A novel low-f-number, wide-field-of-view imaging spectrometer has been designed for measuring the day-glow spectrum over the wavelength range of 260 to 870 nm with spectral resolutions of 0.5 and 0.03 nm. The zero-obscuration all-reflective design is an f/2.0 imaging spectrograph using commercial gratings. The field of view along the spatial direction is 6 deg, with a spatial resolution of 0.1 mrad. The spectrometer is designed to work with a commercially available 1037 x 1340 CCD detector with 6.8 x 6.8-μm pixel size. The imaging spectrometer optics consists of an aspheric toroidal telescope, a slit, an aspheric toroidal collimator, a planar reflective grating, and three off-axis higher-order aspheric imaging mirrors. Significant improvements in the performance have been achieved by introducing aspheric toroidal elements in the design. The weight and size have been reduced by a factor of 20 as compared to previous similar instruments. A virtual prototype of the instrument has also been modeled by using integrated optical and mechanical design software.