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Implementing Auto-Focusing, any advice?

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#1 Noobulosity

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Posted 22 July 2021 - 12:17 PM

I'm building my own electronic focusers based on the MyFocuserPro2 open-source project.  I've finally gotten it functioning on my AT60ED and got my first successful v-curve in N.I.N.A. last night!  (*shakes angry fist at clouds*)  By attaching directly to the 10:1 fine focus shaft on the focuser, I'm able to get about 8.5 steps in the critical focus zone, which I'm hoping is good enough for this scope.  I could also microstep, if need be, but I'm running with full steps right now.

 

A couple things I noticed when I was trying to get it to focus made it easier to repeat getting a good curve:

  • Move the focuser manually, watching looping exposures, and set it so it's just barely beyond the critical focus zone before starting the AF routine.
  • I found it might be better to change the calculation method for finding best focus to "Trends and Parabolic".

But this is so new to me that I just don't have a good grasp on fundamentals or best practices, yet.  So I have a few questions that I'm hoping more-experienced imagers might be able to help answer.

  • Does anyone have any advice for me to have consistent success with the electronic focuser?
  • Am I using too-coarse or too-fine of step increments according to my graph?  (Increments were motor 10 steps.)
  • How many step increments do you recommend I move outside of focus when I start the routine?  (I was using 5, I believe.)
  • Should I consider a different calculation method?  (Trends and hyperbolic?  Just hyperbolic?  Just parabolic?  Just trends?)  Which tends to provide the best results?
  • Without using temperature compensation, how frequently should I be refocusing?  (I also have longer focal length scopes up to 1000mm, which I imagine are more-sensitive and require more-frequent focusing..?)
  • Do you recommend taking more than one exposure per position?
  • Anything else I should know?

Thanks!  smile.gif

 

Here's the graph from my first night of success:

51328246561_9c075eecce_b.jpg

 

My first automatically-focused test shot (with some wispy clouds...):

51329238010_7798cb91f2_b.jpg


Edited by Noobulosity, 22 July 2021 - 12:21 PM.


#2 AhBok

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Posted 22 July 2021 - 12:54 PM

Looks great. I think you have nailed it. Have you tried doing 3-4 focus runs in a row to see if the best focus point changes more than 5-10 steps? If it does, this could indicate focuser drawtube slip or incorrect backlash setting. If not, I would not change anything until problems arise, such as odd “handle bars” on the end (s) of your curve or failure to focus, etc.
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#3 Noobulosity

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Posted 22 July 2021 - 01:00 PM

Looks great. I think you have nailed it. Have you tried doing 3-4 focus runs in a row to see if the best focus point changes more than 5-10 steps? If it does, this could indicate focuser drawtube slip or incorrect backlash setting. If not, I would not change anything until problems arise, such as odd “handle bars” on the end (s) of your curve or failure to focus, etc.

I ran it a couple of times, but didn't watch to see if the best focus location was close.  I'll have to watch that next time I give it a whirl.

 

I did notice if I don't leave the coil power enabled I get some slip as it tries to move before it's fully energized.  I'll have to see if there's some type of pre-move delay I can adjust.  The motor also got a bit hot, so I ordered some heatsinks.  I'll slap one on the end just to help cool it a little bit.  But I used PETG for the printed motor bracket, so I think it'll be able to withstand the heat of the motor okay.

 

51300568845_90e65cc671_z.jpg

 

51298811167_7ba519fcc3_z.jpg

 

51298811397_e5b5a5b949_c.jpg



#4 ac4lt

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Posted 22 July 2021 - 01:07 PM

I'm not an expert here but this has been my experience.

Use hyperbolic if you are going to move fairly far from the focus position. Use parabolic if you are going to stay fairly close to the focus position (that is to say that your furthest movement of the focuser during the run).

For your graph, parabolic seems like a decent choice.

Using trends averages the straight line intersection with the parabolic (or hyperbolic) minimum point. I do that but I really haven't seen a great deal of difference between the two values (which is probably a good thing).

