Onzn is using a telescopic device of his own design, it uses a concave bronze mirror in the bowl and a small pinhole to let the image through. One of the advantages over the larger telescopes of the time is the easily removable lid, allowing Onzn to make markings directly onto the mirror (with a soft chalk-like substance that writes well on metals but is easily and cleanly wiped off) to track the movements and positions of astronomical bodies.