All-reflective optics for use in imaging spectrometers. In one example, an imaging spectrometer includes an entrance slit and spectrometer optics consisting of a primary mirror, a secondary mirror having a diffraction grating formed thereon, and a tertiary mirror, each of the primary mirror, the secondary mirror, and the tertiary mirror having free-form non-rotationally symmetric surface figures defined by Zernike polynomials. The spectrometer optics are configured to receive the telecentric image at the entrance slit, to disperse the optical radiation into its spectral components to provide dispersed optical radiation, and to focus the dispersed optical radiation onto an image plane positioned in a common plane with the entrance slit. The spectrometer optics have a physical length that is twice the length of the entrance slit, a ratio of a distance of the secondary mirror from the entrance slit to a distance of the primary mirror from the entrance slit being approximately 0.32.