Quite a simple yet workable ASCOM-compatible handmade automatic focuser.

The driving part of the focuser is a 28BYJ-48 stepper motor which needs 5V of power and it draws only about 240 mA of the current. Such characteristics of the motor give an ability to use the USB connection to power the motor. Here is a good overview of that stepper motor.

According to that article in full step mode the motor does 2038 steps per revolution. Bearing in mind that the gear ratio is 1:4 the total number of steps in order to the focuser's output shaft to make the full revolution is 2038 * 4 => 8152.

The article also mentions the torque of the stepper motor - 34.3 mN*m which roughly corresponds to the 350 g*force*cm. Again, using the gear ratio of 1:4 the torque of the output shaft is about 1.4 kg*force*cm. Which is quite okay, but ideally it's better to have a better margin between the minimum required value and the maximum torque the focuser can supply. I weighted all the imaging train I put into the focuser and it weights about 700-800 g, so having about 1.4 kg*force*cm torque from the focuser (and some torque we lose due to friction, etc.) might be not enough - actually, when tightening focuser screws up I noticed the focuser almost stopped moving and just skipped steps 😬 Therefore, it's better to have a bigger margin - I believe, something 2x and higher should be a sweet spot.

I think I'll rebuild the focuser entirely reusing some of the 12V stepper motors, which initially might provide the 1.4 kg*force*cm torque and adding some gear system we can drastically increase it. The downside of this we'll need additional wiring to provide a 12V power supply, but anyway we already get it right to the camera, so it's not an issue.

Under the 3DDesign folder there is a Fusion 360 design file. Basically, during the design phase the focuser looked like the following:

The N.I.N.A. astrophotography software provides the autofocusing feature and now having the automated focuser it makes sense to setup the autofocus. There is a great video about how to setup auto-focus in N.I.N.A. quickly and correctly. I tried to follow mentioned steps and that's how the autofocus setup looks in my case:

Once applied all the settings the autofocus routine went correctly and the graph looks quite great: