Theory of an anastigmatic monochromator for vacuum ultraviolet region

Abstract

It is proposed in the present paper to use a concave grating along with a concave mirror to obtain a monochromator in which there is no beam displacement, and the astigmatism is very small. To change the wavelength the concave grating is rotated about an axis passing through itself. The angle between the incident and the diffracted beams are kept small so that the astigmatism is negligible. An analysis of the focussing properties of such a monochromator is made, and it is shown that it can be profitably used in the range 500 Å to 4000 Å. The addition of an extra reflection may not be in itself a disadvantage in view of the fact that reflection properties of coatings are constantaly being improved.