The Extremely Large Telescope (ELT) M1 Local Coherencer to phase mirror segments

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 neighboring ones (reference segments) with an accuracy below 300nm in a range of +/- 250 mu m. This measurement shall be performed while the Local Coherencer is supported by the M1 Segment Manipulator hanging from the M1 Segment Crane. IDOM has developed for the M1 Local Coherencer a lean, compact and robust solution featuring: - Six lightweight and compact Sensing Modules whose main system is a partially coherent light interferometer for the piston measurements that hugely simplifies image processing and avoids any ambiguity in the measurements. - Comprehensive and robust alignment detection and alignment compensation systems that ensure proper positioning and prevent apparent (bias) piston measurement errors. - A lean embodiment in which all the subsystems, including control and safety elements, are mounted on a single support structure and enclosed in the specified design volume, with no need to use the space reserved in the M1 Segment Manipulator - A solution largely based on small COTS and simple electronics, which account for ease of use, high reliability, easy replaceability and high durability of the system. This paper describes the proposed design as presented in the Preliminary Design Review (PDR) of the system held in May 2022.