Performance evaluation of the first modules of the of the gradient-phase interferometer within the ELT M1 Local Coherencer

An instrument for phasing out the segments of the primary mirror of the Extremely Large Telescope (ELT) is being developed by IDOM and UPC in collaboration with ESO. The sensing module includes a partially coherent interferometer with a prism introducing a continuous phase gradient between the interferometer arms, and a stepped glass arrangement to cope with a measurement range of ±250μm with a 2σ accuracy within ±300nm. The setup also allows direct independent measurements of relative tip and tilt. This paper shows the detailed calibration setup, the procedures performed in detail, and the results obtained. The calibration procedures show a 2σ average accuracy of 164nm across the full measurement range, and 16μrad and 28μrad for relative tip and tilt, respectively.