An optical system including an optical element, a positioning mechanism configured to position the optical element into an operational position, and a temperature control mechanism configured to intermittently control the temperature of the optical element between operations. By alternatively positioning the optical element between an operational position and a position in thermal contact with the temperature control mechanism, the two mechanisms for positioning and controlling the temperature of the optical element are de-coupled from one another. As a result, the mechanism for each may be optimized In non-exclusive embodiments, the temperature control mechanism may be used to control the temperature of an individual optical element or a plurality of optical elements, such as for example, a fly's eye mirror used in an illumination unit of an EUV lithography tool.