Twelve-sided mater with a circular riveted back plate, shaped and decorated throne and shackle and suspension ring. The throne is engraved with the arms of Franciscus Paduanius of Forli, Italy. The spandrels formed by the angles of the polygon have foliate decoration typical of Habermel.
The limb of the mater has a scale of hours 0 to 12 twice, divided to 1 hour, subdivided to half, quarter and eighth hours, numbered by 1 hour with Roman numerals. Within this is a degree scale 90 (at the throne) to 0 to 90 to 0 to 90, divided to 10, subdivided to 5 and to 1, numbered by 10.
The central space has a nautical square, with longitude and latitude scales in quadrants 0 to 90 to 0, divided to 10, subdivided to 2, numbered by 10. The cardinal points are named outside the square in Latin, the wind names and 32 compass points engraved inside in Dutch and Latin respectively.
There are three plates for latitudes 39 and 42?; 45 and 48?; and 51? degrees. The other side if the plate for 51? has a tablet of horizons, together with scales usually placed on the back of an astrolabe: a diagram for conversion between equal and unequal hours and a double shadow square. The latitude projections have azimuths for every 5 degrees, numbered by 10, and almucantars for every 2 degrees numbered by 10. There are also east-west and meridian lines and circles for the tropics and equator, and lines for unequal hours, numbered 1 to 12 with Roman numerals, and for the houses of heaven, numbered 1 to 12.
The gilt copper rete is of an unusual form, with a prominent ecliptic, one circular band outside and a crescent inside. There are pointers for 17 named stars. The ecliptic is divided into the signs, with Latin names, each with a scale of 30 degrees, divided to 5, subdivided to 1, and a calendar scale with the names of the months, divide to 5, subdivided to 1 day. The first point of Aries is at 201/2 March. The rete is incorrectly divided equally according to right ascension.
The back has a universal astrolabe projection of the Gemma Frisius type with arcs for declination every 2 degrees and an outer circle of degrees 90 to 0 to 90 to 0 to 90, divided to 10, subdivided to 5, to 1, and to 1/2, numbered by 10. There are arcs for right ascension every 2 degrees, the 10 degree divisions being indicated by additional marks, and the hours arcs being added where they do not coincide with even degree divisions (15, 45, etc), with the hours numbered by 1. Named stars are marked on the projection and the ecliptic line drawn in and divided into the signs, with their symbols, each with a scale 0 to 30, divided to 10, subdivided to 10 and to 1, numbered by 10.
For use with this universal projection there is a rule attached to a pin. This rule has a scale 0 (at the centre) to 90 (right-hand end) to 270 to 360, divided to 10, subdivided to 5 and on a beveled edge to 1, numbered by 10 and marked with the zodiacal symbols. A second rule slides along this one at right angles, with a clamping screw, and has scales 0 to 90 and 90 to 0 on either edge, divided to 10, subdivided to 5 and on the beveled edges to 1, numbered by 10.
There is a second, ordinary pin, a washer and two second horse-shaped wedges, and a counterchanged alidade with a zodiacal scale divided and numbered to 10 degrees.
Signed on the rim: 'Erasmus Habermel fecit'. On the verso of the alidade are the initials 'E ? H ?'.
See R. T. Gunther, The Astrolabes of the World (2 vols, Oxford, 1932), vol. 2, pp. 453-6.