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Figure 1
(a) A schematic diagram of a VLS grating on a concave substrate to achieve an excellent flat-field condition in the meridional plane within the spectral range 2–5 nm (bottom) is compared with a plane grating with constant groove density whose focal spots for the same spectral range lie in a curved line (top). (b) A change of the meridional focal curves of the spectrometer is observed when applying different meridional radii R, while the identical parameters are as follows: D0 = 24000 lines cm−1, rm = 1000 cm, [{ \alpha }={89.124}]° and the image focal length [{{r}}_{{20}}^{\,\prime}{(}{{\lambda }}_{{0}}{)}={200}] cm at the center wavelength [{{\lambda }}_{{0}}={3.5}] nm. The straight fitting lines represent the best detector plane for each R, where the value [{{\delta }}_{{\rm m}}] is the magnitude of `defocus' over the whole spectral range [defined in §2.1 and equation (10)[link]]. The fitting lines for R = 9000 and 52729 cm are depicted, while the diffraction beams of 2 nm and 5 nm are shown simultaneously. The calculated value [{{\delta }}_{{\rm m}}] for each case is: [{{\delta }}_{{\rm m}}={0.01346}] cm (R = infinity), [{{\delta }}_{{\rm m}}={0.0018}] cm (R = 52729 cm), [{{\delta }}_{{\rm m}}={0.22}] cm (R = 20000 cm), [{{\delta }}_{{\rm m}}={0.6381}] cm (R = 9000 cm).

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ISSN: 1600-5775
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