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Design of freeform mirrors in Czerny-Turner spectrometers to suppress astigmatism.

Astigmatism is left uncorrected in traditional Czerny-Turner spectrometers with spherical mirrors, which leads to low throughput of modern instruments applying line-array detectors. By gradually varying the sagittal curvature, freeform mirrors are introduced to suppress astigmatism for a wide wavelength range simultaneously. So as not to reduce spectrum resolution, further calculations of the light path are performed for extra coma compensation. A design example is presented with optimized parameters. The ray-tracing result has revealed a reduction in sagittal spot size from several hundred micrometers to around 10 microm in the wavelength range from 200 nm to 800 nm.