Euclid payload module: telescope characteristics and technical challenges

Euclid is an European Space Agency (ESA) mission to map the geometry of the dark Universe. The mission will investigate the distance-redshift relationship and the evolution of cosmic structures. It will achieve this by measuring shapes and redshifts of galaxies and clusters of galaxies out to redshifts 2, equivalent to 10 billion years back in time. Euclid will make use of two primary cosmological probes, in a wide survey over the full extragalactic sky: the Weak Gravitational Lensing (WL) and Baryon Acoustic Oscillations (BAO). The main goal of the Euclid payload module (PLM) is to provide high quality imaging of galaxies and accurate measurement (less than 0.1%) of galaxies redshift over a large field of view (FoV). The present paper focuses on the telescope of the PLM excluding the instruments. We present a brief introduction to the Euclid PLM system and will report how the constraints of each instrument have driven the definition of the telescope-to-instrument optical interfaces. Furthermore we introduce the description of the telescope optical characteristics and report its nominal performances. Finally, the technical challenges to be faced by ESA's industrial partners are underlined. (authors)