Apparatus for and method of temperature compensating a focusing system in having at least one transmissive optical element with a thermal lens. A reflective optical element is introduced having a thermal lens complementary to the thermal lens of the transmissive optical element so that the combined optical characteristics of the two optical elements are substantially temperature independent. Provision can also be made for a change in the absorption of the transmissive optical element over time. The focusing system is especially applicable to systems for generating EUV light for use in semiconductor photolithography.