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:
