A concave diffraction grating optical spectrometer comprising: - an entry slot adapted to receive and form an incident beam comprising at least one wavelength λ, - a concave diffraction grating adapted to receive the incident beam under an angle of incidence α et to diffract said beam to form a diffracted beam (6, 6a, 6b, 6c) forming an angle β(λ) with the normal to the grating; - at least one concave mirror (8, 8a, 8b, 8c) formed of a cylindrical portion having a circular cross-section of radius r cyl, said mirror (8, 8a, 8b, 8c) being adapted to receive the diffracted beam (6, 6a, 6b, 6c) in the direction β(λ) and to focus it to at least one exit slot (4, 4a, 4b, 4c), and - at least one photodetector (9, 9a, 9b, 9c) optically coupled to said exit slot (4, 4a, 4b, 4c) to measure the diffracted beam (6, 6a, 6b, 6c), focused by said cylindrical mirror (8, 8a, 8b, 8c) characterized in that: - it comprises at least one cylindrical mirror (8a, 8c) of radius of curvature r cyl1 and at least one cylindrical mirror of radius of curvature r cyl2 different from r cyl1; - the centre of the at least one mirror of radius of curvature r cyl1 (8a, 8c) is located at a distance d(λ) towards the diffraction grating with respect to the focus point of the diffracted beam (6a, 6c) in the direction β(λ) on the Rowland circle, such that: d λ = r cyl 1 / 2 3 / 2 / cos β; - the centre of the at least one mirror of radius of curvature r cyl2 is located at another distance d(λ) towards the diffraction grating with respect to the focus point of another diffracted beam in another direction β(λ) on the Rowland circle, such that: d λ = r cyl 2 / 2 3 / 2 / cos β; - a straight line perpendicular to the axis of the at least one cylindrical mirror (8a, 8c) of radius of curvature r cyl1 and contained in the plane of diffraction forms an angle β(λ) with the beam diffracted in the direction β(λ); - a straight line perpendicular to the axis of the at least one cylindrical mirror of radius of curvature r cyl2 and contained in the diffraction plane forms an angle β(λ) with the other beam diffracted in the other direction β(λ); - the diffracted beam (6a, 6c) is focused by the mirror of radius of curvature r cyl1 in an image plane parallel to the diffraction plane located at a distance equal to r cyl1 / 2 3/2 from the diffraction plane; - the other diffracted beam is focused by the mirror of radius of curvature r cyl2 in another image plane parallel to the diffraction plane located at a distance equal to r cyl2 / 2 3/2 from the diffraction plane; - the at least one cylindrical mirror (8a, 8c) of radius of curvature r cyl1 is arranged alternately with respect to the at least one cylindrical mirror of radius of curvature r cyl2 according to the increasing diffraction angle β(λ). - an entry slot adapted to receive and form an incident beam comprising at least one wavelength λ, - a concave diffraction grating adapted to receive the incident beam under an angle of incidence α et to diffract said beam to form a diffracted beam (6, 6a, 6b, 6c) forming an angle β(λ) with the normal to the grating; - at least one concave mirror (8, 8a, 8b, 8c) formed of a cylindrical portion having a circular cross-section of radius r cyl, said mirror (8, 8a, 8b, 8c) being adapted to receive the diffracted beam (6, 6a, 6b, 6c) in the direction β(λ) and to focus it to at least one exit slot (4, 4a, 4b, 4c), and - at least one photodetector (9, 9a, 9b, 9c) optically coupled to said exit slot (4, 4a, 4b, 4c) to measure the diffracted beam (6, 6a, 6b, 6c), focused by said cylindrical mirror (8, 8a, 8b, 8c) characterized in that: - it comprises at least one cylindrical mirror (8a, 8c) of radius of curvature r cyl1 and at least one cylindrical mirror of radius of curvature r cyl2 different from r cyl1; - the centre of the at least one mirror of radius of curvature r cyl1 (8a, 8c) is located at a distance d(λ) towards the diffraction grating with respect to the focus point of the diffracted beam (6a, 6c) in the direction β(λ) on the Rowland circle, such that: d λ = r cyl 1 / 2 3 / 2 / cos β; - the centre of the at least one mirror of radius of curvature r cyl2 is located at another distance d(λ) towards the diffraction grating with respect to the focus point of another diffracted beam in another direction β(λ) on the Rowland circle, such that: d λ = r cyl 2 / 2 3 / 2 / cos β; - a straight line perpendicular to the axis of the at least one cylindrical mirror (8a, 8c) of radius of curvature r cyl1 and contained in the plane of diffraction forms an angle β(λ) with the beam diffracted in the direction β(λ); - a straight line perpendicular to the axis of the at least one cylindrical mirror of radius of curvature r cyl2 and contained in the diffraction plane forms an angle β(λ) with the other beam diffracted in the other direction β(λ); - the diffracted beam (6a, 6c) is focused by the mirror of radius of curvature r cyl1 in an image plane parallel to the diffraction plane located at a distance equal to r cyl1 / 2 3/2 from the diffraction plane; - the other diffracted beam is focused by the mirror of radius of curvature r cyl2 in another image plane parallel to the diffraction plane located at a distance equal to r cyl2 / 2 3/2 from the diffraction plane; - the at least one cylindrical mirror (8a, 8c) of radius of curvature r cyl1 is arranged alternately with respect to the at least one cylindrical mirror of radius of curvature r cyl2 according to the increasing diffraction angle β(λ).