Divergent groove gratings: wavelength scanning in fixed geometry spectrometers

A new geometric scheme translates a diffraction grating along the straight central groove of an exponentially curved pattern. Lit by a stationary incident beam, the two-dimensional pattern scales isotropically, scanning wavelength without change to any angles, macroscopic distances, curvatures or aberrations. This is exemplified by a new class of self-focused grating monochromator, analyzed by rigorous light-path expansion and numerical raytracing. All spectral aberrations in pure meridional powers (including defocus, coma and spherical aberration) cancel for any choice of angular deviation, magnification and translation range. The residual mixed powers yield Δλ/λ = 10 -3  ~ 10 -5 in the soft x-ray for plane and concave gratings at grazing incidence. Over the visible spectrum, Δλ/λ ~ 10 -4 is shown for plane gratings mounted at Littrow and at normal incidence in reflection or transmission.