Introduction: Build a Motorized Barn Door Tracker...

Picture of Build a Motorized Barn Door Tracker...

...shoot stars, planets and other nebulae, with a camera that is.
No Arduino, no stepper motors, no gears, just a simple motor turning a threaded rod, this barn door tracker rotates your camera at the exact same rate as the rotation of our planet, a requirement for taking long exposure photos.

The concept isnt new, its been around since the 70's, back in the days of 35mm film, my version updates it to motor drive and adds a corrective cam to remove the inherent error in the original version.

Briefly, the common ways of doing this are the single hinge 2 boards with a straight threaded rod, the single hinge 2 boards with a curved threaded rod and the doubled hinged 3 boards version.
All versions can be motorised, but the 2nd version with the curved rod has the motor driving a nut through gearing and the curved rod is held stationary.

An example here of Dennis Harper's curved rod tracker.https://sites.google.com/site/distar97/
Gary Seronik's fine curved rod tracker here  http://www.garyseronik.com/?q=node/52
Finally Dave Trott who invented the double-arm tracker. http://davetrott.com/inventions/double-arm-barn-door-drive/






Step 1: Parts and Tools

Mostly hand tools were used with the exception of a mitre saw to get the ends for the hinge mount nice and square. I also used a drill press for drilling the holes for the sliding motor rails so that they are parallel to each other, as well as the hole for the drive rod to ensure it was nicely perpendicular.

Parts
  • A decent hinge with very little play, I went with a solid brass 63mm one seeing as the plank width was 69mm.
  • The main part of the tracker, 500mm pine 22m X 69mm.
  • The camera mount, approx 300mm of 22mm X 44mm meranti (a hard wood, well harder than pine anyway)
  • A brass 1/4" 20 modified machine screw for mounting the camera.
  • M8 nut and bolt for mounting the cam mount to the main body.
  • M6 rod ~ 90mm with wingnuts and washers for the tilt axis in the camera mount.
  • M6 nut and bolt 50mm long for attaching the tracker to the tripod.
  • 16 wood screws, 6 for the hinge and 10 for reinforcements in the camera mount.
  • A 70mm X 50mm section of plastic cutting board for the corrective cam.
  • A 230V AC synchronous 1 rpm motor.
  • 2 x steel rods to fit the motor mounts, 4mm in this case.
  • M6x1mm threaded rod 135mm long out of which I get a usable length of 90mm, @ 1mm pitch that translates to 90min
  • M6 coupling nut to connect the motor shaft to the drive rod with split pins to fit.
  • M6 Tee nut for the bottom board's drive rod.
  • An existing sturdy mount like a camera tripod or a diy contraption to suit, bear in mind some tripods have a plastic pan tilt head assembly and wobble a fair amount.
Something to note with the drive rod, M6 is a nice middle size, M5 would have a smaller board length of 185mm hinge to drive rod distance and possibly very flimsy, M8 would be more robust but would need a hinge to drive rod distance of 285mm which might become very bulky.


Lastly, a camera is also a requirement, preferably a DSLR with remote in order to use the "bulb" setting for long exposures. On my Nikon D70S I use an infrared remote because the camera wont allow bulb setting with the timer, it just overrides with 1/5 sec exposure.
That said, it might be theoretically possible to use a Canon PowerShot (point n shoot range) and load it with the CHDK software to utilise the intervalometer scripts.

Step 2: Some Calculations

Picture of Some Calculations
A mean sidereal day is 23 hours 56 minutes 4.0916 seconds (23.9344696 hours), this is the rate at which the stars appear to revolve around our planet termed diurnal motion and is the rate of travel required in the barn door mechanism.
So, 360°/23.9344696 = 15.041068635170423830908707498578° per hr = 0.25068447725284039718181179164296° per min to match the diurnal rate.

