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Wave-front sensor strategies for SPHERE: first on-sky results and future improvements

SPHERE instrument [1] (Spectro-Polarimetry High-contrast Exoplanet Research) is a second generation ESO instrument dedicated to high contrast imaging, and exoplanet direct detection and characterisation. The overall performance of XAO system of SPHERE, as well as the optimal control law for turbulence correction, are presented in dedicated papers [5,6]. The global performance of the instrument and of all observing modes of SPHERE is done in [4]. The strategy of Wave-front Sensing [WFS] in SPHERE relies on two faces, and is thoroughly discussed in this paper. Firstly, extreme adaptive optics (XAO) is required for both turbulence and quasi-static pattern compensation. Particularly, the high frame rate and large subaperture numbers of the Shack-Hartmann WFS allows SAXO to optimally measure and compensate for atmospherical turbulence. Moreover, the spatial filtering [7,8] allows one to deepen the contrast curve, and is automatically adjusted on turbulence level to provide the best performance. Finally, a dedicated calibration procedure based on focal-plane wave-front sensing is optimized for NCPA compensation on the coronagraphic device, ensuring the best compensation of quasi-static speckle. Secondly, a high robustness to faint magnitude guide star allows SAXO to address a large panel of targets for exoplanet detection and characterization. This is only made possible by the joint use of a dedicated Wave- Front Sensing for turbulence, EMCCD detector capability, and adaptation of the system to the star magnitude. The noise propagation has been carefully monitored and optimized. The weighted center of gravity gives an optimal trade-off between performance with respect to noise, and complexity of implementation. The use of an EMCCD detector allows a powerful noise reduction on the wave-front sensor detector. And finaly, 5 SAXO observing modes are defined in order to cover all star magnitudes up to 16, with systematic optimal performance. During the whole assembly integrations and test period, choices have been done to optimise the trade-off between performance, robustness, and simplicity of use. The self-adaptation and auto-calibration of the instrument has been a strong investment, as well as developing a great simplicity of use. We describe here the actions taken to reach this level of operation for SPHERE. Finally, perspective are withdrawn for improving the strategy of WFS in the framework of future XAO instrumentations in E-ELT.