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Barn Door Tracker.

 
05 Dec

Some old tech, some thoroughly modern.

For a long time I’ve had the itch to go point the camera at the stars, it’s been one of them things that have been in the back of the mind since I moved to where I am and can see loads straight out of the back door.

One of the problems though is the cost, some of these tracking mounts are serious cash and for stuff to use on an irregular basis or if I didn’t get on with standing out in a field for a few hours a time would be a lot of money to write off. On the other side when I was younger I’d built a barn door tracker as a project but…

The down side with a “normal” barn door tracker is that they are not that accurate and if they are manually controlled they can get a bit tedious. So, didn’t build one.

A few months back, whilst I’d got the leg up and not being able to walk anywhere, I was browsing round on the internet and came across a solution by Nick Touran; basically he had taken the idea of a compensation wedge and rewritten it with software on the Espressif ESP8266 module.

Great! Now that’s in my line of things, and the only things I didn’t have kicking around was some wood (2€ from the local carpenter) and some threaded shaft (1,25€ per metre from the local builders merchants), I can see both of the suppliers from my kitchen window.

In the box of bits, I’d got hinges, wood screws, a few arca-swiss plates that had seen better days, an arca-swiss nodal rail, thread bushes for furniture, drive couplers for printers and on my electronics bench some Wemos D1 Mini, steppers and driver boards. To be honest, at this point I’d not really used a lot of ESP chips in projects; usually they make great flippable range extenders for WiFi networks which can be powered with nothing more than a common mobile phone charger.

First thing I did though, whilst Nick’s software is a great idea, there’s some bits that instantly popped in to my head which I thought would work better for my purposes; first was swapping the mechanical side of the calculations from his Python scripts to process internally on the chip. This may not sound a big deal, and in most cases it wouldn’t be, but what I want to do is build a couple of trackers (one for the big cameras, one for the travel) and if I’m out with a pal of mine who’s got one, being able to set the parameters from ArduinoDroid and send them to the chip out in the field.

Then the next, I had a look at the mode switches, and decided to split off a run/stop from the toggle mode and have it on it’s own hard switched (the actual plan is to pair the hard switch with a 3.5mm jack socket so I can control it off other things); and just for good measure I thought (after putting one in during testing so it’d run for an hour, stop and allow me to hit with the protractor) an auto shutdown, which then got extended further to stop Isosceles showing up and making a mess at the party.

On to the build.

The mechanical parts list is :

  • 1x Hinge. Here it’s a 4″/100mm door hinge, mine is a bit sloppy on this, but as the hinge pin is fastened on the top board, it’s tight when mounted as gravity + angle pull it back.
  • 2x Boards. Mine here are 100mm x 300mm x 20mm pine.
  • 1x Drive bolt. Mine is M6 with 1mm thread pitch.
  • 2x Hardened steel rods between 3 & 4mm in diameter, with a length of 200mm or more. We’d got a load of meat skewers that we don’t use, about 0,5€ a piece from the local bazar (thrift store in the US) which I was going to use, but as I’d got some slightly shorter stainless rod sections (200mm), it didn’t seem worth getting the grinder out to knock the point and the loop off for.
  • 2x Small spacers to fasten on the ends of the steel rods so they slide through holes on the board. These would have cost me more than 0,01€ a pop at the local builders merchant, and as they were about the same size as a 1¢ coin, I just used the coins.
  • 1x Threaded insertion nut. These things are made for furniture manufacture mostly, threaded both on the inside and the outside. The inner face should correspond to the threaded rod and the outer would be larger. Drill a hole to the barrel shaft, screw one of these in with a hex wrench and then the rod goes through. Mine are M6 inner, M8 outer and about 20mm in length. I’ve used these a lot in repairing guitars, so had a box; however to buy one or two, some hardware shops don’t stock these and you may need to visit your local carpenter. More on these things in a minute.
  • 1x 60mm arca swiss plate, plus 1/4-20 + 3/8 thread convertor. This is to mount the ball head on the top.
  • 1x 200mm arca swiss nodal rail, goes on the bottom plate.
  • 1x Velcro cable wrap (commonly used in computer builds to keep the cables neat)
  • Glue that will bond metal (araldite, epoxy or similar)

Tools :

  • A good tape measure
  • A steel rule of at least 500mm or 2′
  • Callipers (optional, but so much easier to work with)
  • Drill
  • Drill bits for not only the main drive hole, but also the rod holes and also to create pilots for the screws.
  • Hex wrench for the drive nut.
  • Screwdrivers
    Pencil

So from having all this stuff, what’s the measurements?

