The invention relates to an optical assembly, in particular a projection exposure apparatus for EUV lithography, comprising: a beam generating system for generating radiation (6) at an operating wavelength,at least one optical element (13, 14) which is subjected to the radiation (6) and is arranged in a residual gas atmosphere, and a feed device for feeding at least one gaseous constituent into the residual gas atmosphere in order to suppress a degradation of the surface (14a) of the optical element (14) that is induced by the radiation, wherein either a beam diameter (d) of the radiation (6) at the surface of the optical element, in particular at the surfaces of all optical elements of the optical assembly, lies above a threshold value (d c ) so that a suppression of the degradation by the gaseous constituent is effective, or the beam diameter (d) at the surface (14a) of the optical element (14) lies below the threshold value (d c ) so that a reduction of the effectiveness of the suppression of degradation occurs,and at least one further device (25, 27) for the improved suppression of the degradation of the surface (14a) is assigned to the optical element (14).The invention also relates to an associated method for operating an optical assembly.