AO4ELT5 Proceedings
Optical calibration of the ELT adaptive mirror M4: design, alignment and verification of the interferometric test tower
Pariani, Giorgio (INAF – Osservatorio Astronomico di Brera, via E. Bianchi 46, 23807 Merate, Italy), Tintori, Matteo (A.D.S International, via Roma 87, 23868 Valmadrera, Italy), Briguglio, Runa (INAF – Osservatorio Astrofisico di Arcetri, largo E. Fermi 5, 50125 Firenze Italy), Xompero, Marco (INAF – Osservatorio Astrofisico di Arcetri, largo E. Fermi 5, 50125 Firenze Italy), Riccardi, Armando (INAF – Osservatorio Astrofisico di Arcetri, largo E. Fermi 5, 50125 Firenze Italy), Gallieni, Daniele (A.D.S International, via Roma 87, 23868 Valmadrera, Italy), Biasi, Roberto (Microgate, via Stradivari 4, 39100 Bolzano, Italy), Vernet, Elise (ESO Headquarter, Karl Schwarzschild strasse 2, D-85748 Garching, Germany), Cayrel, Marc (ESO Headquarter, Karl Schwarzschild strasse 2, D-85748 Garching, Germany)
The Optical Test Tower (OTT) is the facility for the interferometric calibration of the M4 Unit (M4U) -the ELT six-segment, 2.4 m flat deformable mirror- now 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 co-flattening 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.
DOI: 10.26698/AO4ELT5.0078- Proceeding PDF
