Approaches to the interferometric test of large flat mirrors: the case of the adaptive M4 for E-ELT

A particular example of meter class flat mirrors is the adaptive M4 Unit of E-ELT, a deformable six petals mirror of 2.4m in diameter. We studied different approaches to the calibration and certification of M4, in a trade-off between stitching and full aperture measurements. Possibilities to test the mirror with a macro-stitching concept, both in normal and grazing incidence have been considered. Approaches reported in the literature, as the Ritchey-Common or the external Fizeau, and different beam expander setups, varying the collimating mirror and the nulling system, both on-axis and off-axis, have been deeply studied to understand performances and sensitivities to fabrication errors, alignment errors and environmental effects.