The traditional solution foreseen for the realization of very large x-ray mirror modules (diameters above 1 m) is the partition of the optics in azimuth and radial modules (like Silicon Pore Optics in Athena). Even if this approach solves the initial problem of the procurement and the handling of very large substrates, it moves the difficulties in the second phase, when thousands of segments have to be assembled without degrading their optical performances. On the contrary, a simpler large mirror module design could correspond to less than a few hundred thin (2 mm to 4 mm for mirror shells between 0.4 m and 3 m diameter) monolithic shells. This configuration keeps both the advantage of the design simplicity and of the high-resolution capability, achievable through the direct polishing approach. Moreover, it guarantees an impressive gain in the polishing time. A technology development road map for this approach is funded in Italy by ASI and led by INAF-OAB. In this paper, we present the advancements obtained in the development of the different phases of the process and in the procurement of new substrates.
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