A thin reflecting disk tightly bears on one edge of a case for defining a closed volume, means being provided in the latter for producing a negative pressure, and the bottom of the case is provided with supports arranged in parallel rows of three supoprts, a small beam being advantageously interposed between the disk and each row of supports when this mirror is used for focusing a laser beam, it is possible, by simply regulating the supports, to modify the shape of the disk so as to, e.g. obtain a rectilinear focal zone and a constant energy concentration of the beam along the zone.