Method of reducing cyclic error effects in a lithographic process, projection system and lithographic apparatus comprising a projection system

A projection system to project a beam, the projection system comprising:

• a first module having a first movement range, the first module being a position controlled mirror of the projection system arranged to reflect the beam,
• a first position measurement system to measure a first position of the first module, and
• a control system comprising a first control loop to control the first position of the first module, the first control loop having a first bandwidth,
• wherein the control system is arranged to continuously move the first module at least during a projection phase of a lithographic process, such that a first main frequency of a first cyclic error of the first position measurement system will be above the first bandwidth of the first control loop.