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The cold shaping (or cold slumping) replication of thin glass foils is an attractive solution for making future high throughput x-ray telescopes. A similar approach is being successfully used for making the normal-incidence mirrors of Cherenkov telescopes like Magic I and II and CTA. The process starts with a thin flat glass sheet that is curved by means of an imposed force on a precision mandrel with the desired nominal shape. After this forming, the shape of the sheet is blocked using a light and stiff structure usually fixed with a glue layer. At INAF - Osservatorio Astronomico di Brera we started to study this process in the past few years for grazing incidence x-ray mirrors with Wolter I (parabola + hyperbola) shape. The availability of flexible glass foils with thicknesses of a few tens of microns now allows for the fabrication of mirrors with very small curvature radii. To implement a mirror assembly, the foils are stacked together using stiffening ribs in order to connect a shell to the next one and form in this way a monolithic stack of curved foil. The accumulated stress during the bending is kept below affordable limits and the associated spring-back after the replication does not strongly affect the angular resolution. Moreover, other major advantages are the cost/time reduction w.r.t. to other approaches and the preservation of the intrinsic good roughness of the float glass sheets. Good targets for the application of this technology are x-ray missions for hard x-rays with angular resolution in the range of 15-60 arcsec, or IAXO (and demonstrator baby-IAXO) ground-based helioscopes for the detection of solar axions. The recent progress in the cold slumping is described in this paper with optical and mechanical design, presenting the measured results on a stack with three layers.
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