Final design for the Extremely Large Telescope (ELT) M1 Local Coherencer

The ELT M1 Local Coherencer is a non-contact metrology system aimed to simultaneously measure the relative pistons on the six sides of a target M1 segment with respect to neighbouring ones (reference segments) with an accuracy below 300nm in a range of ±250μm while it is supported by the M1 Segment Manipulator hanging from the M1 Segment Crane. For this purpose, the Local Coherencer is equipped with six Sensing Modules integrating, - a patented partially coherent light interferometer, - an absolute tip-tilt sensor and a fine local alignment system to orient the system normal to the reference segment, and - a coarse alignment detection system composed of a distance sensor and a border visualisation camera. The Preliminary Design described in a precedent paper has been further optimized to provide a better performance of the interferometer. Additonally, as a part of the Final Design effort, an Early Unit of a Sensing Module has been built and tested to validate the expected performance, check the correct operation of its three measurement systems as well as the local alignment system and test the latency of the measurements. This paper covers the Design and results discussion.