The E-ELT adptive mirror: prototype and optical test

Ground-based optical-infrared 10m-class telescopes, such as Keck, Gemini, and Very Large Telescopes (VLT), are currently leading many of the astronomical research fields due to their unprecedented collecting areas. However, many new astrophysical quests arose from recent observations, justifying the need for even larger telescopes, with diameters > 30 m. The European scientific community, led by the European Southern Observatory (ESO), takes part in this challenging competition whit the "European-Extremely Large Telescope" (E-ELT), a revolutionary project for a 40m-class telescope that will allow us addressing many of the most pressing unsolved questions about our Universe. Building Extremely Large Telescopes asked for innovative technologies, from larger and repeatable optical manufacturing techniques to produce hundredths of aspherical off-axis hexagonal segments and large monolithic mirrors, or large thin optical shell mirrors, to better and faster controls (active and adaptive optics), together with more specialized focal plane instrumentations devoted to address specific astrophysical questions, more likely to large particle physics experiments. Moreover the challenge of building extremely large telescope pushes forward the parallel ability to measure and test optical components of large sizes. Adaptive Optics (AO) techniques allow to obtain enhanced ground-based astronomical observations, partially restoring diffraction-limited spatial resolutions, by compensating the degrading effects of the atmospheric turbulence. The delivered image quality of an AO-assisted telescope depends on the level of the correction of the wavefront error due to the atmosphere. This work focuses on the adaptive deformable mirror unit (M4AU) of the E-ELT. During my Ph.D. study, I took part in the competitive "EELT-M4 project" study, led by an Italian-French consortium (Microgate, ADS, SAGEM, INAF-O.A.Brera). Optical measurements were conceived, designed and performed on a similar, scaled-down, fully-representative prototype of the final system, to fully validate the proposed concept. Due to very stringent requirements in term of wavefront correction (10-100 nm rms), only interferometric optical setups were able to provide enough accuracy and sensitivity to carry out the work. A set of three devices has been designed for the tests: - a Ritchey-Common test setup (RCT), able to cover the full area of the M4AU DP optical surface with a single measurement, taken with a phase-shifting Fizeau interferometer; - a Stitching Interferometric Setup (SIS), where a scanning smaller optical beam is able to reconstruct the same optical surface with increased capture range and spatial resolution, useful in the first steps of the calibration procedures; - a piston sensing unit (PSU), which exploits the optical path difference between adjacent optical shells, after careful analysis of diffraction patterns. Those devices, together with purposely designed control softwares and data reduction softwares, were used during a 6-months measurement campaign, demonstrating the full compliance of the prototype system with the requirements to be obtained in the final system (flattening of each optical shell, co-phasing of the optical shells, optical stroke calibration, stability of calibration under time and changing environmental conditions). Results have been positively reviewed by the EELT project office, and were used as feedback on the design of final optical test system, to be operated on the 2.5m M4AU unit.