A sagittally confined high-resolution spectrometer in the `water window'.

The authors report a novel scheme for a grazing-incidence spectrometer forming an excellent meridional flat field in its detector domain to deliver the desired spectral resolution throughout the full designated spectral range, while reducing the sagittal astigmatism substantially to enhance the spectral intensity. The optical properties of the system are thoroughly investigated and optimized, and the detector plane is fitted well to the meridional or sagittal focal curves. The authors demonstrated that a resolving power of 6000-18000 could be achieved within the `water window' (2-5 nm) for an effective meridional source size of 200 µm (r.m.s.), and it would be further improved to 20000-40000 if the source size was confined to 50 µm (r.m.s.).