Holographic spherical gratings: a new family of quasi-stigmatic designs for the Rowland-circle mounting.
In the context of aberration-corrected holographic spherical gratings used in the Rowland-circle mounting, we have investigated the off-Rowland recording geometries with stigmatic sources that nullify the defocus and meridional coma at all wavelengths. We introduce the additional requirement that astigmatism vanishes at a given wavelength. Then we demonstrate that a family of solutions exists and has a quasi-zero sagittal coma at the wavelength adopted for the astigmatism correction. This simultaneous reduction of sagittal coma and astigmatism greatly enhances the spectral performances of the Rowland mount. We also point out that a subfamily is found when the derivative of astigmatism is equal to zero at the wavelength of correction; the performances are then extended to a wider spectral range. Finally ray traces and performances of two representative examples of the new family of gratings are presented.