CrossRoads:
Ooh, that encoder will be spinning for it's life!
Well it's a 2 r.p.m. turntable.
Sounds like a 3D laser scanner project.
Maybe print an optical encoder wheel to attach underneath the table and use a reflective sensor to detect.
Just google optical wheel encoder and there are a few programs that will print out one with about any degree of complexity.
Since this is just going in one direction no need for direction detection but need to sense a "start" timing mark.
I think gardners suggestion is pretty easy to test without a lot of hardware setup.