I need help with OnStep

[Startoucher101]

Hello stargazers! I would like to make an EQ mount with OnstepX. The microcontroller that I'll be using is the CNCv3.

I also want a NEMA 23 stepper motort(400steps) but others suggested me to buy the Nema 17. 

I want the payload capacity to be at least 20-25kg,so can you tell me what motors to use?

Last but not least what else do I need?

Clear skies,Manos

[AstroKeith]

Assuming appropriate gearing, a Nema17 will be sufficient. I design Dobsonian drives of up to 50kg that use nema17's.

You'll want to use a gear reduction such that the motor micro steps, when reduced and at the telescope axes are small enough for your purposes. This will mostly depend on whether you are wanting to do astrophotography or just visual.

[Startoucher101]

Thank you AstroKeith! Do the Nemas 17 have backlash issues?

And also because you mentiond that you are motorizing a dob what else sould I buy exept the controller and the steppers?

 

Thank you,clear skies!

[Vroobel]

The stepper motors don't have any backlash, it appears in the gearing. Nema 23 steppers usually offer a higher torque than the Nema 17, but it depends on a strength of magnetic field generated inside the stepper - the longer the stepper the higher the torque.

That's right, you have to use a proper total gearing ratio for astrophotography. If you base on 400 steps per revolution and use e.g. 64 micro steps, and the gearing ratio is 100:1 then your theoretical final resolution of the axis equals ( 360 * 60 * 60 ) / ( 400 * 64 * 100 ) =~ 0.5 arcsec which is quite good for astrophotography. In practice, the resolution is smaller, you can find info about it in OnStep(X) documentation.

 

Edited July 13 by Vroobel

[Startoucher101]

Vroobel,I have to tell you something! You are the reason that I begun this project! I just learned so many from your videos and I think that I found the best person to advise me! Thank you providing me all the information needed. I am also designing a rotator because of you! Also what can I do to improve the tracking the tracking accuracy?

 

Thank you and clear skies!

Edited July 13 by Startoucher101

[Vroobel]

That's nice, you made my day better, thank you. ☺️ 

Share all your assumptions here, we can think about your project together with other SGL members.

And good luck, of course! 👍🤞

 

[Vroobel]

27 minutes ago, Vroobel said:

The stepper motors don't have any backlash...

 

 

You may face another problem related to the steppers itself. The torque lowers with a number of micro steps, so in the worst scenario, some micro steps may be lost because the stepper may have not enough power to move a significantly loaded and not balanced axis.

Edit. It may happen rather around 128 or more micro steps.

 

Edited July 13 by Vroobel

[Startoucher101]

Hellon again,thank you responding! How can I face this problem?

[Vroobel]

3 minutes ago, Vroobel said:

Edit. It may happen rather around 128 or more micro steps.

 

  

[Startoucher101]

What do you mean?

[Ratlet]

On the subject of gearing, worth bearing in mind that you need to account for the total gearing of the system, so you can have a gearbox on the stepper but there will probably also be a gearing between the axle you are turning with the motor and that rotates the axis of the mount. 

You can get some pretty nice planetary gearbox fitted steppers from AliExpress. I picked up a 27:1 nema 11 and 99:1 nema 17 and the gearboxes have next to no backlash.

Even from the 11 the torque is wild with 12v.  Here's a video of my playing with it.

https://youtube.com/shorts/QJl4QJZCEEA?si=jx11yXawzu6EIaNb

[Ratlet]

3 minutes ago, Startoucher101 said:

What do you mean?

Because of the way stepper motors work, microstepping reduces the torque output of the motor.  The more microsteps, the less torque.

[Vroobel]

1 minute ago, Ratlet said:

I picked up a 27:1 nema 11 and 99:1 nema 17 and the gearboxes have next to no backlash.

 

 

Indeed, the planetary gearing provided with the stepper as assembly usually has the backlash, but I didn't take this case into account in my calculation above. By the 100:1 gearing, I meant a worm or harmonic gearing.

 

[Vroobel]

9 minutes ago, Ratlet said:

Because of the way stepper motors work, microstepping reduces the torque output of the motor.  The more microsteps, the less torque.

