An x-ray illumination subsystem, comprising: an x-ray illumination source configured to generate an amount of x-ray radiation,the x-ray illumination source having a finite emission area; a focusing optical element having one or more reflective surfaces, wherein a location of an object plane and a location of an image plane associated with a reflection at any location on one or more reflective surfaces are fixed; and a beam selection subsystem including one or more moveable slits or apertures (122A, 122B) located in a beam path between the x-ray illumination source and a specimen under measurement, wherein a position of the one or more moveable slits or apertures with respect to the x-ray radiation in a direction perpendicular to a central axis of the x-ray illumination subsystem controls an optical magnification of the illumination subsystem, and wherein an opening of the one or more moveable slits or apertures controls a divergence of an x-ray illumination beam reflected from the focusing optical element and selected from the reflected x-ray radiation based on the position of the one or more slits or apertures, the x-ray illumination beam incident on the specimen under measurement; said X-ray illumination subsystem being characterized in a computing system configured to: communicate a first command signal to the beam selection subsystem that causes at least one actuator (121A, 121B) to move the one or more moveable slits or apertures with respect to the x-ray radiation in the direction perpendicular to the central axis of the x-ray illumination subsystem to a desired position that corresponds to a desired magnification of the illumination subsystem and to adjust the opening of the one or more slits or apertures to a desired opening that corresponds to a desired divergence of the incident x-ray illumination beam; and communicate a second command signal to the illumination source that causes the illumination source to change a size, shape, or both size and shape of the finite emission area. an x-ray illumination source configured to generate an amount of x-ray radiation,the x-ray illumination source having a finite emission area; a focusing optical element having one or more reflective surfaces, wherein a location of an object plane and a location of an image plane associated with a reflection at any location on one or more reflective surfaces are fixed; and a beam selection subsystem including one or more moveable slits or apertures (122A, 122B) located in a beam path between the x-ray illumination source and a specimen under measurement, wherein a position of the one or more moveable slits or apertures with respect to the x-ray radiation in a direction perpendicular to a central axis of the x-ray illumination subsystem controls an optical magnification of the illumination subsystem, and wherein an opening of the one or more moveable slits or apertures controls a divergence of an x-ray illumination beam reflected from the focusing optical element and selected from the reflected x-ray radiation based on the position of the one or more slits or apertures, the x-ray illumination beam incident on the specimen under measurement; said X-ray illumination subsystem being characterized in a computing system configured to: communicate a first command signal to the beam selection subsystem that causes at least one actuator (121A, 121B) to move the one or more moveable slits or apertures with respect to the x-ray radiation in the direction perpendicular to the central axis of the x-ray illumination subsystem to a desired position that corresponds to a desired magnification of the illumination subsystem and to adjust the opening of the one or more slits or apertures to a desired opening that corresponds to a desired divergence of the incident x-ray illumination beam; and communicate a second command signal to the illumination source that causes the illumination source to change a size, shape, or both size and shape of the finite emission area. communicate a first command signal to the beam selection subsystem that causes at least one actuator (121A, 121B) to move the one or more moveable slits or apertures with respect to the x-ray radiation in the direction perpendicular to the central axis of the x-ray illumination subsystem to a desired position that corresponds to a desired magnification of the illumination subsystem and to adjust the opening of the one or more slits or apertures to a desired opening that corresponds to a desired divergence of the incident x-ray illumination beam; and communicate a second command signal to the illumination source that causes the illumination source to change a size, shape, or both size and shape of the finite emission area.