The stage is controlled with a laser interferometer, which is so accurate that it is used to calibrate beam deflection and distortion. You might think that it works by counting fringes, but no. It actually works just like the State Trooper’s radar gun, measuring a Doppler shift as the stage moves. By integrating the speed of the stage, this interferometer can measure the stage position to less than a nanometer. (It’s really quite interesting how the laser works. The He-Ne laser contains a magnet which splits the optical line with the Zeeman effect. The split frequencies are detected with a mixer that generates a difference signal in the gigahertz range. This is a low enough frequency to be measured with common electronics which measure the frequency shift as the stage moves.)