New Eagle-type monochromator mounting with ruled diffraction grating at 45 degrees off-plane

A new Eagle-type monochromator mounting for ruled concave gratings at off-plane angles of 45 degrees is presented. The monochromator operates at an angle of incidence a range of ~0-15 degrees , where alpha is the angle between the grating normal and the projection of the incident central ray in the Rowland plane. Spherical, cylindrical, ellipsoidal, or toroidal gratings can be used. With toroidal or ellipsoidal grating surfaces one stigmatic image point is obtained, and the amount of astigmatism is strongly reduced in the whole spectral range. In the presence of astigmatism there is no comatic broadening of the line image. With toroidal and ellipsoidal gratings the comatic aberration is small being proportional to the angle alpha and depending only on the groove length. The width-dependent spherical aberration is proportional to alpha(2) and is therefore small. The new monochromator is expected to have applications in the far UV region as the low wavelength limit is expected to be reduced by a factor of radical2 compared to in-plane or near in-plane mountings.