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While on an Aurora watching trip to Abisko, Sweden, i bacame interested not just in watching and photographing the beautiful Aurora borealis, but also the clear skies and sparkling stars and milky way excited me. So much, that i decided to give Astrophotography a severe thought. But if you would like to depict something more on (the digital equivalent of) film, that requires significant more exposure times or a lot of (disk space and time consuming) stacking of individual shots unless you want to use a tracker, which then in return usually requires a sturdy tripod and heavy and expensive mounts. And usually there come add ons on top...
While doing some research i stumbled about an intruiging alternative: The OpenAstroTracker. Not that is a bit unconventional, yet using the same principles ad the commercial mounts, it is also 3D printed, open source and you could even build it yourself for around 200 EUR.
Although I in deed do have a 3D printer, i decided to buy the kit for approx 260 EUR, since i didn't want go through the shopping list and source all the other hardware as motors, boards, belts, screws... myself.
Since the kit at the shop of OpenAstroTech.com was sold out, i ordered via Astroshop.de and after 2 days the package arrived. The next evening i spent assembling the pieces together.
The assembly is straightforward, but you need the online instructions, which are mentioned on the only paper sheet, that is delivered with the kit.
Mainly you start with building the triangular base structure with the ubiquituously used 20x20 aluminium construction profiles, M4 nuts and square nuts,that can self-insert and fasten in the profile grooves. Then you add the chunky rollmounts and rear angle. My kit had two sets of them: the 35-45° and the 45-55° versions. The 35-45° set (one large at the tip of the triangle stud and two smaller left and right handed chunky parts for triangle base) is denoted with a "40" and the other one has an imprint of "50" (the smaller rear angle and the larger front rollmounts). This makes sense, since the equilibration bubble allows adjustments of 5 degrees in every angle. My typical longitude is 49N, so i used the 50 parts. Here one should pay some attention: The 3D parts might not be totally (and i really mean totally) flat. So when tightening them to the 20x20 profiles, there might be some build up of tension, which can cause slight cracks in the rollmounts. Better not to tighten the M4 screws too much. A similar thing with the M3 screws that are used to align and affix the rollmounts to the triangle base connector brackets below by M3 screws. The rollmounts have holes, but they are unthreaded. If you force the M3 screws in, they displace the material rather than cutting a thread into it. So the bottom of the rollmounts becomes even more bendy. Plus, the tension might also lead to cracks. Therfore, either cut threads into the parts or just forget about the M3 screws. I did the latter and paid a little more attention on alignments of all the parts, which worked out nicely in the end. A similar thing happens by the way, when mounting the rear angle. Also here, the M3 screw can easily be omitted.
In between the rollmounts the RA motor bracket goes. This fits really tight. You might want to sand off a little bit from the white motor bracket to make it fit in nicely.
One starts with building the right and the left part of the camera mount separately before attaching them to the RA (rectascension) wheel. This is necessary since you need some space to mount their ball bearings on the studs of the RA wheel. This is a little bit of a wobbly thing until you slide in the 20x20 profile over the 8 square nuts. Just a little patience and things work easily. Or get a pair of helping hands for that step, but i easily did it with just my two hands.
The camera cage essentially consists of four almost(!) identical parts ("poles") plus a center part. Actually, there are 2 identical upper and 2 identical lower parts. The four white bow shaped parts fit into the central part very tight and expose a long lever, so be a little careful. Also, do not yet tighten the screws. Just place the parts, insert the screws ans give them 3-4 turns, not more until all four parts are in place. Then tighten, but don't overtighten.
Now bolt the cage to the RA ring. This an easily be done by one person, but an extra set of hands comes in handy. Add the reinforcement bracket.
Once the gage is finished, it is put on the triangle base, the belts are attached and the electronic box added. And here we go.
Attach a USB power bank to the OAT and it should start working: LCD shows up and asky you wheter the OAT is already homed. You need to get used to how the buttons work, though. I personally don't find it very intuitive.
You need to home the OAT (RA ring arrow pointing to motor bracket arrow and camera mount perpendicular to the RA ring; pretty easy). Make sure the buttons operate in the right directions. Once homed go to polar alignment and then you're ready to go.
This is exactly the noise i heard when switching onthe OAT. And it comes just from the RA drive. First i thought there would be something wrong, but since the OAT is tracking, there need to be some tiny movements all the time. One can get used to it, but it's annoying. A way to fix this is also there. Later more on how to fix that that.
