A deformable mirror for reflecting a light beam incident on the mirror has a flexible section (2) provided with a reflecting surface (2a). and designed to be flexibly deformable. There is a substrate (6) with a reference surface positioned opposite the flexible section (2). A control device (7,8,12) is provided to allow the flexible member (2) to adhere on the reference surface of the substrate (6), so as to enable the flexible surface (2a) of the section (2) to correspond to the reference surface, and in which the reference surface is laid out so that the light beam is provided with a spherical aberration when the reflecting surface adheres to the reference surface.
