Soft X-ray flat-field VLS spectrographs

We have analysed the capabilities of flat-field spectrometers with concave VLS reflection gratings from the standpoint of spectral imaging in the soft X-ray range. Two types of such instruments are shown to exist: spectrographs with as flat as possible a portion of the focal surface (of Harada spectrograph type) and those with a nearly constant distance from the grating centre to the focal surface, which permits reaching a high spatial resolution with the use of a crossed focusing mirror (without a loss in spectral resolution). Three 0.25, 0.5, and 1.5-m long spectrographs of Harada type were calculated for operation in the ranges 90 – 250, 50 – 200, and 20 – 110 Å, respectively. We show that each of the calculated instruments may be converted to an instrument of the second type for operation in the second and fourth diffraction orders without significant changes in scheme geometry. In this case, the theoretical spatial resolution is equal to ~10 μm throughout the optimisation range.