Laser metrology: schematic shows the working principle of measuring the optical path difference between the two beam combiners and the telescope. The metrology laser is launched from the spectrometers and is detected above the primary mirror at the telescope secondary mirror spider arms. To minimize the laser power in the spectrometer and optical fibers, we do not directly measure the interference between the laser coming from the two beam combiners, but measure separately for the fringe tracker and science beam combiner the phase relative to a very bright, third “carrier” beam. The actual phase measurement is implemented by modulating the phase of the fringe tracker and science metrology at different frequencies, and by measuring the cosine and sine components of the respective interference with the third beam through lock-in amplifiers.