A focusing-device for the radiation from a light source, in particular a laser plasma source, has a collector mirror which collects the light from the light source at a second focus in virtual or real terms, in particular for micro-lithography using EUV radiation, and a routing unit and downstream beam formation in an illuminating system. The collector mirror can be displaced in the z-direction (optical axis) and is designed and/or mounted in such a way that the position of the second focus remains unchanged in the event of temperature changes.