I mean the ability to repeat a camera move twice, and have those moves be identical, ala motion control. and maybe that's just not possible for a noob-tinker-setup like this will be--but I can dream smiley So the motion can be duplicated programmatically, just getting the starting point to align each time will be difficult?
This is what you said. You didn't specify how much time might elapse between the first shot and the repeat shot. If your stepper doesn't skip any steps and if you repeat the shot immediately after the first shot, then yes you can probably do it in open loop.
You asked what the difference is and the difference is that with closed loop and especially with an absolute encoder , you can repeat the shot six months later and it will be identical. With a stepper, you can't guarantee the initial starting point unless you have some system to know the starting position with a reasonable amount of accuracy. As I stated previously. Most stepper driven
systems used in the industrial systems , be it a wafer lift in a semiconductor processing tool or an x/y table, have home flags to initialize the system to an exact starting location repeatably , 24/7/365 , year round. Some have more than one opto for reading the flag and can sense the extreme limits of the motion , called end-limits or end-flags. There is no limit (aside from space) to how many flag reading sensors you can have, but if you want to home it , you can only have one unless you electronically disable all but the home flag (which is not hard to do). If the power for all the other flags has a common source or ground you can switch them off with a mosfet leaving only the home flag working. If you have a home flag and you know the first shot was x counts cw from HOME (which is NOT the flag location but some number of steps , like 10 or 15 from the home flag) then when you want to repeat the shot you repeat the distance from Home. Another difference with closed loop or encoder approach is it is immune to skipped steps where a stepper overshoots due to inadequate drive current. If you want to calibrate your system. Buy a laser pointer and stall a clip holder on the turntable with drilled and tapped holes. Mount the clips, snap in the laser pointer, do your calibration then remove the screws and the clip and no one will even know they are there..The accuracy of the calibration is a function of the distance to the walls. If you have a large enough room the accuracy of the position of the dot on the wall is extremely precise, meaning one stepper step changes the position of the dot by several inches to up to a foot on the wall. If you want save money , use a home flag, the laser clips and mark the dot on the wall when you take your first shot and make sure the dot is in the same place when you repeat the shot. It's not rocket science.