Lithographic apparatus, distortion determining method, and patterning device

A method of determining higher-order distortions of a patterning device of a lithographic apparatus, said method comprising:

• imparting a radiation beam with a pattern in its cross-section to form a patterned radiation beam using a patterning device, said patterning device comprising a main imaging field, a perimeter and a plurality of alignment structures;
• detecting transmission of radiation transmitted through said alignment structures of said patterning device and into a device for transmission image detection,
• producing measurement signals from the detected radiation, and
• feed-forward modeling higher-order distortions and/or image plane deviations of said patterning device using measurement signals resultant from radiation transmitted through alignment structures comprised in said perimeter and/or in said imaging field.