Concave holographic gratings function as both a dispersing and focusing element in monochromators and spectrographs. These gratings contain two focusing elements: the substrate, and the groove curvature. As a result, an instrument designer can replace a lens system by using concave gratings, leading to a reduction of optics in a system, and thus, cost. Another advantage of a system based on concave gratings is decreased optical aberrations such as coma and astigmatism. Concave holographic gratings make possible short radii gratings with low ƒ/# and flat-field imaging suitable for array spectrographs.