A gas discharge laser system bandwidth control mechanism and method of operation for controlling bandwidth in a laser output light pulse generated in the gas discharge laser system is disclosed which may comprise a bandwidth controller which may comprise an active bandwidth adjustment mechanism; a controller actively controlling the active bandwidth adjustment mechanism utilizing an algorithm implementing bandwidth thermal transient correction based upon a model of the impact of laser system operation on the wavefront of the laser light pulse being generated and line narrowed in the laser system as it is incident on the bandwidth adjustment mechanism. The controller algorithm may comprises a function of the power deposition history in at least a portion of an optical train of the gas discharge laser system, e.g., a linear function, e.g., a combination of a plurality of decay functions each comprising a respective decay time constant and a respective coefficient.