Optical systems based on position sensitive detectors (PSDs) are used to measure the coil displacement for the unwanted degrees of freedom (2 translations and rotations around three axes). The vertical coil velocity is measured with a heterodyne interferometer. This measurement has recently been improved by replacing the one-axis with a three-axis interferometer.

Moving from a 1-axis to a 3-axis interferometer has improved the voltage to velocity signal to noise ratio by a factor of 10.