Lithographic apparatus and a method to compensate for the effect of disturbances on the projection system of a lithographic apparatus

A lithographic apparatus comprising:

• an illumination system configured to condition a radiation beam;
• a patterning device support constructed to support a patterning device, the patterning device being capable of imparting the radiation beam with a pattern in its cross-section to form a patterned radiation beam;
• a substrate table constructed to hold a:
• subtrate;
• a projection system configured to project the patterned, radiation beam onto a target portion of the substrate;
• an active air mount to support said projection system, the active air mount comprising at least one actuator;
• and
• a feed-forward device, the feed-forward device being configured to receive a set-point signal, the set point signal being based on a difference between a desired position of a moveable, object and an actual position of the moveable object, and. provide on the basis of the set-point signal, a feed-forward signal to the at least one actuator, wherein the feed-forward signal is designed to decrease a disturbance effect on the projection system due to movement of the movable object.