PURPOSE: To provide a reflection optical system capable of being used over a wide wavelength region from ultra violet region to infrared region without interrupting the effective light beam by an image pickup surface and having high optical performance over a whole screen by reflecting/converging a light beam from an object by means of a concave reflection mirror through an aspherical compensating plate and forming the object image on an image pickup plane.

CONSTITUTION: The optical axis of an aspherical reflection compensating plate 1 is made coincident with the optical axis of a main mirror 2 and also the optical axis of curved spherical surface of an image pickup surface 3 is made coincident with them on an optical axis 5. An optical system is composed so that only the region within the range of a main mirror 2 is made to be an effective region and the other region is cut off not to be used. An aperture diaphragm 4 is arranged in the vicinity of the center '0' of a curvature and astigmatic aberration and coma aberration of off-axis light beam are suppressed to be very small. A spherical aberration caused by a spherical shaped main mirror 2 is canceled by means of the aspherical reflection compensating plate 1.