The rete has a large quatrefoil as a base for several star pointers in the ecliptic. The quatrefoil is inscribed the name of the maker and the date in the top lobe: 'Blakene me fecit anno do{mini} 1342'. The central disc is hollowed out in four quadrants, the inner ring is attached to the outer ring by a cross-shaped support. The equatorial bar, which is counter-changed on each side of the ecliptic, is connected with the tropic of Capricorn by three stylized columns and to the ecliptic by a wavy bar. The worm shaped star pointers indicate the following 38 stars: 'Batencaytoz', 'Pantacaytoz', 'Enif', 'Menkar', 'Algon', 'Augethenar', 'Aldebaran', 'Alhayoc', 'Rigil', 'Algeuze', 'Alhabor', 'Algomeyza', 'Egregez', 'Alcuran', 'Equs', 'CorLeonis', 'Alrica', 'Corvus', 'Algorab', 'Deneber', 'Alchimek', 'benenaz', 'Alramek', 'Elfeta', 'Alchi', 'Alhabe', 'Calbalacrab', 'Yed', 'Vega', 'Altair', 'Aldiraz', 'Alrie', 'Libideneb', 'Delfin', 'Alpheraz', 'Menchef', 'Scecor', 'Denebcaytoz'. Each end of the tropic of Capricorn is of dragon head shape. The ecliptic is marked with the usual Latin names of the signs of the zodiac and crudely graduated 12 ? 30?, labelled by 6? and irregularly subdivided. The reverse of the rete bears some markings for a graduation scale on the ecliptic.

The two plates are marked as follows: 1a) laid out and marked for latitude 52? N, with circles for the tropics and the equator, almucantars for every 2?, numbered by 2? from both sides (6 to 24 and 6 to 18, 22, 26 respectively) and along the meridian (16 to 36, 40, 44, 48, 52), azimuths for every 15? and lines for the 12 unequal hours numbered 1 to 12; 1b) marked with circles for the tropics and the equator, almucantars for every 2?, azimuths for every 15? and lines for the 12 unequal hours, but not numbered. The underlying latitude is circa 52?; 2a) marked with circles for the tropics and the equator and the lines for the houses in the manner of Regiomontaus but otherwise blank; 2b) marked with circles for the tropics and the equator, but otherwise blank.

The back is engraved with four altitude scales starting at the top clockwise 90? to 0?, 0? to 90?, 90? to 0?, 0? to 90?, numbered by 10?, subdivided to single degrees. The single degree scale is also used for the circular solar scale, anticlockwise starting at the right, marked with the usual Latin names of the signs of the zodiac (some are abbreviated), graduated 12 ? 30?, numbered by 5?. Further to the inside is a concentric calendrical scale anticlockwise starting at the first point of Aries; the equinoxes correspond to March 12.5 and September 15. The scale is marked with the usual Latin names for the months (some abbreviated; 'Mayus' for May), each month is divided to the corresponding number of days, numbered by 5 and subdivided to single days. In the middle is a concentric calendrical scale consisting of four bands as follows (from the outside):

1) 1 to 28 for the solar cycle.

2) day letters with indication for leap years 567 / 23457 / 123467 / 13456 / 123467B / 24.

3) 7 times 123B (for the leap years).

4) the dominical letters for the leap years corresponding to the 'B' in scale 3: E G B D F A C. In the lower half is a pair of shadow scales, each divided to 12 digits, numbered by 2.

The plane counterchanged alidade has two sighting vanes. The equally ungraduated ruler consists of two tapered arms. The very short lobe-ended wedge is a replacement.

The instrument was donated in 1853 by Joseph Mayer and is described in W. H. Morley, Description of a Planispheric Astrolabe Constructed for Shah Sultan Husain Safawi, King of Persia, and now preserved in the British Museum; comprising an account of the Astrolabe generally (London, 1856, repr. in R. T. Gunther, The Astrolabes of the World (2 vols, Oxford, 1932), vol. 1, pp. xix-50), pp. 45-6 (with fig. 35 on plate XXI); R. T. Gunther, The Astrolabes of the World (2 vols, Oxford, 1932), pp. 468-9, no. 292; and F. A. B. Ward, A Catalogue of European Scientific Instruments in the Department of Medieval and Later Antiquities of the British Museum (London, 1981), p. 113, no. 326 (with plate LII).