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The favorite solution foreseen for the realization of future very large x-ray mirror modules (diameters above 1 m) is the partition of the optics in azimuthal and radial modules. 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 aligned and assembled without degrading their optical performances. The usage of large monolithic shells would provide indubitable advantages with respect to the segmented approach, but poses challenging procurement problems. For example, in the case of Lynx, where superb imaging capabilities are combined with a very large effective area, a mirror module with 3mdiameter is foreseen. One of the three technologies considered by the Study Team for the realization of the mirrors is based on monolithic glass (fused silica) shells, figured with direct polishing technique. The simplicity of the full shell concept is quite attractive: the complete optics module could be composed of few hundreds of parts instead of several thousands of pieces. As a drawback, to be compliant with the mass budget, the shell thickness should be maintained very small even for larger mirror shells. Given that the glass is a brittle material, the procurement of such large raw shells, and of their spares, is considered the major critical issue of the process. In order to overcome the difficulties and the cost related to the assembly of thousands of segments, a straightforward way to realize very large monolithic raw mirrors shells is needed. In this paper we present the first results related to an alternative approach, based on light-weighted metal sheets, which could have an astonishing impact on the full shell concept, providing very large shells at negligible costs. In particular, we focus on spin forming technology, commonly used for the realization of axial-symmetric parts. This process makes use of a rotating machine to deform metal over a pre-shaped mold. As a replica approach, it allows the realization of several back up optics without major impacts on the costs, as it would be the case if the mirror shell realization starts from blanks machining. Aluminium is commonly used in plate turning. Given its low density and good mechanical properties, it may represent an effective and cheap alternative to more exotic low-density materials or to the glass itself. Metal optics, made of Aluminium 6061, are already widely used to fulfil the demands of a-thermal instrument design. Starting from aluminum substrates, diamond turning can be used for figuring the optic. An additional thin layer of electro-less nickel will allow final figuring and polishing. These further figuring processes are nowadays well proven on thick substrates, allowing the realization of optics with cutting-edge performances. Their successful employ on thin shell substrates would open new window on x-ray optics realization.
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