A double monochromator for the extreme ultraviolet

The optical design of a double monochromator suitable for use in the extreme uv is presented. The system employs fixed entrance and exit slits and two concave diffraction gratings which are rotated to vary the wavelength at the exit slit. Adjustment of the optical parameters and use of symmetry produces an approximate focusing condition accurate to fourth order in grating rotation. The defocusing at the exit slit is comparable to the Seya mounting. The astigmatism is calculated and the transmission properties briefly discussed.