The authenticity of this astrolabe is under question. There can be little doubt that the poorly-engraved plate was made to replace a set of now-lost original plates, and its maker has tried unsuccessfully to imitate certain curves from a conventional plane with a stereographic projection. The mater and rete are much more likely to be genuine and could herald from 14th century France or Italy. Letters from the NMM archives explain that a new suspension ring and rule were made in 1939 to accompany the sale of the instrument to the Museum.

The throne, which is of simple design, is cast in one piece with the limb, which is itself linked to the mater by rivets placed at irregular distances. There is only one plate, one side of which is attached to a circular piece of wood, which serves to fill the gap between the plate and the hollow mater. The arrangement of the strapwork on the rete follows the tradition of early medieval astrolabes and it is probable that the same engraver who made the rete is responsible for the engraving on the front and back of the mater. The back of the instrument contains scales for degrees, the zodiac and the calendar. The remainder comprises a series of curves for finding unequal hours and a shadow square.