The differential tip-tilt sensor of SPHERE

The SPHERE instrument aims at detecting giant extrasolar planets in the vicinity of bright stars. Such a challenging goal requires the use of a high performance Adaptive Optics (AO) system, a coronagraphic device to cancel out the flux coming from the star itself, and smart focal plane techniques to calibrate residual uncorrected turbulent and/or static wavefronts. Inside the adaptive optic system, a specific tool is developed in SPHERE to ensure that the star is always well centered on the coronagraph. This tool called Differential Tip-Tilt Sensor (DTTS) measures the position of the star at the same wavelength than the science instruments. It is located very close to the focal plane to minimize drifts between DTTS and the coronagraph. After describing the DTTS, we will describe the tests and laboratory results on stability measurement of the DTTS; stability which is crucial for SPHERE performance.