Coma can be eliminated from a grating spectrometer by a combination of a concave collimating mirror and a convex folding mirror, the angles of incidence being chosen so that coma from the first reflection is balanced by coma of equal magnitude and opposite sign from the second reflection. The principle is demonstrated with a spectrometer designed specifically for measuring the resolution performance of diffraction gratings. The instrument also incorporates a Fizeau flat for wavefront testing.