High acceptance high resolution soft X-ray grating spectrometer: Choice of optical design

Highlights: ► Optical design to maximize angular acceptance of soft X-ray grating spectrometer. ► Comparison between spherical and elliptical focusing mirror using numerical analysis of light path function. ► Resolving power taking into account mirrors aberrations and slope errors. -- Abstract: Elementary excitations in correlated electron systems produce low intensity spectral features that may be observed by resonant inelastic X-ray scattering (RIXS). In the soft X-ray region, ruled gratings are used to obtain optimum energy dispersion. Gratings have to operate at small grazing incidence so that in practice the angular acceptance is small. Schemes have been proposed using varied line spacing plane gratings combined with large focusing mirrors to increase acceptance. Here we analyze the relative performance of spherical and elliptical mirrors by means of numerical calculations of the light path function. We show that the use of an elliptical mirror provides higher resolving power over an extended energy range. Thus the 50–1000 eV energy range can be covered by two gratings only and the resolving power can be adapted to the experimental conditions