and FWIW: The near state of the art that i'm aware of is the renishaw resolute: https://www.renishaw.com/en/resolute-absolute-encoder-system-with-resa30-rotar

and FWIW: The near state of the art that i'm aware of is the renishaw resolute: https://www.renishaw.com/en/resolute-absolute-encoder-system-with-resa30-rotary-angle-ring--10939 https://www.renishaw.com/en/resolute-absolute-encoder-system... Being rotary encoders, they are measuring accuracy/etc in arcseconds. For reference, 1 arcsecond is ~0.0003 degrees (1/3600th of a degree). So the +-5 arcsecond on the largest rings is 0.0015 degrees, and the +-0.5 arcsecond on the smaller is .000015 degrees. Both are fairly ridiculously small angles. To give a sense of scale of this error. On a circle with a radius the size of the earth, 1 arcsecond would only make an arc 30 meters long. On a circle with a 1 meter radius, it's about 5 microns of arc length per arcsecond. So yeah, you can do pretty well now in encoders :)