Monochromator having a toroidal holographic diffraction grating and utilizable in the ultraviolet band

In a monchromator usable at a wavelength .lambda. in the region of ultraviolet light and having a fixed inlet slot, a holographic grating, a concave turnable support carrying said grating and a fixed outlet slot, an improvement wherein the surface of the support of the holographic grating is a concave portion of a toroidal surface, taken at the external periphery of the toroid, and disposed symmetrically with respect to the equatorial plane of the toroid, said equatorial plane containing the point sources C,D for recording the holographic grating at a wavelength of .lambda.o, and the conjugated source-image points A,B, the coordinates of said points A,B,C and D in polar terminology in an XYZ system in which XOY is the equitorial plane being l.sub.A,.alpha.:l.sub.B,B;

• l.sub.C, .gamma.;
• and l.sub.D, .delta.;
• said coordinates being obtained by successively minimizing the terms T,A, and C.sub.1 given hereafter wherein T is a term of defocalization, A is astigmatism and C.sub.1 is coma, ##EQU4## wherein:
• R.sup.1 is the radius of the outer surface of the toroid at the equitorial plane, and
• p is the radius of curvature of the toroid.