| 摘要 |
An imaging optical system includes three reflecting mirrors having first to third reflection surfaces and is configured, such that in an XYZ orthogonal coordinate system using an optical axis at the center of the field of view as Z-axis, the optical axis at the center of the field of view and an optical axis of an image plane are in parallel to each other by changing orientation of the optical axis in a YZ section while maintaining the orientation of the optical axis in an XZ section. At least one of the three reflection surfaces is rotationally asymmetric. Assuming that along the path of the beam traveling along the optical axis at the center of the field of view a distance between the second reflection surface and the third reflection surface is L2, a distance between the third reflection surface and the image plane is L3 and fy1 and an equivalent F-number of the imaging optical system is represented as Fno, the relational expression 0.5<Fno(L2/L3)<1.3 is satisfied.
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