To study the x‐ray laser beam deflection and divergence due to the refraction within the plasma, a specific diagnostic called CADIX has been developed at the Centre d’Etudes de Limeil‐Valenton (CEL‐V). When the beam refraction analysis is required a multilayer mirror is inserted on the x‐ray laser output beam. It selects the lasing wavelengths and reflects them to an off‐axis streak camera which records x rays as a function of time and angle. A detailed description of the instrument is presented including multilayer mirror calibration at the LURE (Laboratoire pour L’Utilisation du Rayonnement Synchrotron, Orsay, France). An experimental measurement on neon‐like silver amplification is presented and analyzed with the response of the mirror. An important refraction effect is observed.