 

I'm not qualified enough to explain it scientifically, but roughly saying, every level of micro stepping (number of micro steps per full step) is based on a fraction of the magnetic field needed for turning the stepper by one full step. A 2-micro-steps mode offers smaller torque than a full-step mode, a 4-micro-steps mode offers less than the 2-micro-steps one, etc.

A 32- or 64-micro-steps mode gives you good fluency and accuracy to the torque ratio.

 

 

Edited July 13 by Vroobel

[Startoucher101]

Ok thank you everyone! So, what stepper motors sould I buy(remember I want a payload capacity of at least 25kg)

Also I will have a complex system for astrophotography that needs to be really acurate! What Onstep microcontroller sould I buy. I really need a good one!

Thank you everyone again!

[Vroobel]

Do you need a mobile app which means you will use BT/WiFi features, or like me, do you want to connect it to a NINA/ASIair using a USB cable?

I used both MKS L-Gen v. 1/2.5 and FYSETC S6 v.2 in my applications. They are custom 3D printer boards which offer at least 5 ready-to-work ports for any 12-24V steppers, but you need to buy stepper motor drivers, at least two of them. I would recommend LV8729 as they are quite cheap and silent.

You should choose your stepper motors depending on your project and the final gear you use, so please, tell us much more about your idea.

Edit.

I have to add that both the board I used are slow which means a GoTo slew will be 1-3 degrees per second. You can find better boards offering more features.

 

Edited July 13 by Vroobel

[Startoucher101]

Thank you Vroober for responding again! I want to control my mount from NINA. I a this suggestion; To use the D1-r32 kit with the CNCv3 sheild. Could this do a good job?

[Vroobel]

I think it's OK, as long as guys from the OnStep recommend it, or as long as it's supported. Take into account that it's deprecated boards, like my FYSETC S6 and MKS Gen-L (L-Gen?).  

 

[Startoucher101]

Ok,so I am going to buy this!

[Startoucher101]

Ok, Also a really impotant question; How can I improve the goto accuracy?

Thank you again, clear skies!

[Vroobel]

You can do it by using e.g. worm gear or harmonic gear together with (not too high) number of micro steps, like in the example provided earlier. 

 

[Startoucher101]

I of course will use worm gears. And acording to you I souldn't use 400 steps? But this will do the tracking worse right?

Thank you again Vroobel!

[AstroKeith]

There are some high performance planetary gearboxes available now. They have quite low backlash ~15 arc minutes. If you use one of these, and then follow up with further reduction - say a worm drive. The further eduction ratio reduces the planetary gearbox backlash.

For my Dobsonian drives, I use 1.8 deg Nema17 steppers (200 step), with 30:1 planetary gearboxes, and then a 30:1 final reduction (roller to main bearings). I use 16 micro steps. The torque output of the motor/planetary gearbox is enough to shift a 42" Dob (100kg) at 6 deg per second. Key to this is acceleration and deceleration.

Goto accuracy is a different issue. You can use step counts to compute angles, but the mechanical accuracy of the mount will seriously degrade the answer. You can add encoders to the scope axes which will be better but not perfect. These days the best solution is to use plate-solving.

[Vroobel]

1 hour ago, Startoucher101 said:

Ok, Also a really impotant question; How can I improve the goto accuracy?

Thank you again, clear skies!

 

My bad, I had to read it twice. The GoTo is a matter of a system controlling your mount like NINA, ASIair or others. As @AstroKeith mentioned above, the system captures an image of the sky and it recognizes the piece of the sky where your scope is aimed. Of course, a supported camera is needed.

I ask a third time: please describe your project, what is your idea. It would be easier if we know what you want to achieve. 

 

[Startoucher101]

Ok,Vroobel! My idea is quite simple; To make a DIY EQ mount(not harmonic). I want to use this mount for;

1.Astrometry 

2.Astrophotograpy

So,you see that I need much accuracy,in order to find faint celestial object. The goto accuracy is really important but I will have to use plate solving. I also need the Onstep controller to support guiding. This is a really good example of what I want