Adjustable aperture stop to control the divergence and orientation of the beam emerging from a vacuum ultraviolet monochromator

An adjustable aperture stop for use in controlling the direction and angular divergence of the beam of radiant flux emerging from a MacPherson 225 VUV monochromator is described. Such a stop facilitates measurements of specular reflectance vs angle of incidence and measurements of diffraction grating efficiency. A discussion of the degree of control exercised by the aperture over the beam cross section is given that is applicable to any monochromator fitted with an aperture stop. For this application, the distance from the grating to the aperture stop is larger than the grating focal length. The result is an imaging effect that provides sharper definition of the beam cross section when the aperture stop is between the entrance slit and grating than if the stop is between the grating and exit slit. This sharp definition persists up to distances of several meters beyond the exit slit and is useful when measuring the efficiency of concave diffraction gratings. A fixed direction of the beam from the MacPherson i strument can only be obtained with the aperture between the grating and exit slit which results in some loss of definition of the beam cross section. However, as long as the pathlength beyond the exit slit is restricted to a few tens of cm, the definition is sufficiently good for measurements of reflectance vs angle of incidence.