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A technique to obtain lightweight and high-resolution focusing mirror segments for large aperture X-ray telescopes is the
hot slumping of thin glass foils. In this approach, already successfully experimented to manufacture the optics of the
NuSTAR X-ray telescope, thin glasses are formed at high temperature onto a precisely figured mould. The formed glass
foils are subsequently stacked onto a stiff backplane with a common axis and focus to form an XOU (X-ray Optical Unit),
to be later integrated in the telescope optic structure. In this process, the low thickness of the glass foils guarantees a low
specific mass and a very low obstruction of the effective area. However, thin glasses are subject to deformations that may
arise at any stage of the production process, thereby degrading the angular resolution. To solve this problem, several groups
are working on the possibility to correct the mirror profile post-manufacturing, using piezoelectric elements exerting a
tangential strain on the non-optical side of the glass mirrors. In this paper we show the results of the approach we have
adopted, based on the application of piezoceramic patches on the backside of thin glass foils, previously formed by hot
slumping. The voltage signals are supplied to the piezoelectric elements by a system of electrodes deposited on the same
side of the mirror via a photolithographic process. Finally, the matrix of voltages to be used to correct the mirror shape can
be determined in X-rays illumination by detection of the intra-focal image and consequent reconstruction of the
longitudinal profile. We describe the production of some active mirrors with different arrangements of piezoelectric
elements and the X-ray tests performed at the XACT X-ray facility to determine the optimal actuator geometry.
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