Fig. 1. Schematic of the Offner imaging spectrometer

Fig. 2. Schematic of meridional focusing curve being tangent to sagittal focusing curve at λ¯ wavelength^[25]

Fig. 3. Schematic of astigmatism correlation for Offner imaging spectrometer^[33]. (a) Focusing curves of the Offner imaging spectrometer; (b) relationship between astigmatism and the diffraction angle β2

Fig. 4. Schematic of spectrometer configuration along optical axis angle of view^[36]. (a) In-plane configuration; (b) off-plane configuration

Fig. 5. Schematic of the quadrature off-plane configuration spectrometer along optical axis angle of view^[38]

Fig. 6. Schematic of an optical path for a conical diffraction off-plane configuration^[42]

Fig. 7. Schematic of concentric catadioptric convex grating configuration^[45]. (a) Convex grating disposed on outer surface of a meniscus lens; (b) convex grating disposed on inner surface of a meniscus lens

Fig. 8. Schematic of ultra-compact concentric catadioptric imaging spectrometer^[49]

Fig. 9. Schematic of a catadioptric configuration with reflective coating^[50]

Fig. 10. Schematic of +1st order diffraction configuration optical path^[52-53]. (a) Conventional configuration; (b) configuration with meniscus lens

Fig. 11. Schematic of +1st order diffraction optical path with three different image plane positions^[56]

Fig. 12. Comparison of dimensions and performance of three different face types of spectrometers^[63]. (a) All-spherical type; (b) compact all-spherical type; (c) compact freeform type

Fig. 13. Merit functions of different surface types^[64]

Fig. 14. Schematic of a multichannel freeform surface imaging spectrometer^[67]