Projection objective of a microlithographic projection exposure apparatus

A projection objective comprising:

• a refractive optical element having a volume and a material related optical property that varies over the volume, variations of the material related optical property of the refractive index refractive optical element causing an aberration during use of the projection objective,
• a first optical element having a first optical surface comprising a first correction mechanism that partially corrects the aberration during use of the projection objective;
• and
• a second optical element having a second optical surface comprising a second correction mechanism that partially corrects the aberration during use of the projection objective,
• wherein:
• the first correction mechanism is located in or intersects a first plane which is perpendicular to an optical axis of the projection objective;
• the second correction mechanism is located in or interests a second plane which is perpendicular to the optical axis of the projection objective;
• a third plane which is perpendicular to the optical axis of the projection objective intersects the volume of the refractive optical element;
• a fourth plane which is distinct from the third plane and is perpendicular to the optical axis of the projection objective intersects the volume of the refractive optical element;
• the first plane is optically conjugate with the third plane;
• the second plane is optically conjugate with the fourth plane;
• and
• the projection objective is a microlithographic projection objective.