An imaging optical system for imaging an object field into an image field with EUV imaging light

An imaging optical system has a plurality of mirrors for imaging an object field in an object plane into an image field in an image plane with EUV imaging light (3). This image is carried out along an imaging beam path that sets a light pipe (24) is guided within the imaging light (3). A light pipe detection means (21) is used for determining the position of the light pipe (24). The light tubes detection means (21) has at least three EUV sensors (25) circumferentially around a main beam (16 This results in an imaging optical system in which an unwanted image loss or an unwanted telecentricity error is avoided.