Variable corrector of a wave front

A method, comprising:

• determining an adjustment to an aberration of a beam of electromagnet (EM) radiation by which a structure is to be measured in an optical metrology tool;
• moving a first aspheric optical element relative to a second aspheric optical element in a direction perpendicular to the beam;
• aberrating the beam differently after the movement than before the movement so as to at least cause a change in position of a common focus point of the rays of the beam with respect to a surface in the optical metrology tool;
• and
• measuring the structure.

An apparatus, comprising:

• an optical metrology tool configured to measure structures with measurement radiation and to determine a focus related parameter of the optical metrology tool using a focus radiation beam separate from the measurement radiation, the optical metrology tool comprising an adaptive optical system disposed in a portion of the focus radiation path and configured to adjust a shape of a wave front of the focus radiation, the adaptive optical system comprising:
• a first aspherical optical element;
• a second aspherical optical element adjacent the first aspherical optical element;
• and
• an actuator configured to cause relative movement between the first optical element and the second optical element in a direction different from a beam axis of the portion of the focus radiation path.