Abstract: A device comprising an energy selecting slit assembly is provided. The slit assembly comprising a slit assembly chassis, opposing slit mechanisms, and an actuator assembly. One of the terminal ends of the actuator arm comprises a relatively fixed terminus, while the other terminal end of the arm comprises a relatively mobile terminus coupled to the rotational slit mechanism via a mechanical coupling configured to translate movement of the mobile terminus into rotational of the rotational slit mechanism. In one embodiment of the present invention, the slit assembly chassis defines a plurality of fixturing datums configured to establish alignment of respective aperture-defining edges of the opposing slit mechanisms along orthogonal X, Y, and Z axes defined by the chassis. Additional embodiments are disclosed.