A method of line narrowing for a narrow band DUV high power, high repetition rate gas discharge laser producing a pulsed light beam comprising output laser light pulses in bursts of pulses, the gas discharge laser comprising a line narrowing module comprising a grating (229 and a beam expander prism assembly comprising four prisms (82. 84, 86, 88), the method comprising: detecting, by means of a center wavelength detector, a center wavelength of at least one pulse in a burst of said bursts of output from the gas discharge laser,; providing feedback based on an algorithm employing the detected center wavelength; selecting an angle of incidence of the pulsed light beam on the grating, by performing the steps of: moving a first prism (86, 88) of the beam expander prism assembly by means of an electro-mechanical positioning mechanism to select an angle of incidence of the pulsed light beam on the grating to thereby have a relatively larger impact on the angle of incidence; moving a second prism (82, 84) of the beam expander prism assembly by means of an actuated material positioning mechanism to select an angle of incidence of the pulsed light beam on the grating to thereby have a relatively smaller impact on the angle of incidence, wherein the first prism has a higher magnification than the second prism and is arranged as the last prism or the next to last prism in said beam expander prism assembly along a direction of an optical path of light into the line narrowing module; and the second prism is arranged as the first prism or second prism in said beam expander assembly along said direction of the optical path of light into the line narrowing module; and wherein moving the first and second prisms is controlled by a center wavelength controller during said burst, based on the provided feedback. detecting, by means of a center wavelength detector, a center wavelength of at least one pulse in a burst of said bursts of output from the gas discharge laser,; providing feedback based on an algorithm employing the detected center wavelength; selecting an angle of incidence of the pulsed light beam on the grating, by performing the steps of: moving a first prism (86, 88) of the beam expander prism assembly by means of an electro-mechanical positioning mechanism to select an angle of incidence of the pulsed light beam on the grating to thereby have a relatively larger impact on the angle of incidence; moving a second prism (82, 84) of the beam expander prism assembly by means of an actuated material positioning mechanism to select an angle of incidence of the pulsed light beam on the grating to thereby have a relatively smaller impact on the angle of incidence, wherein the first prism has a higher magnification than the second prism and is arranged as the last prism or the next to last prism in said beam expander prism assembly along a direction of an optical path of light into the line narrowing module; and the second prism is arranged as the first prism or second prism in said beam expander assembly along said direction of the optical path of light into the line narrowing module; and wherein moving the first and second prisms is controlled by a center wavelength controller during said burst, based on the provided feedback. moving a first prism (86, 88) of the beam expander prism assembly by means of an electro-mechanical positioning mechanism to select an angle of incidence of the pulsed light beam on the grating to thereby have a relatively larger impact on the angle of incidence; moving a second prism (82, 84) of the beam expander prism assembly by means of an actuated material positioning mechanism to select an angle of incidence of the pulsed light beam on the grating to thereby have a relatively smaller impact on the angle of incidence, wherein the first prism has a higher magnification than the second prism and is arranged as the last prism or the next to last prism in said beam expander prism assembly along a direction of an optical path of light into the line narrowing module; and the second prism is arranged as the first prism or second prism in said beam expander assembly along said direction of the optical path of light into the line narrowing module; and wherein moving the first and second prisms is controlled by a center wavelength controller during said burst, based on the provided feedback.