I was thinking about an ultrasonic sensor but I also was wondering how it might detect the shortest route to the opening. I think it could be used to stop the motor when the dome opening was detected, but how might an ultrasonic detect where the opening is when the dome and telescope are off axis. Multiple sensors pointing in different directions?
I get the idea of detecting the scope's azimuth settings, but I don't understand your concept of "measuring the number of turns from the dome turning motor using an opto slit detector". I'm including another diagram to show (kind of) how the motor is positioned to turn the dome. The gears of the motor turn in a continuous track (something like sprockets on movie film) to rotate the dome.
I've thought of creating a continuous tape with positioning codes on the dome but a simple turn left/turn right seems so simple. IOW, the dome should simply move to where ever the scope is pointing.
Another thought is to always start the session with the dome opening in the correct position, then detect a left or right motion of the scope and go from there. For future "remoting" of all the astronomy functions, I'd like the dome to first find the proper position.
Thanks for your advice,