The M6 drive rod has a pitch rate of 1mm in 1 min, so we need to calculate the length needed to achieve that diurnal rate, ie 0.25068447725284039718181179164296° per min.
1/(tan 0.25068447725284039718181179164296°)=228.55589mm

Nice to know:
  • M8 x 1.25 rod would need a rod to hinge distance of 285.69486mm
  • M5 x 0.8 rod would need a rod to hinge distance of 182.8447mm




Step 3: Construction Begins

Picture of Construction Begins
First cut the 500mm length in half and mount the hinge.
Make sure everything is square and moves freely, clap the 2 hinged boards together and shout "action" a few times like they do when making movies, if it makes a nice clack sound it should work well for a star tracker.
  • Now measure 228.55mm from the center of the hinge pin down the middle of the board and mark the drive rod holes, do this on both boards.
  • Only drill the hole in the bottom stationary board and hammer in the M6 Tee nut.
  • On the top board make the 228.55mm mark which will be needed to line up the plastic correction cam.
  • Fit the motor shaft in the drive rod hole and mark the positions for the 2 sliding mounts. These need to be parallel to each other as well as perpendicular to the board to prevent the motor from binding. These were a tight fit in 4mm holes and I forced a M4 nut on top of each one to stop them from dropping out the bottom.
  • At this point I made the pan/tilt hardwood attachment for the camera, also known as AltAz in astro circles.(altitude/ azimuth)

Step 4: The Motor

Picture of The Motor

The motor used is a 230v ac synchronous 1 rpm which is very accurate as it relies on the 50 Hz frequency of the main ac supply.
Using a suitable 12v battery with a small inverter, 100w coke can shaped inverters are more than adequate, allows the whole mechanism a degree of mobility for outdoors use too.
The motor was connected to the drive rod with a M6 coupling nut which had one side drilled out to take the 7mm dia motor shaft, seeing as I will be using this in a clockwise rotation, I pinned the threaded part of the drive rod as well to stop the shaft from unscrewing.
Once power is switched on you need to check which way the motor is turning because it can do clockwise or anti-clockwise too. In use it slides freely down the 2 rails which flex a bit, but with no rotational slack. Where the top of the drive rod will ride on the cam was rounded over sanded smooth and polished.

Step 5: The Case of the Growing Hypotenuse and the Corrective Cam

Picture of The Case of the Growing Hypotenuse and the Corrective Cam

Due to the fact that the boards are moving apart with the drive rod in a 90° fixed position, it is a given that the top board acting as the hypotenuse in this triangle configuration must get longer with time, which causes the boards to open slower as time progresses and is the source of the inherent error in this device.
The last 2 pics of the top board riding on the drive rod illustrate this well.

One of the easiest corrective solutions was discovered by Frederic Michaud and he goes into a nice writeup here. http://www.astrosurf.com/fred76/planche-tan-corrigee-en.html

He proposes a cam which is the involute of a circle, the radius of the hinge pin to drive rod distance of the tracker, and provides  a printable jpg here. http://www.astrosurf.com/fred76/images/tan-sans-derive.zip
He also stresses that the jpg must be printed to the scale according to the drive rod thread pitch which in my case was 65mm square.

There are individuals that have reduced the error by using pivoting points where the drive rod meets the boards to form an isosceles triangle, however these solutions involve a lot more engineering.

My cam was made out of a small section of plastic cutting board, roughly cut to size with a hacksaw and then finished to the curve with a small drum sander bit that would fit in a dremmel tool. I used my drill press to ensure that the curve would be perpendicular to the sides so that the drive rod would track true and not slip off the side.

Step 6: Using and Setting Up

Picture of Using and Setting Up

Here in the southern hemisphere finding the southern polar star is a small mission in itself, maybe better luck once my spotting scope arrives, so my work around uses a protractor and a compass.
The compass indicates true south once I've added in the magnetic declination for my location, and taking my latitude (33°52"), converting to degrees (33.867°) gives me the tilt or altitude I need to aim the trackers hinge to. This I printed out using 2D cad and added a nut and thread for a diy inclinometer to hold against the hinge pin.

