A detachable spectacles-mounted augmented reality (AR) device and clip-on unit wherein the device has a housing (31) configured for detachably supporting the clip-on unit, an exit window (30) and an entrance window (30′) in the housing through which the user observes a scene, a communications interface (71, 74) for coupling to a hand-held device, and a camera (37) inside the housing for imaging the scene observed by the user through a camera window (36) and configured to convey an image of the scene to the hand-held device. A line-of-sight guide unit (39) displays at least one marker at the user's field of view for directing a line of sight of the user toward a designated feature in the scene, and optics (40) within the housing projects the marker at a distance for superimposing on to the scene viewed by the user.