A mirror assembly (332) for directing a beam (28) from an illumination source (26) to a reticle (36) includes a mirror (352) and a back plate (350). The mirror (352) includes a mirror body (352A) that defines a reflective first surface (352B) that directs the beam (28), a mirror mounting region (370), a mirror perimeter region (372) that encircles the mirror mounting region (370), and mirror slot (374) that separates the mirror perimeter region (372) from the mirror mounting region (370). The back plate (350) retains and engages the mirror mounting region (370) of the mirror (352) with the mirror perimeter region (372) spaced apart from the back plate (350). Further, the mirror body (352A) can include a second surface (352C) that is substantially opposite the first surface (352B), and the mirror mounting region (370) extends between the second surface (352C) to near the first surface (352B). Further, the mirror slot (374) extends from the second surface (352C) to near the first surface (352B).