows where the scope is aimed.  Some encoders and install examples here:

 

https://www.astrosystems.biz/dsc.htm


Edited by ngc7319_20, 14 October 2021 - 09:48 PM.


#7 ivazquez

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Posted 15 October 2021 - 08:15 AM

Incremental.  It is easy to just count the ticks.  Various encoders provide 4000, 10000, 32000, etc. ticks per revolution.  Ideally the encoders would mount to the scope axes, and be mechanically independent of the drives.  That way if the drive momentarily jumps, stalls, etc., the computer still knows where the scope is aimed.  Some encoders and install examples here:

 

https://www.astrosystems.biz/dsc.htm

The best encoders I find are only 600-1000 ticks revolution. Where do you find thos high precission encoders? I Know I can use a gearbox to increase the precission but it would be great to have a high precission encoder from start.

 

Thanks. I was thinking about using a toothed belt wrapped over a circle with the teeth facing outside, but I don't know if it would work.

What kind of encoders you suggest? The ones I find are incremental, not absolute.

40000 ticks per absolute revolution or is this the precission of the encoder?

 

Thanks for your help.



#8 astrokeith

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Posted 15 October 2021 - 08:30 AM

The best encoders I find are only 600-1000 ticks revolution. Where do you find thos high precission encoders? I Know I can use a gearbox to increase the precission but it would be great to have a high precission encoder from start.

 

40000 ticks per absolute revolution or is this the precission of the encoder?

 

Thanks for your help.

These are the most popular/common https://www.usdigita...ental/shaft/s6/

 

These are becoming more common, although they dont have the resolution. Note these are quoted as 'pre-quadrature' which means you get x4 the stated resolution. https://www.cuidevic...ar/amt11-series

 

You dont want to use a gearbox as this will introduce errors. Put the encoders directly on the mount axes.



#9 ivazquez

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Posted 15 October 2021 - 10:41 AM

Thanks. I will try to find those in Europe. I think I will start with DSC and then upgrade it to Goto.


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#10 astrokeith

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Posted 15 October 2021 - 11:49 AM

Thanks. I will try to find those in Europe. I think I will start with DSC and then upgrade it to Goto.

I personally would recommend a Nexus DSC, which can be purchased with encoders. It is very flexible and easy to interface to when you add a goto.

 

I use it on my scopes, with SkySafari connected, plus a serial link to my own custom drive. I've also managed to connect a home built digital finder to it via wifi.



#11 Adun

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Posted Yesterday, 12:31 PM

Thanks. I will try to find those in Europe. I think I will start with DSC and then upgrade it to Goto.

 

In that case, I recommend you try this DSC project.




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