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Barn door tracker problems

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Posted (edited)

Hi,

As a newbie in AP I recently discovered that astro imaging isn't as simple as I imagined, it's not just "pointing and shooting".

The main point is that I need a tracker that will follow the sky's movement. But after seeing the prices at which decent trackers are sold I decided to see if I could build one myself from spare parts at my workshop. Turns out that I can, using two planks of wood a hinge, an arduino, a stepper motor, etc. So I decided to get to it, I have built the connection between both planks of wood with the hinge, but have now discovered a problem. After some investigation, I realized it's quite commonly named the "Tangent Problem" (the rod forms a 90º angle with the bottom plank). So the solutions offered vary from curved rods to involute pieces of plastic and tilting the rod.

I also saw another solution that was adjusting the speed at which the rod moves, and since I'm going to be using an arduino-driven stepper motor, I decided that this was the way to go. The problem is that I don't know in what ratio the speed has to increase over time to compensate the "Tangent problem".

That's why I decided to pose my question here as I'm sure someone must have done it already.

Sorry for my bad english, it isn't my first langnuage.

Thanks in advance and clear skies,

S

Edited by feverdreamer1

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Hi

The barn door I built is the simpler type 1 design like the one you have built with two boards. I've got around tagent error I think because my straight rod is topped with a captive nut so the top board is free to slide over the captive nut.

You didn't mention it but the distance from the hidged point to the centre of the drive bolt is calculated based on the thread of the chosen bolt used.

What issues do you have in practice with your barn door build?

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Hi,

Wait so with a captive nut the problem vanishes? That's really cool. Good idea.

And also I'm using an M6 threaded rod with a 1mm/min pitch rate so I figured the distance to be about 228mm. How did you calculate the length needed? Maybe I did a mistake.

I guess the main problem I'm having is the way the top plank moves, as i said, i was planning on using a stepper motor to make things more 'precise' but I'm not sure of the step speed I must set it on,I'm guessing 1rpm but not really sure, if you could give some insight on this I would really appreciate it. Also thanks for the fast answer.

Clear skies,

S

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I used an online calculator for the distance from hinge to bolt. I'll add it later as it's in my barn door build thread.

If the distance is right it's 1rpm but if aiming to program out tangent error then it would vary. I've got a link too on the thread I did on my build to arduino code for that.

The bolt I used was m10.

The reason I don't think I suffer particularly with errors with the mount I built is because even when towards the end of the bolt still get the same reliable images at 3 minutes exposure using a 40mm lens.

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Posted (edited)

Plans etc. are available. You don't have to employ fancy shapes for the clapper board sections rectangular pieces of plywood are fine.

I used the design some years ago to make a purely manual model and it worked very well and served as an excellent introduction to EQ mounts and polar alignment.

Enjoy making the build. That's me in the hammock in the background.

Cheers,
Steve

Edited by SteveNickolls

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7 minutes ago, SteveNickolls said:

Plans etc. are available. You don't have to employ fancy shapes for the clapper board sections rectangular pieces of plywood are fine.

I used the design some years ago to make a purely manual model and it worked very well and served as an excellent introduction to EQ mounts and polar alignment.

Enjoy making the build. That's me in the hammock in the background.

Cheers,
Steve

Hey Steve,

That's a great mount and it looks cool too. The only problem is bending the rod. Like I found the radius I would need but the problem is bending the rod to that radius. I dont have the equipment necessary and I guess that if I do it with a hammer the threaded part of it would just mess it up.

But I can get some ideas from your mount

Keep chilling and clear skies

S

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Here's the online calculator

https://blarg.co.uk/astronomy/barn-dobarn-door-tracker-calculator

Another built with Arduino code I mentioned earlier

For something different I built a table top mounted barn door with it's own wedge. Link here

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20 minutes ago, happy-kat said:

I used an online calculator for the distance from hinge to bolt. I'll add it later as it's in my barn door build thread.

If the distance is right it's 1rpm but if aiming to program out tangent error then it would vary. I've got a link too on the thread I did on my build to arduino code for that.

The bolt I used was m10.

The reason I don't think I suffer particularly with errors with the mount I built is because even when towards the end of the bolt still get the same reliable images at 3 minutes exposure using a 40mm.

Oh is there? Then I'll actually have a look at your profile and find it. I guess ill figure out the varying rpm by trial and error hehe. If I actually manage to do anything I will post it here, if not, I'll just try to do imaging before the tangent problem really arises

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4 minutes ago, happy-kat said:

Here's the online calculator

https://blarg.co.uk/astronomy/barn-dobarn-door-tracker-calculator

Another built with Arduino code I mentioned earlier

For something different I built a table top mounted barn door with it's own wedge. Link here

The links you provide are really helpful. I will definitely take some ideas and notes from them. I will post my results here too.

Clear skies,

S

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Posted (edited)
28 minutes ago, feverdreamer1 said:

The only problem is bending the rod. Like I found the radius I would need but the problem is bending the rod to that radius.

Hi, If you can transfer the radius in the plans onto a piece of scrap wood then cut out the shape with a fine saw. You can then sandwich the rod betwen the two pieces of cut wood and then hammer the top piece of wood to slowly impart a bend into the rod.. If you have a vice so much the easier. Slowly begin to bend the rod a section at a time over the form until you have a curved rod. You don't have to have a very long curved rod, each mm of the rod = a minute so you can afford to damage an end of the rod in bending, hacksaw it off and use the best curved piece of say 100mm length. Hope that helps.