I haven't seen a value in multiple focus readings per point.

You might consider fewer focus points with larger steps to shave some time off the focus run.

No idea on the time interval since I'm using temperature based focus changes but it's really going to depend on your local conditions. The more quickly the temperature changes the more you'll need to refocus. Maybe get an open weather api key and plug it into Nina to get at least the approximate temperature at your location and use that to trigger focus changes. Barring that take a look at some historical data for your location and see how much time elapses between 1 to 2 degree C temperature changes and use that as a guideline.
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#5 Noobulosity

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Posted 22 July 2021 - 01:23 PM

I'm not an expert here but this has been my experience.

Use hyperbolic if you are going to move fairly far from the focus position. Use parabolic if you are going to stay fairly close to the focus position (that is to say that your furthest movement of the focuser during the run).

For your graph, parabolic seems like a decent choice.

Using trends averages the straight line intersection with the parabolic (or hyperbolic) minimum point. I do that but I really haven't seen a great deal of difference between the two values (which is probably a good thing).

I haven't seen a value in multiple focus readings per point.

You might consider fewer focus points with larger steps to shave some time off the focus run.

No idea on the time interval since I'm using temperature based focus changes but it's really going to depend on your local conditions. The more quickly the temperature changes the more you'll need to refocus. Maybe get an open weather api key and plug it into Nina to get at least the approximate temperature at your location and use that to trigger focus changes. Barring that take a look at some historical data for your location and see how much time elapses between 1 to 2 degree C temperature changes and use that as a guideline.

Thanks!  I could add a temperature probe to this setup, but I just wanted to get it up and running.  I also don't know how well taping a temp probe against the focuser body really works for tracking temperature changes.

 

The API key is an interesting idea.  It wouldn't work for imaging at remote locations where I don't have wi-fi or mobile signal.  But I could try that at home.  I would need to see how that integrates with this setup, and get a sense of how much temperature changes affect focus for my scopes.  My Zenithstar 103 is pretty insensitive to temperature drops.  But I bet my Sky-Watcher 190MN would be a lot more sensitive.


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#6 ac4lt

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Posted 22 July 2021 - 01:32 PM

The curve you posted looks great! If it is repeatable you shouldn't have any problems. Your DIY skills far exceed mine so adding in a temperature probe sounds like the best long term solution. Focusing based on time you are either going to focus too often in the stable part of the night or focus too infrequently in the early part of the night. You can tell Nina to focus based on HFR changes but there are too many non-focus related reasons why that could change and cause unwanted focus runs to really rely on (at least for me).
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#7 terrypaula

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Posted 22 July 2021 - 01:56 PM

FYI:  It's been my experience that auto temperature adjustments can be a problem at times; actually getting in the way of an exposure sequence, stopping it in the middle.  Make sure you have all you settings/timings in proper sequence.  That was the problem I first had when I first started using it.  You may have too tweek things a bit...  I've been using a home-made focuser fashioned from a 1" square stepping motor.  Most likely the issue to start with.

 

Oh yeah, yours looks a bunch better then mine too.


Edited by terrypaula, 22 July 2021 - 01:58 PM.

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#8 AhBok

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Posted 22 July 2021 - 02:34 PM

If you live in an area where the temp changes are gradual and your scope has plenty of time to equilibrate before imaging, then focusing on temperature has little value. I focus every 2 hours and my focus stays tight. I live in the midsouth, though where the humidity fights against rapid changes in temperature. If you live in a dry climate with large temperature swings or if you bring your scope out of the house and begin imaging immediately, then a temperature probe would be well worth your effort to implement.
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#9 Noobulosity

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Posted 22 July 2021 - 03:38 PM

The curve you posted looks great! If it is repeatable you shouldn't have any problems. Your DIY skills far exceed mine so adding in a temperature probe sounds like the best long term solution. Focusing based on time you are either going to focus too often in the stable part of the night or focus too infrequently in the early part of the night. You can tell Nina to focus based on HFR changes but there are too many non-focus related reasons why that could change and cause unwanted focus runs to really rely on (at least for me).

Thanks!  I'll give focusing based on time a try for now.  I have an upcoming overnight trip to the mountains, so I don't want to change too many things before then.  I probably won't have time to work out any bugs.  But I may give the temp probe a try.  It wouldn't be terribly hard to add one.

 

FYI:  It's been my experience that auto temperature adjustments can be a problem at times; actually getting in the way of an exposure sequence, stopping it in the middle.  Make sure you have all you settings/timings in proper sequence.  That was the problem I first had when I first started using it.  You may have too tweek things a bit...  I've been using a home-made focuser fashioned from a 1" square stepping motor.  Most likely the issue to start with.

 

Oh yeah, yours looks a bunch better then mine too.

Thanks!  I spent a lot of time designing all the pieces that I've printed, but I think it'll be worth all the effort.  Mine is using a NEMA14 stepper, so a little smaller than the average at around 35x35x25mm for the stepper body.  It's nice and small, not adding a ton of weight to the rig.

 

I've still been getting used to the newer sequencer in N.I.N.A., so I may have to take a little time and practice everything before my trip.

 

If you live in an area where the temp changes are gradual and your scope has plenty of time to equilibrate before imaging, then focusing on temperature has little value. I focus every 2 hours and my focus stays tight. I live in the midsouth, though where the humidity fights against rapid changes in temperature. If you live in a dry climate with large temperature swings or if you bring your scope out of the house and begin imaging immediately, then a temperature probe would be well worth your effort to implement.

I do live in an arid climate, and unfortunately there are quite large temperature swings between day and night here.  I've seen upwards of a 40°F drop from a day's hi and low temperatures.  I do try to get the scope out early if it's cold so it can acclimate before I start shooting.  Sounds like a temp probe might be worth the effort.

 

Any advice on how to attach it for good thermal contact?  Thermal adhesive/tape, maybe?  Am I measuring focuser temp or ambient temp when triggering a refocus?



#10 AhBok

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Posted 22 July 2021 - 03:47 PM

Perhaps others from a similar climate as yours will chime in with better advice than I can give on positioning the sensor. When I relied on temperature compensation, I positioned the sensor to measure ambient temperature (probe not touching anything).

#11 Oort Cloud

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Posted 22 July 2021 - 07:26 PM

I'm by no means an expert in this area, but since the temperature of the telescope is what would drive the need for refocus (by way of expansion/contraction), would it not make the most sense to attach the probe to the OTA itself?
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#12 ac4lt

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Posted 22 July 2021 - 07:46 PM

To be clear when I was talking about temperature based focusing, I wasn't talking about real-time adjustment but focusing every 1 or 2 or whatever is needed for your telescope degrees C change.

#13 Noobulosity

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Posted 22 July 2021 - 07:57 PM

My impression was that the focuser temperature typically drives when to refocus, but I could see OTA temp as an option, as well.



#14 sbharrat

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Posted 22 July 2021 - 10:12 PM

I'm building my own electronic focusers based on the MyFocuserPro2 open-source project.  I've finally gotten it functioning on my AT60ED and got my first successful v-curve in N.I.N.A. last night!  (*shakes angry fist at clouds*)  By attaching directly to the 10:1 fine focus shaft on the focuser, I'm able to get about 8.5 steps in the critical focus zone, which I'm hoping is good enough for this scope.  I could also microstep, if need be, but I'm running with full steps right now.

 

A couple things I noticed when I was trying to get it to focus made it easier to repeat getting a good curve:

  • Move the focuser manually, watching looping exposures, and set it so it's just barely beyond the critical focus zone before starting the AF routine.
  • I found it might be better to change the calculation method for finding best focus to "Trends and Parabolic".

But this is so new to me that I just don't have a good grasp on fundamentals or best practices, yet.  So I have a few questions that I'm hoping more-experienced imagers might be able to help answer.

