One of the concerns with the Megajoule Laser design is the transport mirrors laser damage threshold. Earlier studies have shown that the main constraint on laser damage threshold comes from nodules at the mirror surface. It is therefore important to restrict both the number of such nodules and their dimensions: the larger they are, the greater is the resulting damage. SFIM-ODS, in close collaboration with CEL-V, has started a special-purpose study to characterize these nodules as precisely as possible. The objective of the study is twofold: to determine the origin of the nodules and subsequently to adapt the mirror fabrication process so as to restrict their formation, and to analyze their shapes and dimensions so as to determine which nodules are critical for laser damage. To understand the origin of the nodules and their effect on laser damage threshold, the mirrors are characterized by different methods: (1) absorption and scattering mapping: does the presence of nodules result in specific absorption?, (2) surface analysis by atomic force microscopy: to characterize nodule shape and dimensions, (3) Focused Ion Beam cutting of nodules: to locate the seed initiating the nodule (on the substrate or in the stack), and characterize the seed shape and composition (contamination, material flaw during evaporation, etc...), and (4) laser damage threshold measurements: to determine the mirror laser damage threshold and study the behavior of nodules under laser in accordance with their dimensions and shape.