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Double-Arm Barn Door Tracker

astrophotography DIY equipment
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#1 SkilledSpark

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Posted 12 July 2020 - 06:47 AM

Hello!

 

I am very new to the forums community. I have decided to post this here since I wasn't really able to get enough info on the topic.

I have been doing Astrophotography for a while now, and have actually been able to produce some really good images only with a DSLR and tripod.

But I thought it's time I get a tracker, and since most if not all commercially available ones are out of my pocket, I have seen that Barn door trackers seem to be a really good alternative to that.

 

I have been studying them for a couple of days now, with there being the single arm version and the double arm barn door tracker. I have so far been able to gather a stepper motor and an arduino since that seemed to be a common electronics setup and some gears and threaded rod.

Seeing that there are a lot of different schematics available for the single arm barn door tracker, I was going to stick to that until I realised that they don't exactly give a long duration (without errors) at higher focal lengths (200mm+).

Bended threaded rod one was my next go to, but then I stumbled upon the benefits of the double arm.

 

So, I decided to look at the double arm version, and it seems that Dave Trott's type 4 is the way to go. But I feel the plans for building it aren't really out there.

I am seeing, that the most commonly mentioned error is the tangent error, that's eliminated with the double arm version.

 

My first question, is it possible to eliminate the tangent error on a single arm, with changing the speed of the stepper motor periodically with the help of the arduino (in accordance to the calculations), if so, would it work even better than a double arm that way? [Double arm already seems to be really good]

If not, any help with the dimensions for building the double arm type 4 would be appreciated.

Any other suggestions regarding my next step are appreciated too!

 

[I am sorry if I am unaware of any of the facts, I am a noob and much of this seems new to me]

 

Thanks!



#2 *skyguy*

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Posted 12 July 2020 - 07:09 AM

Welcome to the CloudyNights Forums .....

 

Have you read this topic on CN .... "Analysis of Type 4 barn door mount."

 

https://www.cloudyni...arn-door-mount/

 

It seems the double arm tracker needs to be constructed to very tight tolerences to achieve a superior tracking accuracy beyond the standard single arm tracker. This might be beyond the wood working abilities of most builders.

 

Good Luck with your project ...


Edited by *skyguy*, 12 July 2020 - 07:11 AM.


#3 SkilledSpark

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Posted 12 July 2020 - 07:40 AM

Thanks for the reply skyguy!

 

I will definitely read the topic that you have shared...

Yes, it seems that there should be almost no errors during construction in double-arm tracker, hence I wanted to know if having variable speeds on the motor in the single-arm would benefit the tracking at higher focal lengths and how?

 

Regards



#4 Oberon

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Posted 12 July 2020 - 08:06 AM

Yes, in principle variable speed will enable a single arm tracker to track as well as any single axis tracking system. You'll just need to do the math. In principle it sounds simple enough for an Arduino, sorry I've not done so in practice so can't offer much help beyond encouragement that you're on the right track.



#5 SkilledSpark

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Posted 12 July 2020 - 08:25 AM

Thanks for the reply Oberon!

 

I have been able to programme it to be able to change the speed on the go using the lcd shield. If you say that it's possible, then I will definitely go deeper into the math involved in it and automate it to do so. I just wasn't sure whether it would have worked or not.

 

Thanks and regards



#6 *skyguy*

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Posted 12 July 2020 - 09:42 AM

It's certainly possible to adjust the motor speed to compensate for the tangent error in a straight rod tracker. However, I think using a curved rod to avoid the tangent error is a more elegant and simpler solution. Here's an interesting site that describes a curved threaded rod tracker using an arduino programed to control a stepper motor. The source code is also included in the article.

 

https://kukuruku.co/...-in-two-nights/



#7 cuzimthedad

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Posted 12 July 2020 - 10:04 AM

Welcome to Cloudy Nights SkilledSpark!



#8 SkilledSpark

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Posted 12 July 2020 - 10:28 AM

Thanks for the suggestion skyguy!

Nice to meet you all, feel like this is a very well experienced community here.

 

I will be going with the curved rod tracker in that case smile.gif

 

How long exposures do you think I will be able get without any major errors at let's say 50mm and 200mm focal lengths?

 

Regards


Edited by SkilledSpark, 12 July 2020 - 10:30 AM.


#9 Ed Jones

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Posted 12 July 2020 - 04:42 PM

Although I've always made curved bolt trackers I wonder if it would be simpler yet to use a straight threaded rod and put an auto tracker on it to take care of the tangent error.  idea.gif



#10 msheald

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Posted 12 July 2020 - 05:25 PM

Hello! A single arm tracker with variable speed stepper controlled with an Arduino is the way to go.

 

    Making a curved rod sounds simple. However, I was never entirely happy with mine. It will unlikely be a true circle no matter how careful you are in bending the rod. So, in principle, you would have to adjust the speed (or PEC train the rod) anyway if the goal is to perform telephoto shots of any significant exposure.

 

    A single arm is mechanically easier to build (and sturdier, in my opinion). Rather than calculating the equation for speed adjustment, I've seen folks create a table for the speed adjustment. With the latter, you can get the data in the field - essentially PEC train the threaded rod in addition to getting the speed adjustment. The initial speed values can be obtained from trigonometry, if you like (or do it all in the field), and adjust it with real world data. Running it several times will fine tune it. Best regards.

 

Mike



#11 SkilledSpark

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Posted 13 July 2020 - 12:21 AM

Thanks for all the help everyone!

 

I will try building the single arm soon and post my progress once I am done building it.

 

Thanks once again!
Regards



#12 Bill Kocken

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Posted 13 July 2020 - 07:42 PM

I built a single arm tracker using an Arduino to drive a stroller. The Arduino programmed to correct for the tangent error. I followed the plans found here. https://fstop138.ber...-motor-control/

The Arduino code is kind of complex and I had trouble finding the code libraries that he installed. The tracker works well for short exposures. I haven't tested it enough to know if the tracking remains good for long exposures. I suspect lots of the precision comes from the ability to build the mechanical pretty well.




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