Hi @jankaiser,
My machine is also based on an old modified projector.
I had never used a stepper motor, but from the get-go I decided on a large motor.
This is the motor -driver set that I am using:
I have reason to be very satisfied. Despite the considerable size of the motor and the driver that came with it, there was enough room inside the original case to house them.
At first, the motor-projector mechanical coupling was done with a toothed wheel on the projector shaft and a pinion on the motor shaft. With this mounting, two full turns of the motor shaft were required to advance a single frame.
Later I decided to mount the gear on the motor shaft and the pinion on the projector shaft. With this new assembly, the transport speed of the film was multiplied by four. It only takes half a turn of the motor shaft to advance one frame.
The motor works at the maximum resolution of 32 microsteps per step, the operation is very smooth, silent and precise, although of course slower than working at full step.
It has never given me any problems. The software that controls motor movement is about the only thing that I have never modified from the beginning.
As a power supply I use a 12 V source recovered from an old analog flat screen TV, which complies perfectly.
By the way, your optical system served me as a model for the one I have now in operation, with very good results. Thanks for sharing.
Regards.