  • Does anyone have any advice for me to have consistent success with the electronic focuser?
  • Am I using too-coarse or too-fine of step increments according to my graph?  (Increments were motor 10 steps.)
  • How many step increments do you recommend I move outside of focus when I start the routine?  (I was using 5, I believe.)
  • Should I consider a different calculation method?  (Trends and hyperbolic?  Just hyperbolic?  Just parabolic?  Just trends?)  Which tends to provide the best results?
  • Without using temperature compensation, how frequently should I be refocusing?  (I also have longer focal length scopes up to 1000mm, which I imagine are more-sensitive and require more-frequent focusing..?)
  • Do you recommend taking more than one exposure per position?
  • Anything else I should know?

Thanks!  smile.gif

 

Here's the graph from my first night of success:

 

 

My first automatically-focused test shot (with some wispy clouds...):

 

Silly me. I read the headline and assumed you just bought and installed an autofocuser lol.gif But you built your own autofocuser!! Well, i haven't done that. But I have figured out that different curve fitting (parabola vs hyperbolic) works better for different setups. My curve mostly looks like yours and for mine, the parabolic match is a much better fit than hyperbolic. So I think one of your degrees of freedom you first need to reduce is which fitting curve you need to decide on. Have you tried asking on NINA Discord. They clearly have had to deal with these issues and have beeen very helpful with my debugging....


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#15 Noobulosity

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Posted 22 July 2021 - 10:56 PM

Silly me. I read the headline and assumed you just bought and installed an autofocuser lol.gif But you built your own autofocuser!! Well, i haven't done that. But I have figured out that different curve fitting (parabola vs hyperbolic) works better for different setups. My curve mostly looks like yours and for mine, the parabolic match is a much better fit than hyperbolic. So I think one of your degrees of freedom you first need to reduce is which fitting curve you need to decide on. Have you tried asking on NINA Discord. They clearly have had to deal with these issues and have beeen very helpful with my debugging....

Good suggestion!  I haven't asked them about it, but I can certainly do so.

 

Building the autofocusers has been quite an adventure.  I've learned quite a bit about soldering, and got to put some of my CAD and 3D printing skills to use.  I still have one more to finish up, but I'm waiting on that one until after I take this little guy up into the mountains to shoot the Veil Nebula complex.  I'm excited!



#16 Noobulosity

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Posted 22 July 2021 - 11:13 PM

LOL, guess I should've read the documentation on configuring electronic focusing on the N.I.N.A. site.  It answers some of my questions:

 

Auto Focus Initial Offset Steps:  This is typically good at the default value of 4 for Star HFR. For contrast measurement, a value of 6 is preferable.

........

 

To determine the correct Auto Focus Step Size, a user could start with the focuser slightly out of focus outwards from best focus. Then move the focuser outwards by say 10 steps. If there an obvious difference to the eye, and the star diameter has increased by around 20-30% or so, this is likely a right step size. Another method is the following: starting from a good focus position (e.g. by focusing with a Bahtinov mask) the user can progressively move the focuser inward (or outward) until N.I.N.A. is not able to determine stars HFR in the image. The total increment represents the maximum initial offset. To be on the safe side the user can take 80% of the max initial offset and divide it by the Auto Focus Initial Offset Steps (default = 4). The resulting value represents a good Auto Focus Step size. Star HFR detection and Annotate Image must be turned ON during the process.

........

  • Parabolic: an error-weighted parabolic fit will be done on the focus points, and its minimum determines the point of best focus. This is most appropriate for users whose autofocus step size and offset steps keep them in the vicinity of the CFZ (critical focus zone), so that the asymptotes of the focus curve are usually not reached.
     
  • Hyperbolic: an error-weighted hyperbolic fit will be done on the focus points, and its minimum determines the point of best focus. This is appropriate for most users, for whom the focus curve will resemble a hyperbola, with clear asymptotes on each side of focus.

Based on that last info and the graph I posted, it would seem that either "Hyperbolic" or "Trends and Hyperbolic" would be the better choice.



