Ground-based observations of the terrestrial dayglow emissions require the measurement of a small signal embedded in a large background. A high sensitivity imaging spectrometer has been developed to perform these measurements. The instrument is capable of operating over a full diurnal cycle with photometric like sensitivity to provide simultaneous measurements from the ground of the rich spectral content of the daytime mesosphere, thermosphere and ionosphere spectra. This low-f-number wide field-of-view imaging spectrometer is capable of measuring the day-glow spectrum over the wavelength range of 300 to 880 nm with a very high spectral resolution of 0.5 nm. The spectrometer has a field-of-view of 14 degrees without any vignetting. The spectrometer is designed to work with a 1024 by 1024 pixel UN-enhanced CCD detector having a 24.6 mm by 24.6 mm imaging area. A five-element telecentric lens group has been designed to achieve a compact and low cost package. The symmetric configuration of this lens group eliminates the optical distortions to permit the use of a simple curved slit. Three identical lens groups have been used for the telescope, collimator, and imaging optics to simplify the design and to reduce the fabrication cost. The optical design and performance analysis results for this imaging spectrometer are presented. The optomechanical design and modeling of the instrument is also discussed.