| 摘要 |
PROBLEM TO BE SOLVED: To avoid a shortcoming that power gets so strong that aberration easily occurs by arranging an aperture diaphragm at a position satisfying a specified arrangement condition between a concave reflection mirror near to a 2nd surface and a convex reflection mirror near to the 2nd surface. SOLUTION: The basic constitution of this optical system is constituted of four mirrors, that is, the concave reflection mirror M1, the convex reflection mirror M2, the convex reflection mirror M3 and the concave reflection mirror M4 in order from a 1st surface R side. Namely, the mirror M1 is provided at the rear of the 1st surface R, and the aperture diaphragm S is provided through two opposed mirrors M2 and M3, so that an image is formed on the 2nd surface W by the mirror M4. In such a case the diaphragm S is arranged at the position satisfying the arrangement condition of 0.1<=x/L<=0.9. In the expression, L shows a distance on an optical axis from the mirror M4 near the 2nd surface W to the mirror M3 near to the 2nd surface W, and (x) shows the distance on the optical axis from the mirror M3 near to the 2nd surface W to the diaphragm S. |
