Abstract

The optical properties of plane-grating monochromators such as the Ebert and Czerny—Turner types are examined by geometrical optics. The grating equation and the expression for the focal surface are given; by using them, expressions are found for the image form; the optical system that minimizes wavelength impurity is discussed; and such residual aberrations as coma and astigmatism are examined collectively. In any monochromator, the focal surface depends upon the grating position and the off-axial properties of the collimator. In an Ebert-type monochromator with a circular-arc-shaped entrance slit the image form is a circular arc only in a symmetric arrangement; it is elliptic in other arrangements; the light through a circular-arc-shaped exit slit is free of wavelength error; and even if the slit is long, the image will not be much blurred by residual aberrations. Therefore, it is advisable to use long circular-arc slits if the system is of the Ebert type. In the Ebert- or Czerny—Turner-type monochromator with a straight entrance slit the image is parabolic. In the Czerny—Turner type both the image form and focal surface are affected by its nonconcentric character.

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