A system comprising: a detachable spectacles-mounted augmented reality device and a clip-on unit attached or attachable to the AR device, said AR device comprising: a housing configured for supporting the clip-on unit, an exit window and an entrance window in the housing through which the user observes a scene, a portable communications device, a communications interface (71, 74) coupled to the portable communications device, a micro-display within the housing coupled to the communications interface (71, 74) for receiving from the portable communications device an image for displaying at the user's field of view, and optics within the housing including the entrance window, a see-through optical element, the exit window and an objective (41, 41') configured for projecting the image at a distance for superimposing on to an external scene viewed by the user through the see-through optical element; a camera inside the housing coupled to the communications interface (71, 74) for imaging the scene observed by the user through a camera window and being configured to convey an image of said scene to the portable communications device; the portable communications device having a touchscreen configured to display a superposition of the camera image and a digital mark and is configured to provide input means for dragging the image on the touchscreen relative to the digital mark to determine the x-y offset of the center of the camera field-of-view; and the clip-on unit having an attachment for detachably mounting the AR device in front of a spectacle frame worn by the user and allowing for lateral displacement of the AR device relative to the spectacle frame without moving the attachment for aligning the exit window with a line of sight of the user. a housing configured for supporting the clip-on unit, an exit window and an entrance window in the housing through which the user observes a scene, a portable communications device, a communications interface (71, 74) coupled to the portable communications device, a micro-display within the housing coupled to the communications interface (71, 74) for receiving from the portable communications device an image for displaying at the user's field of view, and optics within the housing including the entrance window, a see-through optical element, the exit window and an objective (41, 41') configured for projecting the image at a distance for superimposing on to an external scene viewed by the user through the see-through optical element; a camera inside the housing coupled to the communications interface (71, 74) for imaging the scene observed by the user through a camera window and being configured to convey an image of said scene to the portable communications device; the portable communications device having a touchscreen configured to display a superposition of the camera image and a digital mark and is configured to provide input means for dragging the image on the touchscreen relative to the digital mark to determine the x-y offset of the center of the camera field-of-view; and the clip-on unit having an attachment for detachably mounting the AR device in front of a spectacle frame worn by the user and allowing for lateral displacement of the AR device relative to the spectacle frame without moving the attachment for aligning the exit window with a line of sight of the user.