Two approaches to reducing optical aberrations of concave grating spectrographs have been used, holographically controlling the groove curvature and spacing and reshaping the optical substrate while ruling the grooves conventionally. The latter approach, slightly deforming an ellipsoidal grating blank, can lead to diffractionlimited performance at a single far ultraviolet wavelength. When such a grating is used in a slitted Rowland circle spectrograph, the result is an extremely efficient spectrograph with spectral resolving power of ∼30,000 and low astigmatism. Optical fabrication technology has advanced to the point where these exotic surface gratings are becoming practical.
David Content, Catherine Trout, Pam Davila, and Mark Wilson, "Aberration corrected aspheric gratings for far ultraviolet spectrographs: conventional approach," Appl. Opt. 30, 801-806 (1991)