Optical calibration of the E-ELT adaptive mirror M4: Testing protocol and assessment of the measurement accuracy

Abstract

The M4U is the adaptive mirror of the european Extremely Large Telescope (E-ELT), composed by six thin shell segments and controlled by 5136 voice coil actuators. During the project phase C we addressed the concept of the optical calibration and verification of the mirror; the outcome of such study is a testing protocol on the large facility named Optical Test Tower (OTT). In this work we give an overview of the test facility and of the calibration concept; then we present the analysis results, with a particular focus on the environmental noise (e.g. Convection and thermal drifts), procedure criticalities and processing errors. In the end we summarize their impact on the requirements verification of the adaptive mirror. The M4U project is led by the Italian consortium AdOptica under an ESO contract.