A projection objective of a microlithographic projection exposure apparatus comprises a manipulator for reducing rotationally asymmetric image errors. The manipulator in turn contains a lens, an optical element and an interspace formed between the lens and the optical element, which can be filled with...http://www.google.ca/patents/US7990622?utm_source=gb-gplus-sharePatent US7990622 - Projection objective of a microlithographic projection exposure apparatus
