On the Tender X-Ray Flat-Field Spectrograph with a Concave VLS Grating

We analyze the way of advancing to tender X-rays the short-wavelength bound of the operating range of flat-field grating spectrographs. We design two VLS grating spectrographs with flat fields covering a range of 2 – 13 Å, using spherical ( R = 30 and 90 m) VLS gratings mounted at grazing angles of 1.5° and 0.5°. Numerical ray tracing suggests that the width of the spectral image of a point monochromatic source is well within 26 μm, which corresponds to the resolution limit of a CCD detector with a pixel size of 13 μm. For the 1.5° instrument, a sufficiently high reflectivity may be provided by a multilayer coating (e.g., Cr/C) with a period gradient aligned with the grating lines, which eliminates the overlap of different diffraction orders due to the wavelength dispersion of the optical constants.