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To address application demands for long slits, high spectral resolution, and compact system design, traditional Offner-type freeform surface imaging spectrometers face challenges of excessively high grating line density and manufacturing complexity. This paper proposes an innovative design for an off-axis three-mirror imaging spectrometer featuring a grating-prism combination with dispersion-corrected freeform surfaces. Building upon a modified Offner structure, the secondary mirror employs a freeform surface grating while prisms are systematically integrated into the optical path to share dispersion and synergistically correct aberrations. By configuring varying numbers (1-3) of prisms (only one being a freeform surface prism) at three critical locations-the input arm, secondary mirror, and output arm-12 distinct design configurations are achieved. A grating imaging spectrometer was concurrently designed for comparison. The resulting designs were rigorously compared against requirements: a wavelength range spanning from 400 to 1000 nm, a spectral resolution of 3 nm, a slit length of 60 mm, and an F-number of 3. Among these designs, a particular configuration demonstrated modulation transfer (MTF values) surpassing 0.7 at the Nyquist frequency of 34 lp/mm in full wavelengths while maintaining the smile and keystone less than one-tenth that of the pixel size. Tolerance analysis confirms that these freeform surfaces can be manufactured effectively. This design fulfills the criteria for both long slit functionality and compact architecture.
Zhang et al. (Wed,) studied this question.