In use I set the boards open at the maximum angle, I then sight along the hinge pin to the south and tilt it up at the required angle for my latitude, the hinge will be on my left in the east with the motor on the right hand side to the west. Then when switching on the motor I make sure its running in a clockwise direction with the boards closing. Once the device is fully closed, I switch off the power and remove the splitpin from the shaft and twirl the drive rod back up by hand.

In the Orions Belt closeup,(1st pic) a F11 shot @ 100 sec @ iso 200 would have been enough to show some elongation of the star if not a definite trail, the tracker was aligned with a compass and protractor so quite happy even though I havent found the southern polar star yet.

Two examples of tracking on and off over a 5 min exposure.

 Last pic of Orions belt is from my Canon PowerSHot A480 using CHDK, 161secs @ iso 200 F4 which the camera saved as a *.DNG raw file fortunately, I was then able to process it in Adobe and saved the result as a jpg.

Comments

LDyson (author)2017-11-13

Made this over the weekend and had a beautifully clear night straight away. First attempt yielded this photo of Orion's Nebula.

Thanks for this great guide!

petercd (author)LDyson2017-11-15

Most welcome, looks good.

jackcohn (author)2017-07-01

Thank you! I have completed this project but am finding that a lot of vibration is coming from the motor up through the guide rods and the threaded rod into the timber and onto the camera. Did you experience this issue? Do you have any suggestions as to how to best combat this ? Thanks in advance for your advice!

petercd (author)jackcohn2017-07-03

Not at all, as can be seen from my sample pics at the end.

Grease
all metal on metal parts in case its friction related and ensure the
tripod mounts are solid in case they are amplifying small movements.

Next would be to remove any burrs on the drive rod, slap some metal polish on it and run it through the T-nut a couple of times using an electric drill, that should do the trick.

Only other thing to check would be smooth rotation of the motor in an unloaded scenario.

DIYdweeb (author)2016-09-09

I have built a few trackers over the last few years, using a slightly different design and a curved rod for greater tracking accuracy. I like this article, the camera mount in particular is very ingenious. You can get a lot of parts for a tracker from this guy - google bencooper.karoo barn door tracker. He uses the design first published by Gary Seronik.

Far_Echo (author)2016-08-31

I think I love you... :p

vonheist (author)2016-06-29

I really love this instructable! I've almost finished mine, will post pics when it's ready. The links to the cam are not working at the moment. Is there anyone who can help me out with the correction cam? Thanks!

petercd (author)vonheist2016-06-29

Thanks for the heads up, those links are now dead. Google didnt help, other than to find a site that had copied my ible and those links didnt work either.

Here's the jpg which I saved to an external h/d, fortunately. :)

DominickV2 (author)petercd2016-08-16

I'm glad that I found this image but the translating it is a bit off....I've figured out by the rest of the comments I'm supposed to print this so the arrows are 65mm long, but I can't figure out the placement exactly.....can anyone help?

petercd (author)DominickV22016-08-16

Placement of what?

The subsequent plastic cutout is placed at the initial contact point of the threaded rod.

DominickV2 (author)petercd2016-08-16

Yep that was it...thanks...I kind of figured but just wanted some confirmation

vonheist (author)petercd2016-06-30

You are my hero! Thanks!

jackcohn (author)2015-10-29

Thanks so much for these instructions! I have nearly collected all the pieces needed but after testing the 1 rpm motor it seems to be taking about 65 sec to complete the rotation instead of 60sec. Will this affect the outcome too much? Or should I go down the arduino path (on which I have never trod) and try to change that?

Your advice is greatly appreciated.

petercd (author)jackcohn2015-10-30

It will be problematic because the calcs assume 1 rotation and not 1.05 etc.
You will need new calculations regarding thread size and board length.
Try a speed check with it running off an inverter, if its better then that would be a convenient way to go.

mrastrom (author)2015-08-17

Hi,

I am almost done making this, but need to know the exact position where I need to place the camera mount, in this case I am using a 360deg Ball head instead of a ALT-AZ mount.

Thanks

petercd (author)mrastrom2015-08-20

There is no "exact" position, the entire length travels at the same time/speed.

