Aberrations of Plane Grating Monochromators with Off-axis Paraboloidal Mirrors

Aberrations of plane grating monochromators with off-axis paraboloidal mirrors are studied analytically by the use of the light-path function and the Burch plate diagram extended to off-axis systems. Residual wave aberration of the third order becomes vanishingly small by compensation of two aberration terms in the U configuration, but remains fairly large by accumulation in the Z configuration. This explains an excellent image quality of the U configuration. It is shown that exact slit-image-curvature invariance with wavelength exists in the Z configuration but does not in the U configuration. A suitable value of the distance between the grating and off-axis paraboloidal mirror can be determined to eliminate certain residual aberration terms of the fourth order.