Focus of a laser optical system can be corrected using a variable radius mirror having a focusing cavity and a separate cooling cavity. Pressure of a focusing material at a sufficiently low mass flow in the focusing cavity deforms a reflective surface mounted to the focusing cavity, changing its radius. Cooling material provided to the cooling cavity cools the variable radius mirror. A laser beam is reflected by the deformed reflecting surface to focusing optics, focusing the reflected laser beam on an EUV-emitting target, and minimizing a laser focus error by one or more of: maximizing a measured EUV power or minimizing a measured laser beam divergence. Providing focusing material at a deformation pressure and at a sufficiently low mass flow, and providing a separate cooling cavity, avoids perturbations in the reflective surface which would otherwise affect laser beam focus.