Method of reducing a wave front aberration, and computer program product

A method of reducing a wave front aberration of an optical wave traversing a projection system of a lithographic apparatus constructed and arranged to expose a radiation sensitive layer on a substrate to an image of a pattern, the method comprising:

• obtaining information on the wave front aberration;
• calculating an adjustment of an optical element of the projection system for reducing the wave front aberration;
• and
• applying the calculated adjustment to the projection system, wherein the calculating includes:
• obtaining information on a spatial distribution of radiant intensity in a pupil of the projection system as present during the exposure of the radiation sensitive layer,
• selecting a threshold intensity in a range between maximum intensity of the distribution of intensity and zero,
• defining areas in the pupil within which a local intensity is higher than the threshold intensity, and
• limiting the reducing to areas of the wave front corresponding to the areas in the pupil.