Calibrating SPHERE, the exo-planet imager for the VLT

One of the main challenges to obtain the contrast of >15mag targeted by an extra-solar planet imager like SPHERE lies in the calibration of all the different elements participating in the final performance. Starting with the calibration of the AO system and its three embedded loops, the calibration of the non-common path aberrations, the calibration of the NIR dual band imager, the NIR integral field spectrograph, the NIR spectrograph, the visible high accuracy polarimeter and the visible imager all require sophisticated calibration procedures. The calibration process requires a specific extensive calibration unit that provides the different sources across the spectrum (500-2320nm) with the stabilities and precisions required. This article addresses the challenges met by the hardware and the instrument software used for the calibration of SPHERE.