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, a fine 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: a superluminiscent led (SLED) with a higher brilliance and spatial coherence has been selected to enhance the radiometry and contrast, the optical layout has been optimized to improve both the radiometric and wavefront degradation performance, additionally a detector with a bigger sensor area has been integrated to avoid the need of an afocal system to fit the output beam, thus further reducing the number of elements inducing beam degradation. 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 the three measurement systems contained in the system as well as the local alignment system and tests the latency of the measurements. This paper describes the Final Design and the first results obtained with the aforementioned Early Unit of the Sensing Module.
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 ±250μm. 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.
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