Design and realization of a XUV plane-grating monochromator at variable subtended angle

A plane-grating monochromator for the extreme-ultraviolet region that uses a plane grating operated at variable included angle and illuminated in converging light has been realized. The configuration consists of three optical elements: the focusing mirror, the plane grating and the plane mirror. In the proposed design, the spectral focusing is kept on the exit slit plane by changing the grating included angle through the use of an additional plane mirror after the grating. The coma aberration given by the grating is compensated by an opposite coma introduced by the focusing mirror, that is used in a very asymmetrical configuration. The feasibility of the configuration has been demonstrated on a monochromator for the 10-80 eV region, with sub-10-meV output bandwidth. The correction of defocusing and coma at the proper included angle has been clearly demonstrated. The monochromator efficiency has been measured to be in the 0.08-0.18 range.