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Zero distortion three mirror telescope designed for the Dark Universe Explorer (DUNE) space mission

A feasibility study is presently led by CNES for the Dark UNiverse Explorer (DUNE), a space mission designed to provide unprecedented constraints on dark matter and dark energy using weak gravitational lensing measurements over a very large region of the sky (20 000 square degrees). To achieve this scientific goal, the instrument requires a stable 0.23 arc second FWHM PSF with very low ellipticity (6%) over a field of view of 0.55 square degree. To perform the survey we adopted the drift scan mode which brings many advantages in this type of mission but adds the unusual requirement of a zero distortion optical design. We will present the optical performances of a 1.2 meters Korsch three mirror telescope concept (called NODI for NO DIstortion) where the third mirror works with an atypical large magnification and a slightly curved focal surface to obtain zero distortion over an annular 1.7 degrees field of view. A mosaic of 64 CCD detectors of 8 Mega pixels each is shown to pave the curved focal surface without significant degradation of the distortion.