An imaging optical system includes a primary reflecting mirror, a secondary reflecting mirror, a tertiary reflecting mirror, and an aperture stop. The imaging optical system has a field of view, a focal length of the field of view that is defined as FFL, and an effective aperture of a field of view entrance pupil that is defined as FEPD. The FFL and the FEPD at a central field of view are greater than that at the edge field of views, and the FFL and the FEPD change continuously.