A method of thermally conditioning an optical element operating in a vacuum environment. The optical element includes a first body having at least one optical surface and at least one heat transfer surface. The first body is dynamically controlled in position and/or orientation. The method includes controlling a temperature of a second body to a desired temperature, the second body including a second heat transfer surface; positioning the second body adjacent the first body and dynamically controlling the second body in position and/or orientation so as to maintain the first and second heat transfer surfaces in a substantially constant arrangement without contact between the bodies; and delivering a gas as a heat transfer medium into a heat transfer space defined by the first and second heat transfer surfaces, while controlling the pressure of the gas in the heat transfer space to between about 30 Pa and about 300 Pa.