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An X-ray Observatory, with superb imaging capabilities and with large throughput, has been recognised as a strategic missions in the Astro2020 decadal survey. The traditional solution foreseen for the realisation of very large x-ray mirror modules (diameters above 1 m) is the partition of the optics in azimuthal 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 monolithic shells. As an example, the complete opto-mechanical design, compliant with the Lynx mass budget and based on fused silica, foresees that the shell thickness ranges between 2 and 4 mm (for mirror shells between 0.4 and 3 m diameter). The conceptual design of such an mirror module could be refined for smaller scale mission, keeping both the advantage of the design simplicity and of the high-resolution capability, achievable through the direct polishing approach. A technology development roadmap 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 realisation of two new single-reflection shells (SR shells), almost representative of the final optical configuration foreseen for the mirror assembly. The first shell will be used to prove the figuring process in a lab-mount, built upon elements of the previous supporting structure concept. The second shell will be hosted in an upgraded lab-mount structure, which guarantees better performances (frequencies, gravity and thermo-elastic response) and which is suitable to test the transfer of the shell to a spider-like configuration.
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