A new formula has been developed and verified to account for the location of possible anomalous behavior of a plane diffraction grating when illuminated at skew incidence as in the Grieg and Ferguson mounting. It is shown that for their mounting, in contrast to both Littrow and Ebert mountings, blaze angles can be chosen which eliminate possible anomalous behavior in the observed spectra.
C. Harvey Palmer, "Anomalous Behavior of Gratings at Skew Incidence," Appl. Opt. 7, 353-356 (1968)