First optical tests of the Euclid grisms made by photolithography

The ESA mission Euclid is designed to explore the dark side of the Universe and to understand the nature of the dark energy responsible of the accelerating expansion of the Universe. One of the two probes carried by this mission is based on the Baryonic Acoustic Oscillation (BAO) method that requires the redshift measurements of millions of galaxies. In the Euclid design the spectroscopic channel uses slitless low resolution grisms. Classical grisms, manufactured by replication of a ruled master on the hypotenuse of a prism, are extremely difficult to make for Euclid due to the combination of low groove density and small blaze angle. Two years ago we started an R&D program to develop grisms by the photolithography process that is well adapted to coarse gratings and allows introducing aberration correction by ruling curved and non parallel grooves. During the Euclid Phase A, we developed several prototypes made by photolithography and we present in this paper the test results done in the specific environment of the Euclid mission.