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The Optical Test Tower (OTT) is the facility for the interferometric calibration of the M4 Unit (M4U) -the ELT sixsegment, 2.4 m flat deformable mirrornow in the final design phase by AdOptica. It is composed by two main subsystems: a large aperture setup, which accomplishes a macro-stitching approach where each mirror segment is flattened and co-phased to the other petals, and a sub-aperture setup to certify the mirror quality at the high spatial frequency scales. The accurate calibration of the interferometric cavity is of capital importance to meet the tight coflattening accuracy of the segmented adaptive mirror. We will analyze the critical aspects affecting the optical performances of the OTT, in particular: the absolute measurement of the optical alignment, the calibration strategy we foresee for the main optical components, the nulling of 1.5 m interferometric cavity, and the management of the drift of the optical surfaces. The M4U project is led by the Italian consortium AdOptica under an ESO contract.

Optical calibration of the ELT adaptive mirror M4: design, alignment and verification of the interferometric test tower