cancels the pressure changes from the stage motion and fluid injection removal system as specified by equation 3 below thus the effect of dynamic pressure changes on both the last optical element 22 and the wafer stage are minimized in other embodiments signals from the velocity sensor or the stage controller may be absent or information from the fluid injection removal system may be provided to the controller in reducing the pressure fluctuations affecting the last lens element and wafer stage the controller 36 is likely to also reduce somewhat the amplitudes of the waves 23 c this in turn may improve the performance of the fluid injection removal system it may also reduce the chances of breakdown of the leading edge meniscus and thus avoid the formation of isolated fluid droplets on the wafer the above description is appropriate for a linear system where the pressure change u03b4ps created by the stage motion is independent of the pressure change u03b4pf created by the fluid injection removal system and the pressure change u03b4pa from the actuator in reality the fluid motion may make the system response non linear so that u03b4ps u03b4pf and u03b4pa are functions of one another however eq 3 remains valid also if the pressure changes are small enough the system response may be approximated as linear satisfying eq 3 is complicated by the fact that the controller can 39 t respond instantaneously to the pressure sensor signal nor can the separate contributions to the pressure sensor.