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We present the current results of the astrometric characterization of the VLTplanet finder SPHERE over 2 years of on-sky operations. We first describe thecriteria for the selection of the astrometric fields used for calibrating thescience data: binaries, multiple systems, and stellar clusters. The analysisincludes measurements of the pixel scale and the position angle with respect tothe North for both near-infrared subsystems, the camera IRDIS and the integralfield spectrometer IFS, as well as the distortion for the IRDIS camera. TheIRDIS distortion is shown to be dominated by an anamorphism of 0.60+/-0.02%between the horizontal and vertical directions of the detector, i.e. 6 mas at1". The anamorphism is produced by the cylindrical mirrors in the common pathstructure hence common to all three SPHERE science subsystems (IRDIS, IFS, andZIMPOL), except for the relative orientation of their field of view. Thecurrent estimates of the pixel scale and North angle for IRDIS are12.255+/-0.009 milliarcseconds/pixel for H2 coronagraphic images and-1.75+/-0.08 deg. Analyses of the IFS data indicate a pixel scale of7.46+/-0.02 milliarcseconds/pixel and a North angle of -102.18+/-0.13 deg. Wefinally discuss plans for providing astrometric calibration to the SPHERE usersoutside the instrument consortium.

SPHERE IRDIS and IFS astrometric strategy and calibration