| 摘要 |
PROBLEM TO BE SOLVED: To obtain excellent image-forming performance while the number of reflection surfaces is restrained to be comparatively small and further to improve the image- forming performance of an actually produced optical system while securing substantial telecentricity on an object side by constituting the system so that a specified condition may be satisfied. SOLUTION: This optical system is constituted of a 1st reflection mirror (concave mirror) M1 having a concave reflection surface, a 2nd reflection mirror (plane mirror having an aspherical surface) M2 having an aspherical reflection surface whose power is zero in a paraxial area, a 3rd reflection mirror (convex mirror) M3 having a convex reflection surface, an aperture diaphragm AS having a specified shape aperture part, and a 4th reflection mirror (concave mirror) M4 having the concave reflection surface in order to form the image of an object on a 1st surface R on a 2nd surface W. Then, it is constituted to satisfy the condition 0.7>s/D>0.1. In the expression, (s) shows a distance along an optical axis from the apex of the mirror M3 to the aperture diaphragm AS and D shows the distance along the optical axis from the mirror M3 to the mirror M4.
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