Ok good idea. I use a light sensor. This could be an easy way for me to physically move the mount to match where I want the start of my line. Then I can have something to compare oscillioscope probing, and find the lead on thr motor driver that has the motor angle.
This is not a common project, it took me a long time to gather what I could about reverse engineering a laser printer for re-purposing. Most all older laser printers have the same hex mirror motor assembly. It would actually be possible to modify the printer to accept sensitized pcb and just print the etch pattern using its own steppers and feed rollers.