A Dyson imaging spectrometer comprising: • an entry port extending in a direction X in an object plane of the spectrometer, said entry port being adapted to emit an incident light beam; • a diffraction grating comprising a set of lines on a concave support; • a refractive optical system comprising a lens, said lens comprising a plane first face and a convex second face, the convex face of the lens and the concave face of the diffraction grating being concentric, said optical system being adapted to receive the incident light beam and to direct it toward the diffraction grating, to receive a beam diffracted by the diffraction grating, and to form a spectral image of said diffracted beam, said spectral image lying in an image field in an image plane of the spectrometer; and • an exit port in the image plane of the spectrometer adapted to receive an image of the entry port spatially resolved in a direction X' and spectrally resolved in a direction Y'; characterized in that: said diffraction grating comprises a set of non-parallel and non-equidistant lines and/or the support of the diffraction grating is an aspherical ellipsoidal surface of revolution in order to improve the image quality of the spectral image in the image field at the same time as minimizing spectral and field distortions, • the optical system further comprises a hemispherical lens and a plate with plane and parallel faces, said plate lying against the plane face of the lens and being disposed on the optical path of the incident beam between the entry port and the diffraction grating, the object plane of the spectrometer being situated on one face of the plate and the image plane of the spectrometer being separated from the lens by a gap, • the materials of the lens and the plate being chosen to compensate chromatic dispersion over the image field. • an entry port extending in a direction X in an object plane of the spectrometer, said entry port being adapted to emit an incident light beam; • a diffraction grating comprising a set of lines on a concave support; • a refractive optical system comprising a lens, said lens comprising a plane first face and a convex second face, the convex face of the lens and the concave face of the diffraction grating being concentric, said optical system being adapted to receive the incident light beam and to direct it toward the diffraction grating, to receive a beam diffracted by the diffraction grating, and to form a spectral image of said diffracted beam, said spectral image lying in an image field in an image plane of the spectrometer; and • an exit port in the image plane of the spectrometer adapted to receive an image of the entry port spatially resolved in a direction X' and spectrally resolved in a direction Y'; said diffraction grating comprises a set of non-parallel and non-equidistant lines and/or the support of the diffraction grating is an aspherical ellipsoidal surface of revolution in order to improve the image quality of the spectral image in the image field at the same time as minimizing spectral and field distortions, • the optical system further comprises a hemispherical lens and a plate with plane and parallel faces, said plate lying against the plane face of the lens and being disposed on the optical path of the incident beam between the entry port and the diffraction grating, the object plane of the spectrometer being situated on one face of the plate and the image plane of the spectrometer being separated from the lens by a gap, • the materials of the lens and the plate being chosen to compensate chromatic dispersion over the image field. • the optical system further comprises a hemispherical lens and a plate with plane and parallel faces, said plate lying against the plane face of the lens and being disposed on the optical path of the incident beam between the entry port and the diffraction grating, the object plane of the spectrometer being situated on one face of the plate and the image plane of the spectrometer being separated from the lens by a gap, • the materials of the lens and the plate being chosen to compensate chromatic dispersion over the image field.