Patent ID: 8717538

Claim:
An imaging optical system which during operation directs light along a path to image an object field in an object plane to an image field in an image plane, the imaging optical system comprising: a plurality of mirrors arranged to direct the light along the path, the plurality of mirrors including first and second mirrors each having a surface in the path that reflects the light but does not surround an opening for passage of the light, wherein: the first mirror is closer to the object plane along the optical axis than any other mirror in the imaging optical system which has a surface in the path that reflects the light but does not surround an opening for passage of the light; the second mirror is closer to the image plane along the optical axis than any other mirror in the imaging optical system which has a surface in the path that reflects the light but does not surround an opening for passage of the light; the imaging optical system has a region having a first end which contacts the first mirror and a second end which contacts the second mirror; the region includes two spaced-apart planes parallel to the image plane and/or the object plane; one of the two spaced-apart planes contacts the first end of the region and the other of the spaced-apart planes contacts the second end of the region; between the two spaced-apart planes, the imaging optical system includes at least one sub-region where the path passes multiple times; the imaging optical system comprises at least one pupil plane; each plane which is parallel to and between the two spaced-apart planes and where the path passes through multiple times is not in a pupil plane of the imaging optical system; and neither of the first and second mirrors is in a pupil plane of the imaging optical system.