Resolving power of multigrating spectrometers

A theoretical and experimental study of the resolving power of spectrometers employing several gratings with coacting dispersion is performed. It is concluded that the resolving power is equal to the sum of the resolving powers of the separate gratings in contradiction to a recent result obtained by Mazzacurati et al., Opt. Commun. 67, 399-403 (1988); J. Phys. E. 21, 798-804 (1988). Experimental verification of our theoretical prediction is obtained with the MEGA spectrometer containing four echelle gratings. The measured resolving power for the He-Ne 6328-A laser line is (2.05 +/- 0.03) x 10(6), in good agreement with the theoretical value of 2.06 x 10(6).