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The European Extremely Large Telescope (ELT) is a first-of-its-kind project, putting together a range of unique engineering challenges. The performances of its mirror cells are particularly demanding, and their verification has required the development of innovative methodologies. This paper addresses the developed verification methodologies for the most critical performances of the Secondary and Tertiary Mirror Cells (M2 and M3) and the Fifth Mirror Cell (M5). Concerning the monolithic 4-meter class M2 and M3 Cells, that is the maximum surface error (SFE) allowed in operational conditions. The SFE due to manufacturing and integration tolerances is singularly complex to evaluate, and testing is required to ensure compliance of the Cell supports as built. Moreover, such testing must be performed with a non-reflective Dummy Mirror. Hence, a methodology to obtain the SFE from the forces measured on all Mirror supports was developed. The application of uncertainty reduction techniques was essential to obtain reliable conclusions. The M5 Cell supports the largest fast steering mirror (2.7 by 2.2 metres) ever employed in a telescope. The most challenging performances regarding dynamic response are its control bandwidth above 10 Hz, a phase lag below 30 degrees up to 4 Hz, and a cross-coupling below 1% between degrees of freedom. After investigations with an actuator prototype, a method based on application of optimised frequency sweep commands was selected. The test setup was designed to avoid artificial resonant modes, and dedicated postprocessing methods to extract the frequency response were developed. This paper describes in detail the SFE verification methodology for the M2 and M3 Cells, and the tip-tilt capability verification for the M5 Cell. The obtained test results are presented and discussed.
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