A compact spectrograph based on a VLS grating for the range of 3–20 nm

A compact spectrograph for the soft X-ray range has been developed based on a Variable Line Spacing (VLS) diffraction grating operating in the first internal order of diffraction. The design incorporates a number of innovative solutions that allow for a reduction in the size of the spectrograph for greater mobility, as well as an increase in light collection efficiency. The main optical elements are assembled on a single base, which eliminates misalignment of the optical scheme within the spectrograph. A commercial CMOS matrix GSENSE2020BSI with dimensions of 13.3 $\times$ 13.3 mm and a pixel size of 6.5 $\mu$m, featuring high quantum efficiency in the soft X-ray range, is used as the detector. The optical scheme of the spectrograph includes a focusing mirror with a dual-layer reflective coating of Cr/C, which enhances the light collection efficiency of the spectrograph. Measurements of the diffraction efficiency of the grating in the first internal order have been conducted, which, along with the calculated dependence of the resolution on the wavelength, indicate a sufficiently high sharpness of the spectral image of the source. The article also presents the X-ray optical scheme of the spectrograph, calculated X-ray optical characteristics, a 3D image, and a photo of the spectrograph.