Radetzky: a new, large grazing incidence interferometer for large plane surface testing

The ability to manufacture larger adaptive optics mirror rely also on the availability of metrology tools suitable for the purpose to validate the mirrors. In the framework of the ELT M4 adaptive, 2.5 m diameter, plane mirror we devised a series of tools able to certify that the plane wavefront is achieved and kept in place. A stitching series of small beams is conjugated with a whole image of the mirror thanks to a big, grazing incidence interferometric cavity. The set is completed by a novel piston sensing device which collate different segments of the (possibly) multiple mirror. We describe here the foreseen procedure and the status of the prototype realization.