Secondary spectrum aberrations are especially destructive in high performance refractive optical systems having large apertures and long focal lengths. It is derived that secondary spectrum level of such systems is dependent on a certain structural sum and a deviation of the optical materials used in the system. The structural sum is the main reason of the secondary spectrum aberrations in the achromatic refractive optical systems built with ordinary materials. Reduction of secondary spectrum aberrations is based on the minimization of the structural sum and use of optical material deviation. Essence of the matter is both sign and magnitude of the deviation for the given spectrum direction. A coefficient of secondary spectrum reduction has been introduced. This coefficient receives values in interval from 0 to 1, where value 1 respects the theoretical correction. Medium contributions of this coefficient indicate, whether applied extraordinary optical materials are effective for the secondary spectrum reduction.