Ther is a list of around 20 targets in the sky already included. However, to me this list is a little short with missing some of the targets one would think of being standard such as Sirius, Castor, Pollux, Beteigeuze etc. But don't worry; there's a way to add to the list easily in the source code.
Another thing i didn't find was how to set date (time works, though) and the present location with the controller, but...
There's a beautiful tool called OAT control, which you can run on a laptop and attach to your OAT. This allows a more user friendly control of your OAT plus setting date and location. Plus you can theoretically even add targets by yourself. However, i experiences some cold exits (crashes) of the software when trying to add them through the software. But there's another simple way, too.
You can also add your location here and it's updated on the OAT, too. You have to update the position via OAT Control whenever you significantly move the position of your OAT, though
But the annoying sound of the RA otor persists.
Just suggestions; No warranty, Follow at your own risk
I decided i wanted to have the GPS add on. It's a small 10 EUR sensor, that can easily be retrofitted. The wiki mentions and depicts an older board (4 pins) and a newer board (5 pins plus micro USB connector) at two different pages. Both work and seem to be compatible; just pay attention to connect the right pins (GND, VCC and TX)
If you performed the modification, recompiled and uploaded the new firmware to the OAT, it might take veeeery long when you fire up the GPS for the first time. For me it took almost half an hour (place a 5x5cm aluminium foil under the tiny ceramic GPS antenna as antenna "counterweight", by the way to improve signal gain.
Once everything runs and the GPS initialized itself, then the next times the GPS signal is found much much faster: Typically it takes 3-10 seconds to find 3+ satellites
As i found out, there are lots of features already implemented in the software, that can be added. For me the most interesting one would be the GPS sensor. This requires toggeling one configuration bit in the software, though. So i need to recompile. The wiki suggests to use platformio, but somehow i always failed with the installation of Visual Studio plus all the required add ons and am more familiar with the Arduino IDE. So let's see wheter i get the software compiled under the Arduino IDE, too.
With the IDE already installed since years, i just needed to locate the repository of the source code on github. The following description refers to March 2025 with Version 13.11.14 of OAT on github (from the Astrotech wiki via useful links and then github). On the main repository there are two firmware folders: .OpenAstroFirmware (this is for the telescope! mount) and OpenAstroTrackerFirmware (the one for the foto mount). We need the latter one. Then download the whole repository as zip file and unzip it to the location where all your Arduino sketches are. For me it's C:\A_Daten\Arduino\OpenAstroTracker-Firmware (the folder might be called OpenAstroTracker-firmware-development after unpacking. Rename to assure that the folder name matches the name of the .ino file inside).
In that folder, there are some files with the extension .md. Delete them. You can open the IDE and load OpenAstroTraker.ino. The whole project should now come up with several tabs.
For the steppers you should install the AccelStepper library by Mike McCauley (mine: V1.64) and for the GPS you need the TinyGPSplus by Mikal Hart (mine is V1.0.3). You probably also need to install the TMC2209 library.
There's a handy online configurator tool that creates a local configuration for your OAT. Although you could also do everything by hand, the tool comes in really handy. Just call it, make your selections and copy&paste the code into a new tab called Configuration_local.hpp in your project. My suggestion is to go with the silent or stealth mode to get rid of that annoying sound right away. If you want to use the GPS, select it in the tool, too.
Make sure to unplug the capacitor in the middle of the MKS board if you bought the kit version and assure that you're connecting to the right COM port. You want to select an Arduino Mega 2560 board, too.
then compile the code. It should compile without issues.
There's some more memory available and we can use a portion of it to add new targets directly to the controller. Meanwhile i have around 50 targets. You just need to add a textfile and fill in according to the examples given there. The file is called c722_menuPOI.hpp and located in the subfolder src of your project.
The OAT Control comes with around 40 predefined targets or points of interest. You might want to sooner or later add some more. As mentioned, the software tends to crash, when adding targets via software directly.
But help is near: The software reads the targets from a human readable XML file. You can locate the file yourself and open with editor, word pad or notepad. For my configuration, the file is PointsOfInterest.xml. Just add targets as you like following the intuitive format
When you recompiled your software, you probably need to recalibrate your OAT. Independent on how you set the microsteps in the code, there's (only?) one way tha really works.
Follow these steps:
I observed the following: When the target selection window is open, the manual adjustment buttons on the main window behave quirky: either with long delay or running by themselves even if the mouse is no longer pressed. How to fix? Pressing home works instantly and moves the OAT to home position. Then select a target (you can doubleclick and it's taken to the target entries for RA and dec, by the way). then press "slew!". Avoid having the target window open when doing manual adjustments.
