Meter-class mirror figure metrology using a 24-channel fiber interferometer

We measured the influence functions of a 9-actuator warping harness of an ELT primary mirror segment prototype using a fiber interferometer. The compact setup consists of a stiff, lightweighted aluminium plate with a similar diameter as the segment (1.25 m), holding 24 fiber-fed collimators arranged in three concentric rings. We measure simultaneously the 24 absolute distances to the mirror surface with a nominal precision of 0.5 ppm. The recorded noise level in a quiet environment was below 50 nm rms. We found good agreement between the measured influence functions and the finiteelement model by the segment support manufacturer. These measurements were performed at ESO Garching in June 2017, with the “Absolute Multiline” metrology system.