Accurate Barn Door Tracker - finished project

[Legion Of Andromeda]

Here she is..... Attention! the images do not correspond directly to the paragraphs, i just put them in to break up the text.

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Essentially, it's 2 x birch ply slabs, a control box, hinges, counterweight, mounting plate, tripod ball head, red dot finder, stepper motor, curved threaded brass driver rod

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The door and base were going to be in solid oak as i wanted high mass to supress any motor vibrations, but went with 18mm birch ply as it was far easier to machine. I chose stainless steel ball bearing hinges for their smoothness and zero play tolerance, placed as far apart as poss but made sure they were dead inline with each other. i also recessed the hinges into the birch to ensure the inside faces lined up with the hinge pivot point. I then drilled a pilot hole through both boards that was 183mm from the hinge axis and on a centre line for the curved brass rod to pass through. ..The maths is as follows; based on M5 bar giving 0.8mm travel per thread / rotation / 1 minute... therefore 60 mins is 15 degrees of earth rotation and gives 48mm of travel, (15 x 24 = 360 degrees) so; 48mm x 24 = 1152mm circumference / Pi = 366mm diameter = 183mm radius....or if you have no control over the rpms, adjust the values accordingly; faster = larger radius, slower=smaller

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It made perfect sense to go with a curved threaded brass rod to ensure a constant rate of rotation and no tangent error. M5 bar gave me sufficient strength but still able to be bent by hand around circular objects of decreasing in size until i got it to retain its diameter when laid over a template. Please note that one should only use the best 30cm from the middle of a 50cm rod. i secured one end to the door using SS wing nuts and washers, the hole needs to be more than 5mm as the rod not only curves through but is angled back so that the working part of the rod starts at a tangent to the inside face of the door and ends up passing through the corresponding hole in the base without to much resistance

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I used an existing small Skywatcher dovetail mounting plate which thankfully has a 1/4"-20 threaded hole in the centre to screw onto the top of my existing camera mount, the ballhead is removed and fixed to the door for the camera of course. The mounting plate was offset so the curved rod didn't clash with the tripod pole. The position of the ball head needs to be near the hinge to lessen the weight on the threaded drive nut/rod, but not so close that weight of camera goes past the centre of balance and then pulls on the drive, or worse, the whole assembly flips over and does some serious damage if it goes past that point.

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I partially recessed a donated 19mm bearing in the baseboard centred over the drive gear hole. i used a 19mm flat bit for the recess and opened up the hole on the underside to 10mm so that the centre of the bearing could rotate freely and allow the rod to pass without any hindrance. i temporarily added one of the wing nuts to the bottom end of the rod to keep it from flapping around too much whilst i did other things

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The gears are at a 1 - 4 ratio. The motor spindle turns a 16 tooth spur gear 4 times for one rotation of the 64 tooth spur gear. The general sizing is in good proportion to the overall kit. In this case i used module1 plastic gears from RS components; 64 teeth with a pitch diameter of 64mm / 8mm bore in conjunction with a 16T/16dia/ 5mm bore... I also bought a spare set of 72 /18 as a back up. I used an M5 insert or 'T' nut fixed to the big gear- not inside the bore as one would think, but inverted so that the threaded shaft of the 'T' nut sits snugly down into the centre of the bearing. I removed the spikes with an angle grinder so that the flange was flat and smooth which i then superglued to the underside of the big gear...getting it central was quite tricky though. I then installed the big gear onto the curved rod most of the way up and it spun pretty well on the bearing and then made finer tweaks to the rod so it stopped touching the bearing but since the gear was very close to the baseboard , i added a small spacing washer which not only lifted it up but got the gear level with the baseboard - which is very important as you want both of the gears to be in line and level with each other. I was tempted to place two insert nuts back-to-back but they would form a straight bore over a curved rod, plus there was a high chance of the two threads binding.

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I then installed the NEMA 17 stepper motor. First I transposed the motor dimensions onto the baseboard, I drilled a 25mm clearance hole for the 16T gear, fitted the 16T gear onto the 'D' profile driveshaft but i had to install a small grub screw into the gear hub to hold it in place. i then drilled 4x 4mm countersunk holes for the 25mm machine screws to hold it in place. Its best to slightly oversize these holes if you need to twist it around so the gears mesh nicely. also, if the motor is too low then you can easily raise it up by removing a layer or two of ply from underneath - rather than shifting the gear to the end of the shaft.