Closer to the hinge but keeping the center of gravity above the tripod mount is a good pratical solution.

DavideTeci (author)2015-08-11

if I had to use another engine with 12v 3rpm I should change something about your project right?

petercd (author)DavideTeci2015-08-11

Yep, you would need to redo all the claculations involving rpm and threads per inch etc.
http://davetrott.com/inventions/double-arm-barn-door-drive/

mrhenryhenryhenry (author)2015-05-09

Hi,

Thank you for sharing the design of the star tracker.

Could you please tell me where you got the M6 tee nut and the M6 coupling nut?

Thanks!

Hardware store http://www.brightsonline.co.za/

mrastrom (author)2015-02-17

"He also stresses that the jpg must be printed to the scale according to
the drive rod thread pitch which in my case was 65mm square."

I am little confused about the corrective cam print size. I am using 1mm pitch thread rod, could you please clarify...

thanks in advance.

petercd (author)mrastrom2015-02-17

If you open up that jpg and just plain print it you might get a scale error, (you might have print to fit page as default) so he has included 2 red arrows, vertical and horizontal so that you can measure the final printout to make sure it is 65mm both vertically and horizontally, ie 65mm square.

Makes more sense if you've looked at the pic.

mrastrom (author)petercd2015-02-17

I tried Irfan View like you mentioned in another commet, that seems to have worked... thanks again.

mrastrom (author)2015-01-01

I am trying to source the motor for this setup and found this

http://www.audiomate.co.uk/proddetail.php?prod=Bla...

Could you please tell me if this motor is what you have used?

petercd (author)mrastrom2015-01-01

That's the one yes, same front pic too.
http://www.fort777.co.za/index.php?main_page=product_info&cPath=269_270_423_1093&products_id=6939

Sometimes when I switch it on it goes CCW and I just flick the power to make it go clockwise again.

mrastrom (author)petercd2015-01-01

Could you please tell me how much the motor weighs?

petercd (author)mrastrom2015-01-02

116 grams

mrastrom (author)petercd2015-01-02

Thanks a ton!

Regards

mrastrom (author)petercd2015-01-01

awesome, thank you Sir for a quick response..... and for sharing this in such detail...

wi11ywonka (author)2014-01-29

Hello,

This is so awesomely simple. I'm a noob in Astronomy, DoItYourSelfery and even this forum, but Im going to give it a go! THANKS for this! However, I'd like to or have to use US Standard nuts, bolts and rods. So which calculations must I use or attributes about the rod must I know? The part under calculations:

Nice to know:


  • M8 x 1.25 ...
  • M5 x 0.8...

the 1.25 and the .8, what are those numbers? It seems if I knew those numbers of an M5 equivilant in US Standard I can find everything else.

Thanks again!

Will

petercd (author)wi11ywonka2014-01-30

The M8 is 8mm dia and the 1.25 is the thread pitch, ie 1.25 mm per full thread
Alas you cant convert because then the other dependables wont match up.
I've seen imperial thread of 1/4" 20 on some of the other builds.
this site is using imperial threads in the build http://www.philharrington.net/scotch.htm

Ironwave (author)2013-12-21

Hi.
Thank you for the answer.
I got quite a bit done.
Few questions remain if you please.
https://www.instructables.com/files/deriv/FD1/9XIY/HPFZR1CA/FD19XIYHPFZR1CA.THUMB.jpg
A green- Is this length important, in example, must the camera sensor be directly over the hinge?
B yellow- I think this relates to question 1, my fault.
C blue- The length of upper part?
D purple- Where on the arm do you put the alt-azi upper part? Near end, directly over the part where you will mount it on the tripod I guess?
E white- Length of the lower part.
F black-How do you know how many degrees to turn this?

