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Reduction of Aberrations

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Reduction of Aberrations

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In the design of grating systems, there exist several degrees of freedom whose values may be chosen to optimize image quality. For monochromators, the locations of the entrance slit A and exit slit B relative to the grating center O provide three degrees of freedom (or four, if no plane of symmetry is imposed); the missing degree of freedom is restricted by the grating equation, which sets the angular relationship between the lines AO and BO. For spectrographs, the location of the entrance slit A as well as the location, orientation and curvature of the image field provide degrees of freedom (though the grating equation must be satisfied). In addition, the curvature of the grating substrate provides freedom, and the aberration coefficients H'ij for a holographic grating (or the equivalent terms for a VLS grating) can be chosen to improve imaging. Even in systems for which the grating use geometry (the mount) has been specified, there exist several degrees of freedom due to the aberration reduction possibilities of the grating itself.

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