| 摘要 |
PURPOSE:To obtain a high optical performance having a prescribed image forming power by constituting two concave mirrors so that their centers of curvature coincide with each other and their focus positions have conjugate relations. CONSTITUTION:A focus position 4 of a concave mirror 1 continuously exists on a dotted-line circle 11 which has a point 3 as the center and has a radius R1/2, and its focal length f1 is R1/2. Similarly, a focus position 5 of a concave mirror 2 continuously exists on a circle 12 which has the point 3 as the center and has a radius R2/2 and its focal length f2 is R2/2. Properties of this mirror optical system are used, and an object point is set in one area on the circle 11 corresponding to the focus position of the concave mirror 1 and an image point is set in one area on the circle 12 corresponding to the focus position of the concave mirror 2 when the mirror optical system is used as a reduction system. That is, two concave mirrors are so constituted that their focus positions have conjugate relations. Thus, the mirror optical system is obtained where various aberrations are less generated in case of the use as an expansion system as well as a reduction system and which is easy to produce and has a high optical performance.
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