A spectral imager suitable for providing an image of a scene within a detection plane along a slit-along direction, and for producing simultaneously a spectral spreading along a slit-across direction which is perpendicular to the slit-along direction in the detection plane, said spectral imager comprising: - a radiation collecting optics, adapted for producing an intermediate image of the scene within an intermediate focal plane; - a spectrometer, suitable for imaging a content of a slit track in the detection plane along the slit-along direction, and for producing the spectral spreading along the slit-across direction with radiation that originates from the slit track; - a photodetector which comprises a two-dimensional array of photosensitive elements, and which is arranged in the detection plane so that rows of the photosensitive elements are parallel to the slit-along direction; and - an optical fibre bundle, which comprises a plurality of optical fibres (301, 302, 303...) extending each from an entrance optical head to an output optical head, the optical fibres having respective entrance ends aligned within the entrance optical head, and respective output ends aligned within the output optical head, the optical fibre bundle being arranged so that the entrance optical head is located in the intermediate focal plane, and the output optical head forms the slit track, and a length of the optical fibres (301, 302, 303...) in the optical fibre bundle between the entrance optical head and the output optical head being comprised between 40 cm and 2 m, the spectral imager further comprising: - bundle shape control means, that are suitable for fixedly determining a shape of the optical fibres (301, 302, 303...) between the entrance optical head and the optical output head, the bundle shape control means being adapted for maintaining curvatures of each one of the optical fibres (301, 302, 303...) within two planes (41, 42) that are angularly separated, with curvature radii comprised between 0.5 cm and 20 cm for each plane, the spectral imager being characterized in that the bundle shape control means are adapted for maintaining each one of the optical fibres (301, 302, 303...) along a wavy path in each of the planes (41, 42), with two or three turns in each plane, two successive turns in each plane being separated by a change in a curvature direction of said optical fibre. - a radiation collecting optics, adapted for producing an intermediate image of the scene within an intermediate focal plane; - a spectrometer, suitable for imaging a content of a slit track in the detection plane along the slit-along direction, and for producing the spectral spreading along the slit-across direction with radiation that originates from the slit track; - a photodetector which comprises a two-dimensional array of photosensitive elements, and which is arranged in the detection plane so that rows of the photosensitive elements are parallel to the slit-along direction; and - an optical fibre bundle, which comprises a plurality of optical fibres (301, 302, 303...) extending each from an entrance optical head to an output optical head, the optical fibres having respective entrance ends aligned within the entrance optical head, and respective output ends aligned within the output optical head, the optical fibre bundle being arranged so that the entrance optical head is located in the intermediate focal plane, and the output optical head forms the slit track, and a length of the optical fibres (301, 302, 303...) in the optical fibre bundle between the entrance optical head and the output optical head being comprised between 40 cm and 2 m, the spectral imager further comprising: - bundle shape control means, that are suitable for fixedly determining a shape of the optical fibres (301, 302, 303...) between the entrance optical head and the optical output head, - bundle shape control means, that are suitable for fixedly determining a shape of the optical fibres (301, 302, 303...) between the entrance optical head and the optical output head, the bundle shape control means being adapted for maintaining curvatures of each one of the optical fibres (301, 302, 303...) within two planes (41, 42) that are angularly separated, with curvature radii comprised between 0.5 cm and 20 cm for each plane, the spectral imager being characterized in that the bundle shape control means are adapted for maintaining each one of the optical fibres (301, 302, 303...) along a wavy path in each of the planes (41, 42), with two or three turns in each plane, two successive turns in each plane being separated by a change in a curvature direction of said optical fibre.