Interferometry at the pillars of Hercules: or measurements in high holes density, low reflectivity, low modulation regime

We investigate the interferometric measureability of the silicon carbide Reference Body of the ELT adaptive mirror M4. The sampling is technically challenging, because of the low fringes modulation due to poor surface finish and to the extremely large number of holes in the a perture. We describe our approach to face and solve such criticalities, based on laboratory experimentations with a Twyman Green interferometer on a dedicated optical setup; we comment the feasibility of such measurement in a real environment and present in the end a checklist to enable interferometer measurements in such un-favourable conditions.