I think F is maybe the only really important thing, the rest don't really matter?

petercd (author)Ironwave2013-12-21

A & B not important, i just located them in the middle so I could easily access the thumbscrew underneath.
C can be as short as practically convenient.
D can be anywhere bearing in mind the closer you put it to the screw rod the more weight will be at that point=more strain on the motor, but the closer you have it to the hinge the more unstable it becomes in wanting to open the hinged boards through over balance, so somewhere near the middle is fine.
E must = or > the length of the lower hinged board, ( I made mine 228mm plus 20mm extra for mounting the motor, so like 250mm total ) but having a longer top board is just a waste of wood and adds unnecessary weight.
F, (basically the altaz mount) is just used to rotate to a part of the sky you're interested in.
The entire contraption is mounted on a tripod and that tripod mounting is used to aim at the south pole and once set south you dont touch it again, hence the need for the altaz to mount the camera so you can rotate to different parts of the sky.

Ironwave (author)petercd2013-12-21

Hey!
Thank you very much for the answers.
I was afraid it might be too complicate to describe.

Ill post pics of the results if I can get the gismo going.
Still waiting for the motor. It may take some time this time of year.
For interest sake, where did you find it?

petercd (author)Ironwave2013-12-21

You're welcome, I got mine at Yebo Electronics Boston Branch, R115. It can go either way CW or CCW on startup so you just need to check which way its going when you switch on, if its the wrong way then you just flick the power switch till it goes your required direction. Yebo electronics link http://www.fort777.co.za/index.php?main_page=product_info&cPath=269_270_423_1093&products_id=6939&zenid=u2puk32cu2lu0j5qqch6vf0ete3l13cb

Ironwave (author)2013-12-18

Hi
This is awesome.
Im building it as of today. Already made a few amateurish shots on my canon sx 40 mod with chdk. were ble to get Jupiter and its Galilean moons with it alone.
Also from South Africa, so this is too good.
Anyhoo.
Can you provide some info on the cam?
I don't exactly understand how big to print the cam. Im using same parts and specs as you. The 65mm square' don't help me much, since I have no idea what to do with it.
Any help will be greatly appreciated.
Thank you.

petercd (author)Ironwave2013-12-19

hiya, when you print it out, the pic must be 65mm x 65 mm on paper, the red lines on the rhs and bottom edge with arrows give the place to measure the 65mm. I use Irfan view and insert 6.5 with aspect ratio in "custom check block" in print settings. I almost read someones mind once with a Panasonic FZ10 (12x) and I thought that was serious zoom, but a Canon sx40 with 35X zoom... you could probably see what brand of cigarettes they smoke on Jupiter. :)

cfs0527 (author)2013-12-07

Thanks! I have been wanting to make one of these gizmos since I first saw one in S&T. It's about time I got my butt in gear. I like the simple camera mount yours has. Thanks for the inspiration. I know my old Pentax 35mm SLR will work but I do have a point and shoot digital. . Can you explain this CHDK thing? Also, how long are your exposires? I figure a 10 min shot would open the boards only 2.5 degrees but your photos show them much wider than that.

petercd (author)cfs05272013-12-08

You're correct, a 10min shot would only open the boards 10mm = 2,5 degrees, my pics arent actual working pics but merely glamour shots for the 'ible.
Due to my light polluted location I can only get about 2 min shots before the orange haze sets in.
CHDK is for Canon PowerShot series cameras only and because its software you dont need a remote unlike DSLR's where you have to use either a cable or an IR remote.
Basically you load the software onto a SD card and then run a script within that enviroment which manually overrides the shutter timer. Its fully reversable, when you switch off the cam all settings reset back to factory. There somewhat of a learning curve with lots of reading, site here..http://chdk.wikia.com/wiki/CHDK
Theres a list of supported cameras on the rhs of that website.

mfarooqi (author)2013-12-05

WOw.. really aowsome.. i have been just thinking ... who to track starts for loooonog exposures.. i tried at my end.. always either star trails are too long.. or environment becomes too bright..

I'll do this practice in my earliest free time :) thanks

audreyobscura (author)2013-12-02

Woah! I never knew about this. So cool! Great work, you should post some more exposures!

petercd (author)audreyobscura2013-12-02

Thanks, have to delete a lot cos Im still learning the tricks.
Naturally, building one of these devices will ensure cloud cover for some time. :)

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