A three-aspherical-mirror anastigmat telescope comprising at least a concave first mirror, a convex second mirror, a concave third mirror and a detector, and having an optical axis, - the three mirrors being arranged so that the first mirror and the second mirror form, from an object at infinity, an intermediate image between the second mirror and the third mirror, the third mirror forming, from this intermediate image, a final image in the focal plane of the telescope, in which plane the detector is placed, the first, second and third mirrors being of a set form characterized by at least a radius of curvature and a conicity, the telescope furthermore comprising: - means for moving the third mirror linearly along the optical axis of the telescope so as to make the focal length of the telescope change between at least a minimum focal length (fmin) and a maximum focal length (fmax), the telescope having, at the minimum focal length, a first exit pupil in a first position, and the telescope having, at the maximum focal length, a second exit pupil in a second position, characterized in that the telescope further comprises: - a deformable and controllable mirror having a deformable surface and placed in a fixed median position (Pm) between the first and second positions, - means (10, 10') for changing the optical path between the deformable mirror and the detector, which means are configured so that the detector remains positioned in the focal plane of the telescope, - the third mirror having a conicity (c'3) determined from an initial conicity, the initial conicity being determined from the Korsch equations, the conicity (c'3) being determined so that the telescope has, in the absence of said deformable mirror and for the minimum and maximum focal lengths, astigmatism and coma aberrations that are compensable by said deformable mirror, - said fixed median position (Pm) of said deformable mirror and the form of its surface, for the minimum focal length (Smin) and maximum focal length (Smax) respectively, being determined so as to correct said compensable aberrations and to optimize image quality in the focal plane of the telescope according to a preset criterion, said form of the deformable mirror surface comprising at least a first and a second aberration category, the first aberration category being the first-order spherical aberration (Z9 MD) and the second aberration category being focus (Z4 MD). - the three mirrors being arranged so that the first mirror and the second mirror form, from an object at infinity, an intermediate image between the second mirror and the third mirror, the third mirror forming, from this intermediate image, a final image in the focal plane of the telescope, in which plane the detector is placed, the first, second and third mirrors being of a set form characterized by at least a radius of curvature and a conicity, - means for moving the third mirror linearly along the optical axis of the telescope so as to make the focal length of the telescope change between at least a minimum focal length (fmin) and a maximum focal length (fmax), - a deformable and controllable mirror having a deformable surface and placed in a fixed median position (Pm) between the first and second positions, - means (10, 10') for changing the optical path between the deformable mirror and the detector, which means are configured so that the detector remains positioned in the focal plane of the telescope, - the third mirror having a conicity (c'3) determined from an initial conicity, the initial conicity being determined from the Korsch equations, the conicity (c'3) being determined so that the telescope has, in the absence of said deformable mirror and for the minimum and maximum focal lengths, astigmatism and coma aberrations that are compensable by said deformable mirror, - said fixed median position (Pm) of said deformable mirror and the form of its surface, for the minimum focal length (Smin) and maximum focal length (Smax) respectively, being determined so as to correct said compensable aberrations and to optimize image quality in the focal plane of the telescope according to a preset criterion, said form of the deformable mirror surface comprising at least a first and a second aberration category, the first aberration category being the first-order spherical aberration (Z9 MD) and the second aberration category being focus (Z4 MD).