A high repetition rate gas discharge laser system comprising a master oscillator adapted to provide a master oscillator output beam and a power amplifier, adapted to receive the master oscillator output beam, to amplify the master oscillator output beam and to provide a power amplifier output beam comprising a pulsed output of greater than or equal to 15 mJ per pulse, sub-nanometer bandwidth tuning range pulses having a femtometer bandwidth precision and tens of femtometers bandwidth accuracy range and a spectral analysis module, for measuring bandwidth on a pulse to pulse basis at pulse repetition rates of 4000Hz and above, the spectral analysis module comprising: a primary beam splitter in the path of the power amplifier output beam operative to pass the vast majority of the power amplifier output beam and to reflect a first small portion of the power amplifier output beam, the primary beam splitter oriented at an angle to sufficiently reduce the fluence on the primary beam splitter, and creating overlapping Fresnel reflections in the first small portion of the power amplifier output beam wherein the master oscillator output beam also passes through the primary beam splitter; a secondary beam splitter made from a material having a damage threshold sufficiently high to tolerate the fluence created by the overlapping portion of the Fresnel reflections in the first small portion of the power amplifier output beam, the secondary beam splitter reflecting the vast majority of the first small portion of the power amplifier output beam and passing a second small portion of the power amplifier output beam; and a telescoping optic in the path of the second small portion of the power amplifier output beam operative to demagnify the second small portion of the power amplifier output beam onto a first stage diffuser receiving the demagnified second small portion of the power amplifier output beam, the demagnification selected to keep the fluence in the overlapping Fresnel reflections in the second small portion of the power amplifier output beam below the damage threshold of the first stage diffuser, the telescoping optic comprising a cylindrical telescoping optic oriented to demagnify a long axis of the second small portion of the power amplifier output beam more than a short axis of the second small portion of the power amplifier output beam, redistributing the fluence of the second small portion of the power amplifier output beam across the first stage diffuser to keep any portion of the first stage diffuser from exceeding the damage threshold for the material from which the first stage diffuser is made. a primary beam splitter in the path of the power amplifier output beam operative to pass the vast majority of the power amplifier output beam and to reflect a first small portion of the power amplifier output beam, the primary beam splitter oriented at an angle to sufficiently reduce the fluence on the primary beam splitter, and creating overlapping Fresnel reflections in the first small portion of the power amplifier output beam wherein the master oscillator output beam also passes through the primary beam splitter; a secondary beam splitter made from a material having a damage threshold sufficiently high to tolerate the fluence created by the overlapping portion of the Fresnel reflections in the first small portion of the power amplifier output beam, the secondary beam splitter reflecting the vast majority of the first small portion of the power amplifier output beam and passing a second small portion of the power amplifier output beam; and a telescoping optic in the path of the second small portion of the power amplifier output beam operative to demagnify the second small portion of the power amplifier output beam onto a first stage diffuser receiving the demagnified second small portion of the power amplifier output beam, the demagnification selected to keep the fluence in the overlapping Fresnel reflections in the second small portion of the power amplifier output beam below the damage threshold of the first stage diffuser, the telescoping optic comprising a cylindrical telescoping optic oriented to demagnify a long axis of the second small portion of the power amplifier output beam more than a short axis of the second small portion of the power amplifier output beam, redistributing the fluence of the second small portion of the power amplifier output beam across the first stage diffuser to keep any portion of the first stage diffuser from exceeding the damage threshold for the material from which the first stage diffuser is made.