Patent ID: 8576376
Filing Date: 2013-11-05
Classification: G02B,G03F

Abstract:
1. 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: at least six mirrors positioned along an optical axis of the imaging optical system, where the at least six mirrors direct the light along the path, wherein at least one of the mirrors has an opening through which the light passes, and a distance along the optical axis between a fourth-last mirror in the path and a last mirror in the path is at least 10% of the distance between the object field and the image field along the optical axis, wherein the distance is between a piercing point of the optical axis and the fourth-last mirror and a piercing point of the optical axis and the last mirror, wherein the piercing point of the fourth-last mirror is a point where the optical axis passes a reflection surface of the fourth-last mirror or a point where the optical axis passes a surface which carries on continuously from the reflection surface of the fourth-last mirror based on the optical design of the fourth-last mirror, and the piercing point of the last mirror is a point where the optical axis passes a reflection surface of the last mirror or a point where the optical axis passes a surface which carries on continuously from the reflection surface of the last mirror based on the optical design of the last mirror.