Abstract: An optical illuminator assembly for an analytical instrument, such as a clinical hematology or a flow cytometer instrument, and method of aligning the components of the illuminator assembly. The illuminator assembly includes a plurality of optical components, such as a laser source, e.g., a laser diode, optionally a spatial filter, a beam shaping aperture, and a focussing lens. The optical components are mounted in mounting plates, frames or mounting cylinders, which are in turn coupled to a mounting surface on the assembly. Each of the mounting plates or frames is movable in the x and y directions, perpendicular to the laser beam, by micrometer adjusters having opposing spring plungers or the like, to decenter components in the desired laser beam path of the illuminator assembly. The micrometers and spring plungers are removably secured in the mounting surface so that after the x-y alignment, the mounting plates or frames can be secured to the mounting surface by, e.g.