A mirror assembly (32) for directing a beam (28) includes a base (450), and an optical element (454) that includes (i) a mirror (460), (ii) a stage (462) that retains the mirror (460), (iii) a mover assembly (464) that moves the stage (462) and the mirror (460) relative to the base (450), and (v) a thermally conductive medium (466) that is positioned between the stage (462) and the base (450) to transfer heat between the stage (462) and the base (450). The thermally conductive medium (466) has a thermal conductivity that is greater than the thermal conductivity of air. The thermally conductive medium (466) can include an ionic fluid or a liquid metal.