Well… This web page is a good start, as it will do all the calculations for you https://blarg.co.uk/astronomy/barn-door-tracker-calculator. It also shows the equations if you want to go through and do it yourself.
With having a M6 thread for the drive, that gave me a hinge centre to drive centre distance of 228.55mm, in build, though, given that I was using hand tools and nothing else, I ended up with 228.85mm between the two centres.

First thing to do would be fit the hinge as snug to the bottom board as possible (but if like mine you are using a fixed pin hinge, I’d recommend the end that fastens to the pin being fastened to the top board).

Then measuring the hinge centre to drive centre; yeah, not fun eh? What I did with this was get the callipers out (it’s possible to buy plastic ones for a couple of coins which seem to be good to within 0.25mm) and then measure the barrel of the hinge and divide that in two.

If you’ve got the barrel butted right up to the board then from there it’s just using a straight rule to measure the distance to where the drive should be whilst subtracting the previously acquired figure.

Take your time measuring this out, three or four runs with it before you drill the hole as this here is the most important mechanical calculation of the lot.

After that is in, fasten the nut for the drive rod through the hole you’ve just made, and then if you decide to use a motor on this, slide the axle of the motor through the hole and then mark the mounting points.

Pull the motor off after marking the mounting points and then drill the holes out for sliding the rails through. It doesn’t matter if there’s a bit of slop here as the spacers will stop the rods from slipping out (or if you are fancy and have steel rod with a thread on the end, just a nut would suffice)

Once this is done, fasten the top board with the hinge.

Now this is where I went a bit loose cannon compared to most build guides; on the bottom plate, I measured the centre out and then got an arca-swiss nodal rail (200mm) and fastened that so it sits down the centre line of the board; and because these things have holes in them just screwed it down with some wood screws. Now that will just slot in to any of the tripod heads I’ve got without issue and it makes balancing the centre of gravity easier than just trying to rely on one nut to do it all.

On the other side of it, on the top board, same again; I measured down the centre line, and then got an arca-swiss (60mm, but a 100 would probably have been better in hindsight) and then popped the guider nuts out the bottom with a hex wrench. Then I put the 1/4-20 screw through it, the 3/8 adaptor on the other side, added a drop of thread lock and tightened them together whilst making sure the screw was in the centre. After that, a wood screw the either side of the nut and fastened that about 3 finger widths away from the hinge edge. Now that will accept any tripod head that has a 3/8 thread on the bottom.

A bit of glue on one end of each steel rod to stick the spacers on to and that’s it.

Even using hand tools instead of the noise makers (it’d probably have taken me the same amount of time what with unpacking, setting up, packing down the electrical stuff) it took about ten minutes end to end to put the mechanical together; but two caveats there, I’m used to getting the sleeves rolled up and attacking tasks like this so I knew what tools I’d need afore AND I also had all the measurements down on paper before I started.

I know I’ve not mentioned the velcro wrap yet, and it may be confusing to some folk to why I needed one; but (aside from keeping electronics tidy later), one thing I found in testing with using the inserted nut was whilst it was very smooth when the boards were flat, turning the board to working angle (here it’s 41º), the thread was binding slightly too much for my taste; so with wrapping the velcro wrap around the drive rod and then feeding the drive rod in to the nut, the wrap was just sitting inside the hole in the bottom plate and just centring the rod. Perhaps next time I’d use a lower profile nut with less thread on it and bind it in to the hole somehow, or build in some way to take the tension off the drive rod.

Electronics

The electronics side of isn’t too complicated, currently I’ve a Wemos D1 Mini attached to an ULN2004A and the 28YBJ-48 stepper motor and a couple of switches connected to D6 and D7.

At the moment, I’m not going to go in to building the electronics I’ve used here as those things are still covered in bodge tape and elastic bands. I’m still not decided what I’m doing with the electronics as the back of the head is saying get a PCB cut for it, which would give a bit more robust nature than using jumpers, and maybe I could do with some more pins on there (how about a push button attached to the motor so when it hits the bottom of the drive board, it turns off? Or perhaps a small focused diode on the hinge pin to align the pole star?) and… That voice in the back of the head is saying “Eh man… You know all these big old 7.2v camera batteries clogging up the desk drawer?”

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