#17 tosjduenfs

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Posted 23 July 2021 - 02:07 AM

I'm also using a MyFocuserPro2 on an AT60ED with a nema 14 stepper.  For the motor temp you'll just need to lower the stepper current from the driver. You should be able to find information on your stepper driver on how to set current based on a reference voltage you can probe with a voltmeter on the driver. 

 

I added a temp probe on mine but I no longer use it.  I focus only on a change in HFR  (typically works great except when it finds an occasional stray cloud you'll be doing a couple AF runs in a row).

 

Like others have said you may want to lower the number of focus points to speed up the AF run.  

 

Also make sure to set backlash to "Overshoot" and then add 25 or 50 steps (or whatever number of steps is enough to clear backlash) or so to either IN or OUT in order make sure the focuser moves always end by pulling the draw tube into the telescope.  This way the weight of the camera won't pull the backlash out and cause you to lose focus.

 

I put a piece of painters tape on my lens cap with the approximate focus position so that I can get it in the ball park at the start of an imaging run.

 

Check out this video for some more tips on configuring AF: https://www.youtube....h?v=lHrfaR7A0L0


Edited by tosjduenfs, 23 July 2021 - 02:13 AM.

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#18 ac4lt

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Posted 23 July 2021 - 08:29 AM

My impression was that the focuser temperature typically drives when to refocus, but I could see OTA temp as an option, as well.


It's the OTA contraction that causes the need for refocusing. But the exact location of the probe probably isn't critical. It's really the ambient air temperature you care about.
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#19 Noobulosity

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Posted 23 July 2021 - 10:18 AM

I'm also using a MyFocuserPro2 on an AT60ED with a nema 14 stepper.  For the motor temp you'll just need to lower the stepper current from the driver. You should be able to find information on your stepper driver on how to set current based on a reference voltage you can probe with a voltmeter on the driver. 

 

I added a temp probe on mine but I no longer use it.  I focus only on a change in HFR  (typically works great except when it finds an occasional stray cloud you'll be doing a couple AF runs in a row).

 

Like others have said you may want to lower the number of focus points to speed up the AF run.  

 

Also make sure to set backlash to "Overshoot" and then add 25 or 50 steps (or whatever number of steps is enough to clear backlash) or so to either IN or OUT in order make sure the focuser moves always end by pulling the draw tube into the telescope.  This way the weight of the camera won't pull the backlash out and cause you to lose focus.

 

I put a piece of painters tape on my lens cap with the approximate focus position so that I can get it in the ball park at the start of an imaging run.

 

Check out this video for some more tips on configuring AF: https://www.youtube....h?v=lHrfaR7A0L0

That's a good tip!  I'll have to add a note somewhere on the scope for getting me close to focus right off the bat.

 

I haven't touched backlash, yet.  I haven't noticed any issues, but it's something I should investigate, for sure.  There's very little backlash in this setup, but if I'm going to do this I may as well do it right.  :)  Backlash seems pretty small for this little scope.



#20 Kevin Ross

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Posted 23 July 2021 - 12:44 PM

I did notice if I don't leave the coil power enabled I get some slip as it tries to move before it's fully energized.  I'll have to see if there's some type of pre-move delay I can adjust.  The motor also got a bit hot, so I ordered some heatsinks.  I'll slap one on the end just to help cool it a little bit.  But I used PETG for the printed motor bracket, so I think it'll be able to withstand the heat of the motor okay.

I also made a DIY focuser. It consists of a NEMA 14 stepper connected directly to the fine focus knob, just like you've done. It works quite well for me. I'm not using the MyFocusPro2, but rather a stepper motor HAT for my Raspberry Pi, which keeps everything very nicely self contained. Imaging computer and focus controller in one box. smile.gif

 

You shouldn't need to keep power applied to the motor. I only have power applied to my stepper when actually doing a move. Otherwise, power is off. I have no problems with any slipping or whatever it is you're seeing.

 

You mentioned microstepping to increase focuser resolution. I don't think that is a wise idea. For one, you'd be forced to keep power applied to the stepper to hold position when on a microstep, which will heat the motor and use more battery power (if you're running on batteries). And two, stepper motors have very little holding torque when not on a full step. I use microstepping, but only for smoother, quieter movements, not for extra resolution. My focus moves always stop on a full step, and then power is turned off to the motor.

