Optical calibration of the ELT adaptive mirror M4: strategy for optical measurement error estimation

For the optical characterization of M4 Unit (M4U) is mandatory to set up an opto-mechanical solution that can guarantee excellent measurement quality over the whole 2.4 m clear aperture. The final design of the test structure foresees the use of a dynamic interferometer with 1200x1200 pixel resolution that generates a test beam collimated by a 1.5 m diameter parabolic mirror to image a single M4U segment. The test tower provides the capability to illuminate each single segment plus the center of the entire unit. A reference flat of 600m can be inserted to decouple the M4U from the alignment aberrations. In the paper we will present the strategy for the budgeting of the optical measurements in the Optical Test Tower (OTT). Following a bottom up approach, we identified a set of possible noise sources, then we computed their contribution to the optical measurements, reporting in the end the global effect on the calibration. The M4U project is led by the italian consortium AdOptica under an ESO contract.