a projection exposure apparatus for transferring an image of a patterned reticle onto a substrate comprises an illumination optical system for generating and directing an exposure beam onto the reticle , and a projection optical system provided between the reticle and the substrate . the projection optical system has a plurality of imaging mirrors each having a mirror support made of a support material . the support materials are subject to thermal expansion during projection that induces imaging aberrations at substrate level . the support materials are selected such that an aberration merit function , which is indicative of the overall amount of at least one type of the thermally induced aberrations , is minimized by mutual compensation of contributions of the mirrors to the one type of thermally induced aberrations . as a result , the mirror supports will then generally be different and have , when heated during exposure , different coefficients of thermal expansion .