Abstract
Monochromators having a very single scanning mechanism similar to the Seya-Namioka type [1] have been designed in the past for the V. U. V. spectral range. They are working generally at normal incidence, the angle between the two fixed beams being chosen less than 70° in order to reduce the amount of astigmatism (ASM 50 [2] or H 20 monochromators manufactured by Jobin-Yvon). However such type of mountings are presently limited at wavelengths longer than 800 A by the low efficiency of the reflecting power of the coatings generally used. At high incidence angle, we find low efficiency caused by astigmatism, accompanied by an equivalent reduction in energy throughput. Efforts have been made to correct this astigmatism in part by using a holographic grating. However, for short wavelengths at grazing incidence, a spherical concave holographic grating cannot have much effect on the astigmatism. This has led us to using toroidal blanks.
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