Comparison of a compact imaging spectrometer with long-slit designs utilizing various modified Offner structures and freeform surface grating-prism dispersion elements

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.