In the field of optical checking of optical instruments of telescope type, an instrument comprises an optical objective, a photo-detection housing arranged at the focus of said optical objective and at least one light source arranged in the vicinity of said photo-detection housing, the optical pupil of the optical objective having a first diameter. The checking means comprise a plane mirror having a second diameter that is smaller than the first diameter and means to arrange the plane mirror such that the image of the light source given by the optical objective and reflected by said plane mirror is focused on the photo-detection housing, means for analyzing said image received making it possible to determine the optical quality of the telescope. The plane mirror may be associated with an autonomous checking bench. It may also form part of the telescope and be incorporated into the protection hood for the optic.