MORFEO@ELT: extreme large size dichroic mirror

MORFEO is the Multi-Conjugated Adaptive Optics (MCAO) module for the European Extremely Large Telescope (ELT). It provides a one to one imaging of the telescope focal surface and it is equipped with two post-focal deformable mirrors together with the Laser Guide Star (LGS) and Natural Guide Star (NGS) channels for wavefront sensing and tomographic reconstruction. One of the key optical element is a meter scale flat dichroic mirror, which is a sort of extended hot mirror. Indeed, it has to reflect the NIR spectral range up to the K band used for both science (primarily MICADO) and the NGS; Moreover, it has to efficiently transmit the sodium line used in the LGS. In this paper, the specifications of the dichroic are discussed reporting the manufacturing possibilities and the expected performances. In addition, the main issues related to the production and test of the optical element are discussed.