Cheers,
Steve

Edited by SteveNickolls
clarity

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19 minutes ago, SteveNickolls said:

Hi, If you can transfer the radius in the plans onto a piece of scrap wood then cut out the shape with a fine saw. You can then sandwich the rod betwen the two pieces of cut wood and then hammer the top piece of wood to slowly impart a bend into the rod.. If you have a vice so much the easier. Slowly begin to bend the rod a section at a time over the form until you have a curved rod. You don't have to have a very long curved rod, each mm of the rod = a minute so you can afford to damage an end of the rod in bending, hacksaw it off and use the best curved piece of say 100mm length. Hope that helps.

Cheers,
Steve

That's actually a great idea Steve. I think that I will try making a normal barn door tracker (as in without a bent rod) and get to know it properly before I take on the bent one.

Thank you very much,

S

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Posted (edited)

I would also like to ask anyone who has a diy mount if they're actually reliable when using long lenses (about 150mm) and how much exposure time can you get without showing star trails

Edited by feverdreamer1

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I have never used a manually operated barn tracker with long FL lenses my reasoning being that if you had a 150 mm lens on a static tripod you would be looking at very short exposures to avoid star trails, 500/150 seconds (just over 3 seconds) with a full frame or  2 seconds with an APS-C sensor. So with your barn door tracker you would be needing to hand move the device slightly every 2-3 seconds with a 150 mm lens, very tedious very fast and you need to make the adjustments regularly. You can see in one of the images a battery clock to help time when the disc needed to be moved. I tended to use short FL lenses and move the disk every 15 seconds a quarter turn. Obviously a motor controlled tracker would perform the movement continually hence its attraction. So for a hand cranked device best stick to shorter FL lenses for now but a motor driven device would work with longer FL lenses. Oh, also consider the need for better polar alignment when using longer FL lenses.

Cheers,
Steve

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29 minutes ago, SteveNickolls said:

I have never used a manually operated barn tracker with long FL lenses my reasoning being that if you had a 150 mm lens on a static tripod you would be looking at very short exposures to avoid star trails, 500/150 seconds (just over 3 seconds) with a full frame or  2 seconds with an APS-C sensor. So with your barn door tracker you would be needing to hand move the device slightly every 2-3 seconds with a 150 mm lens, very tedious very fast and you need to make the adjustments regularly. You can see in one of the images a battery clock to help time when the disc needed to be moved. I tended to use short FL lenses and move the disk every 15 seconds a quarter turn. Obviously a motor controlled tracker would perform the movement continually hence its attraction. So for a hand cranked device best stick to shorter FL lenses for now but a motor driven device would work with longer FL lenses. Oh, also consider the need for better polar alignment when using longer FL lenses.

Cheers,
Steve

Actually I'm building an arduino driven mount, so the rod will move thanks to a stepper motor with a speed of 1rpm (please correct me if I'm wrong). I saw you made a motor driven mount, so I wanted to know how much maximum exposure (like without star trails) did you get with the one you made (the one you posted the link of).

Also what focal length would I need to get crisp images of nebulae and galaxies with this barn door tracker.

Clear skies,

S

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85mm lens is long enough for m42 for example. Many DSO are quite large, even 50mm lens will pick some up and a modified camera could show more.

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2 hours ago, feverdreamer1 said:

I saw you made a motor driven mount, so I wanted to know how much maximum exposure (like without star trails) did you get with the one you made (the one you posted the link of).

Hi, I followed the construction plans but only made a manually operated tracker no motor was ever added.  I'd have to look back in my astronomy logs as to the exposure times I managed but that would be purely manually operated.

As happy-kat rightly says 50-85 mm lenses will show detail of nebulae though using an astro-modified camera, having a dark location and lots of sub exposures all help in the final image.

Cheers,
Steve

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2 hours ago, SteveNickolls said:

Hi, I followed the construction plans but only made a manually operated tracker no motor was ever added.  I'd have to look back in my astronomy logs as to the exposure times I managed but that would be purely manually operated.

As happy-kat rightly says 50-85 mm lenses will show detail of nebulae though using an astro-modified camera, having a dark location and lots of sub exposures all help in the final image.

Cheers,
Steve

Oh sorry, I think I confused happy-kat's design with yours. My bad

But thank you both for answering so quickly, I thought it would take days till I got an answer but the SGL community is really cool. I see myself asking a lot of questions in this forum as I am green when it comes to AP.

Anyway, thank you very much,

Clear skies,

S

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I get away without a bent rod because my top board is not rigidly fixed to the bottom board by the rod. When we are back in late autumn I might try a test at the end of my rod wind and take a 3 minute exposure to see if the theoretical is proven or not as I don't remember winding my board back unless I reached the end but I didn't make a point of recording exactly when I rewound it. I should add that my mount is very stable and the boards quite heavy which probably all helps.

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Hi,

I made the build (as in the planks with the hinge) for the moment, as I dont have much time. But the other day I was thinking that I may have an issue using the arduino stepper motor.

Is the torque on this motor enough to lift the plank and the DSLR (a Canon550D) as well as the alt az mount for the cam?

Has anybody made a barn door tracker with the normal arduino stepper motor (28BYJ-48)? And if so, any suggestions? Is it necessary to use some gear reduction to "improve" torque?

S

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