A mirror system has four mirrors with a common optical axis, i.e. a first collecting mirror (1), a second smaller dispersing mirror (2), a third collecting mirror (3) and a fourth collecting mirror (4), seen in the direction of light incidence. Mirror (1) reflects incident light on mirror (2) that forms an intermediate image (5) of the light. The dispersed light then reaches mirror (3) that acts as a collimator and reflects parallel light on mirror (4), that in turn reflects the light on image field (6) in which a detector (7) is arranged. Mirror (1) has a central bore (10). Flat mirrors (8) fold the path of the light and are preferably arranged in the plane of intermediate image (5). An image field flattening lens (9) can flatten image field (6). Both classical Cassegrain systems and Ritchey-Chretien systems that consist of mirrors (1) and (2) may be aplanatically and anastigmatically corrected with a flat image field (6) by means of the disclosed two-mirror corrector made of mirrors (3) and (4).