 

My first attempt at a bracket used PLA. It wasn't the motor that got too hot, it was the sun! Leaving it out during the day, either under a Telegizmos cover, or just in direct sunlight without a cover, caused the PLA to soften enough that I could easily bend it with my fingers. I reprinted it in PETG and no longer have that problem.

 

20190714_143158.jpg

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#21 unimatrix0

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Posted 23 July 2021 - 02:19 PM

I also made a DIY focuser. It consists of a NEMA 14 stepper connected directly to the fine focus knob, just like you've done. It works quite well for me. I'm not using the MyFocusPro2, but rather a stepper motor HAT for my Raspberry Pi, which keeps everything very nicely self contained. Imaging computer and focus controller in one box. smile.gif

 

You shouldn't need to keep power applied to the motor. I only have power applied to my stepper when actually doing a move. Otherwise, power is off. I have no problems with any slipping or whatever it is you're seeing.

 

You mentioned microstepping to increase focuser resolution. I don't think that is a wise idea. For one, you'd be forced to keep power applied to the stepper to hold position when on a microstep, which will heat the motor and use more battery power (if you're running on batteries). And two, stepper motors have very little holding torque when not on a full step. I use microstepping, but only for smoother, quieter movements, not for extra resolution. My focus moves always stop on a full step, and then power is turned off to the motor.

 

My first attempt at a bracket used PLA. It wasn't the motor that got too hot, it was the sun! Leaving it out during the day, either under a Telegizmos cover, or just in direct sunlight without a cover, caused the PLA to soften enough that I could easily bend it with my fingers. I reprinted it in PETG and no longer have that problem.

 

I bet some people would be ready to kill me, knowing that I just threw away about 50-100 stepper motors , similar to what people use in their DIY projects for focusers and rotators etc..
All sizes, all voltages imaginable. 

It's the company I work for, they were doing a spring cleaning in the storage. 



#22 Noobulosity

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Posted 23 July 2021 - 02:26 PM

I bet some people would be ready to kill me, knowing that I just threw away about 50-100 stepper motors , similar to what people use in their DIY projects for focusers and rotators etc..
All sizes, all voltages imaginable. 

It's the company I work for, they were doing a spring cleaning in the storage. 

Man, I would've kept all of those, tossed them in my car, and sold or gave them away.  Even kept a bunch for my own future projects.  That's crazy!


Edited by Noobulosity, 23 July 2021 - 02:29 PM.


#23 unimatrix0

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Posted 23 July 2021 - 02:37 PM

Man, I would've kept all of those, tossed them in my car, and sold or gave them away.  Even kept a bunch for my own future projects.  That's crazy!

I know, I was ready to keep some thrown away, but anyhow you ever want to find some, some older/larger laserjet printers (HP, Xerox, etc) have some in them. I you can't find these stepper motors individually , try finding an old printer, either free or sold cheap, you can harvest out a lot of parts, including screws, cables and stepper motors.  Also look up printer parts on ebay, these motors are listed there for relatively cheap like $5-$15


Edited by unimatrix0, 23 July 2021 - 02:39 PM.


#24 Kevin Ross

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Posted 23 July 2021 - 03:25 PM

Also, one more thought. I used .9 degree stepper motors, instead of the more common 1.8 degree, for extra resolution. You might want to give that a try, if you find you don't have enough resolution with your current stepper. But I suspect even 1.8 degree steppers offer enough resolution.



#25 Noobulosity

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Posted 23 July 2021 - 04:01 PM

Also, one more thought. I used .9 degree stepper motors, instead of the more common 1.8 degree, for extra resolution. You might want to give that a try, if you find you don't have enough resolution with your current stepper. But I suspect even 1.8 degree steppers offer enough resolution.

Thanks!  I actually used a NEMA14 with 0.9°/step already.  Without gearing down, I probably wouldn't get enough steps in the CFZ to focus reliably.




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