Prototyping of differential optics for the SPHERE IRDIS dual imaging planet finder camera

The SPHERE (Spectro-Polarimetric High-contrast Exoplanet Research) planet finder instrument for ESO's VLT telescope, scheduled for first light in 2011, aims to detect giant extra-solar planets in the vicinity of bright stars by the aid of an extreme-AO turbulence compensation system and to characterize the objects found through spectroscopic and polarimetric observations. Dual imaging observations within the Y, J, H and Ks atmospheric windows (~0.95 - 2.32μm) will be done by the aid of the IRDIS cryogenic camera. Ensuring sufficiently low level of differential aberrations between the two parallel images is a major concern for this instrument. We describe the prototyping efforts made in order to validate our concept.