A three-aspherical-mirror anastigmat telescope comprising at least a concave first mirror, a convex second mirror, a concave third mirror and a first 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 disposed between the second mirror and the third mirror, the third mirror forming, from this intermediate image, a final image in a first focal plane (P F) of the telescope, in which plane the first 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 to a plurality of focal lengths (fi) 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 plurality of aspherical optical components (CAi) respectively being associated with the plurality of focal lengths (fi), placed in a plurality of positions (PCAi) respectively located between the first and the second position, each aspherical component being placed on an optical path of a beam corresponding to said associated focal length when the telescope is working at said associated focal length, and off the optical path associated with another focal length when the telescope is working at said other focal length, - means for changing the optical path placed between the aspherical components and the first detector, and configured so as that said first detector stays in in the first focal plane of the telescope, - the third mirror having a conicity (c'3) determined from an initial conicity, the initial conicities being determined from the Korsch equations, the conicity (c'3) being determined so that the telescope has, without said aspherical components being present and for the minimum and maximum focal lengths, compensable astigmatism and coma aberrations by said aspherical components, - the position (PCAi) and the form of the surface (Si) of each aspherical component being determined so as to correct said compensable aberrations of said telescope for the associated focal length (fi) and to optimize the image quality in the first focal plane of the telescope according to a pre-set criterion, the form of the surface (Si) of each aspherical component comprising at least a first and a second aberration type, the first aberration type being the first-order spherical aberration (Z9 CA) and the second aberration type being the focus (Z4 CA). - the three mirrors being arranged so that the first mirror and the second mirror form, from an object at infinity, an intermediate image disposed between the second mirror and the third mirror, the third mirror forming, from this intermediate image, a final image in a first focal plane (P F) of the telescope, in which plane the first 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 to a plurality of focal lengths (fi) between at least a minimum focal length (fmin) and a maximum focal length (fmax), - a plurality of aspherical optical components (CAi) respectively being associated with the plurality of focal lengths (fi), placed in a plurality of positions (PCAi) respectively located between the first and the second position, each aspherical component being placed on an optical path of a beam corresponding to said associated focal length when the telescope is working at said associated focal length, and off the optical path associated with another focal length when the telescope is working at said other focal length, - means for changing the optical path placed between the aspherical components and the first detector, and configured so as that said first detector stays in in the first focal plane of the telescope, - the third mirror having a conicity (c'3) determined from an initial conicity, the initial conicities being determined from the Korsch equations, the conicity (c'3) being determined so that the telescope has, without said aspherical components being present and for the minimum and maximum focal lengths, compensable astigmatism and coma aberrations by said aspherical components, - the position (PCAi) and the form of the surface (Si) of each aspherical component being determined so as to correct said compensable aberrations of said telescope for the associated focal length (fi) and to optimize the image quality in the first focal plane of the telescope according to a pre-set criterion, the form of the surface (Si) of each aspherical component comprising at least a first and a second aberration type, the first aberration type being the first-order spherical aberration (Z9 CA) and the second aberration type being the focus (Z4 CA).