THE DESIGN OF ABERRATION-CORRECTED CONCAVE HOLOGRAPHIC GRATINGS FOR THE LYMAN-FAR-ULTRAVIOLET SPECTROSCOPIC EXPLORER HIGH-RESOLUTION SPECTROGRAPH

A design procedure for two aberration-corrected spherical concave grat ings operating in the spectral range 91-103 nm (extreme-ultraviolet) i s presented. The first configuration is a Rowland circle mount whereas the second is based on the conditions of stationarity with respect to wavelength for the second-order aberration coefficients, as well as t he condition for stationary meridional coma. The first design has bett er resolution whilst the latter design has a better overall degree of aberration correction. Holographic gratings recorded with two divergin g stigmatic sources are considered for both designs. The theoretical p erformance of these gratings is very good across the spectral range of interest. The combination of simplicity and comparatively low cost to gether with very good aberration correction performance makes these gr ating designs attractive for the Lyman Far-Ultraviolet Spectroscopic E xplorer.