Astigmatism in noncentered mirror systems, including gratings, is briefly discussed. A formula for meridional coma is derived. It is shown how Seidel aberration terms can be calculated for the sagittal cross section, namely spherical aberration, Petzval curvature, and others. The advantages and limitations of this approach are pointed out.
GOTTFRIED R. ROSENDAHL, "Contributions to the Optics of Mirror Systems and Gratings with Oblique Incidence. II. A Discussion of Aberrations," J. Opt. Soc. Am. 52, 408-411 (1962)