Aberration-reduced holographic spherical gratings for Rowland circle spectrographs

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Abstract
We used a numerical minimization method to design a Rowland holographic spherical grating that is recorded with two stigmatic laser sources. The method aims at simultaneously reducing all aberrations up to fourth order over a significant spectral range. In the context of a high spectral resolution, far-ultraviolet spectrograph, an original solution is found that implies a nonclassical recording geometry with one virtual source. This solution satisfies the requirement of a resolving power of ~ 30 000 with the unquestionable advantage of manufacturing and testing simplicity. Finally, another example, which is obtained in a different context, shows that the properties of this recording geometry probably have a general applicability.