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Electrics; I fitted an existing decent sized project box to the underside of the baseboard at the hinge end, this contained the speed controller and the stepper motor driver. The input is 12v dc, the 4 motor wires come out of one corner to the motor, i also have a broken + wire from the power socket that goes to a long-armed microswitch, the power is automatically cut when the drive rod gets to certain point for, i hope, obvious reasons. I used an old padlock key clamped between two wing nuts to hit the kill-switch... I'm not going to get into the stepper motor stuff here as it's pretty complex and there are a ton of fantastic tutorials on youtube.

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I added a steel counterweight to the other side of the motor...it was needed!.... The ball-head camera mount is attached to a slotted steel plate (butchered from a radiator bracket) with a corresponding 1/4"-20T thumbscrew underneath that sits over a 32mm dia hole in the door, circa 75mm up from the hinge axis

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It's very important that the axis of the red dot finder is parallel to the hinge axis. My method was to make a drawing of the section thru the kit that had both axes set out, i then stuck this to a wall 10m away, i used a small laser level against both the x and y axis of the hinge to project that axis onto the drawing in the other room and moved the drawing around until it was correct. i then turned on the red dot finder and adjusted it until the dot lined up with its own position on the section drawing.

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I taped over the dials on the red dot finder once i was happy with their position. The proof of the pudding is once you have polar-aligned the tracker, you swing the door between its two furthest points and the red dot shouldn't move relative to the North Celestial Pole.

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Speed is of the essence. Mark one of the teeth on the big gear and a corresponding line on the baseboard and using a stopwatch, get as close as you can to 60 seconds for one rotation by altering the dial. then take it outside and set it up, point your camera at something towards the east as this part of the sky moves the fastest relative to us, get your lens focused as best you can, set an exposure of 30 seconds without the motor running and zoom in on the resulting photo and make a note of the trail length, then repeat with the unit turned on...if you're lucky, you will have round stars this time. if not, make tiny adjustments until you do. In my case i ended up with it running slightly slower at 62 seconds on one rotation.

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I set my tracker up so that it will do 3 hours / 45 degrees rotation. Any more than that and it will be past it's centre of balance. One could start with the base board angled down from the hinges at 30 degrees to avoid this problem at the end of a long session but i struggled enough with my cheap tripod angling back at 52 degress as it is.

i use a basic 1 amp wall transformer at home and a car battery out in the field

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The dial controller i have used is a bit of a faff, somewhat hit-and-miss. i might upgrade to Arduino Uno for more precise control when i find the time. but the test results, so far, are very encouraging

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I might dismantle and add a lick of paint next weekend.

All of the materials / components came to £110. I already had a tripod, small dovetail plate, project box, birch ply, bearing and steel counterweight,

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Edited June 8, 2020 by Legion Of Andromeda

[Xsubmariner]

A thoroughly enjoyable article and great learning project that draws on simple engineering principles to explore the art of tracking celestial bodies, whilst creating a working mount. I am sure it will  generate much conversation at any star party.

Made my day, thanks.

[Tomatobro]

A wicked piece of kit. love it

[Stub Mandrel]

The curved brass rod approach is so simple and obvious - once someone else has thought of it first!

Well done! Most impressive.

[Legion Of Andromeda]

i completely disassembled, gave it a lick of paint and reassembled today

[Chriske]

Oh...! Nooooo.....!   PINK...!!😳

[Chriske]

Just kidding...😁

[Stub Mandrel]

12 hours ago, Legion Of Andromeda said:

gave it a lick of paint

Sammy Hagar fan?

[sloz1664]

Great project, well thought out and as the images show, works perfectly. I want to make one of these when I have finished my other projects, but mine will be painted fluorescent yellow so I can see it in the dark

Steve

[happy-kat]

Great project and looks very nicely made.

The paint was a good idea as repeated dewy evenings might have damaged your birch ply.