* evernet@eso.org , phone: +498932006322, fax: +498932006562 Extreme adaptive optics system optimization with the High Order Test bench Elise Vernet *,a , Markus Kasper a , Christophe Vérinaud a , Enrico Fedrigo a , Sébastien Tordo a , Norbert Hubin a , Simone Esposito b , Enrico Pinna b , Alfio Puglisi b , Andrea Tozzi b , Alastair G. Basden c , Stephen J. Goodsell c , Gordon D. Love c , Richard M. Myers c a European Southern Observatory, Adaptive optics Department – Telescope System Division - Karl Schwarzschild Str. 2 - D-85748 Garching Bei München - Germany b INAF – Osservatorio Astrofisico di Arcetri - Largo Enrico Fermi 5 - I-55125 Firenze – Italy c Durham University - Department of Physics - South Road - Durham DH1 3LE ABSTRACT Extreme adaptive optics systems dedicated to the search for extrasolar planets are currently being developed for most 8- 10 meter telescopes. Extensive computer simulations have shown the ability of both Shack-Hartmann and pyramid wave front sensors to deliver high Strehl ratio correction expected from extreme adaptive optics but few experiments have been realized so far. The high order test bench implements extreme adaptive optics on the MACAO test bench with realistic telescope conditions reproduced by star and turbulence generators. A 32×32 actuator micro deformable mirror, one pyramid wave front sensor, one Shack-Hartmann wave front sensor, the ESO SPARTA real time computer and an essentially read-noise free electron multiplying CCD60 (E2V CCD60) provide an ideal cocoon to study the different behavior of the two types of wave front sensors in terms of linearity, sensitivity to calibration errors, noise propagation, specific issues to pyramid or Shack-Hartmann wave front sensors, etc. We will describe the overall design of this test bench and will focus on the characterization of two essential sub-systems: the micro deformable mirror and the phase screens. Keywords: Extreme Adaptive Optics, high angular resolution, Shack-Hartmann, pyramid, wave front sensing, micro deformable mirrors, L3 CCD 1. INTRODUCTION Direct detection of extra solar planet is one of the most exciting but also one of the most challenging aim of the next 20 years. The search for extrasolar planets requires contrast levels better than ~10 -6 a few tenths of an arcsecond from the central star. Scattered light from optical aberrations introduced by atmospheric turbulence (seeing) and the diffraction pattern of the telescope’s aperture limit this contrast by masking the faint planet with light from the star. Optical aberrations can be effectively corrected by an high order (or eXtreme) Adaptive Optics (XAO) system. Currently, the best way to diminish the diffraction or Airy pattern is by use of a coronagraph. Virtually all high contrast instrument concepts for large ground-based telescopes such as the VLT Planetfinder 2, , 46 (called SPHERE) or EPICS 9 for the Future ELT use a combination of XAO and a coronagraph. At least two wavefront sensor concepts, Shack-Hartmann (SHS) and Pyramid wavefront sensor (PWS), appear to be suitable to deliver the high Strehl ratio correction expected from XAO systems. The different behavior of the two types in terms of linearity, sensitivity to calibration errors; noise propagation etc. has been extensively studied mostly by computer simulations. Astronomical XAO has never been realized in an experiment so far.