This composite astrolabe is made from a set of medieval 15th century plates and an early 17th century rete and mater. The set of plates probably once belonged to a French astrolabe, as is evident from the fact that the side of the 49º plate is labelled with the place name 'PAR9': a detail typical of early 14th century French astrolabes. The provenance of the rete and the mater is more problematic and looking at their style and design, Italy seems most probable as its country of origin in the light of present knowledge. It has been suggested that the location of the stars on the rete represent the sky in 1625, thus providing an approximate date for these components. There seems to be at least one other astrolabe by the same maker, now in the Adler Planetarium, Chicago.

The throne is cast in one piece with the limb, which is riveted to the mater. Near the throne are two engraved letters 'S' in mirror image, which could be an owner's mark or a clue towards the identity of the maker. There are four plates representing the following latitudes: 30º/36º, 39º/66º, 42º/45º and 49º/51º. The rete comprises a solid structure made from a thick and strong plate of brass. This type of construction allowed the star pointers to be quite long while remaining sufficiently strong. On the back of the instrument are four concentric scales for hours, degrees, the zodiac and a calendar, in the middle of which is a double, unequal hour diagram with Arabic numerals, and a shadow square.