An optical monitoring device for an optical imaging system having a focal plane, an optical axis and an entry pupil, forming an image of a scene substantially at infinity on an image detector disposed substantially in the focal plane, comprises: a virtually point-like emission source at the periphery of the detector and substantially in the focal plane; a reflecting element with a plane surface upstream of the imaging system relative to the direction of light rays coming from the scene, and according to a position and inclination where a monitoring image of the source produced by the optical system and reflected by the reflecting element is substantially in the focal plane on a detection element connected to a monitoring image analyzer allowing potential optical defects to be identified; the reflecting element having an annular shape allowing passage of light rays coming from the scene and passing through the entry pupil.