A design for a new plane grating monochromator is described. The system employs a symmetrical configuration of off-axis paraboloids for both the collimator and camera mirrors. The entrance and exit slits are mounted on the same side of the grating and are curved in order to eliminate wavelength errors due to spectral line curvature. There is, in fact, a remarkable coincidence between the curvature which is required to avoid wavelength errors and the curvature which is required to obtain equal curvatures in the object and image planes. Ray tracing calculations for a symmetrical configuration of off-axis paraboloids show that the image of a curved slit with a length equal to 0.05 times the focal length of an f/5 mirror is diffraction limited for wavelengths λ > 0.2 μ.
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