Method of obtaining stigmatic imaging in spectral analysis using a concave grating

A method of obtaining stigmatic imaging of an object on a focal suurface at wave-length dispersion of radiation using a concave grating, comprising placing on one hand the object, an entrance aperture, an image and a virtual image considerable as the object, and on the other hand the focal surface in such a manner with respect to the concave grating that the following conditions are satisfied ##EQU6## at a value for .phi. that differs from zero, in which formulae the various symbols denote the following quantities in an orthogonal three dimensional coordinate system the origo of which lies in the middle of the grating surface of the concave grating and where the X-axis runs along the normal to the grating surface and the Z-axis is parallel with the grooves of the grating:

• .phi. = the angle between the XY-plane and a line from a selected point on the object to origo
• .alpha. = the angle between the X-axis and a line connecting origo and the projection point in the XY-plane of said object point,
• r = the distance between origo (O) and the projection point in the XY-plane of said object point,
• .beta. = the angle between origo and the projection point in the XY-plane of that point on the focal surface, which is hit by a ray emerging from said object point to origo after reflection through diffraction,
• r' = the distance from origo to said projection point in the XY-plane of said point on the focal surface,
• R.sub.1 = the radius of curvature in origo of the curve of intersection between said grating surface and the XY-plane,
• R.sub.2 = the radius of curvature in origo of the curve of intersection between said grating surface and the XZ-plane, the angles .alpha. and .beta. having the same sign if said object point and said point on the focal surface lie on the same side of the XZ-plane.