The six plates are engraved with stereographic projections for latitudes 24°, 30°, 37°, 29°, 35°, 42°, 33°, 32°, 22° and 36°, where 32° would be for Isfahan and 22° for Mecca.
This finely engraved astrolabe very closely resembles another signed by the same maker, dated to circa 1660, now in the Museum of the History of Science in Oxford (inv.46886; http://www.mhs.ox.ac.uk/collections/imu-search-page/record-details/?TitInventoryNo=46886=field=on=1665). The position and the wording of the signature are identical on both astrolabes. The retes with their elegant cusped arabesque pointers and the calligraphy on the plates are remarkably similar. J.A.Billmeir, a renowned collector of scientific instruments, wrote of another astrolabe signed by Muhammad Mehdi ‘in spite of the most elaborate ornamentation the mathematical accuracy of the engraving is unimpaired’ (Scientific Instruments (13th-19th Century): The Collection of J. A. Billmeir Esq., Frank Patridge & Sons, London, 1954, no.5, p.11). The same could imply to this instrument, and to that offered as lot 53 in this sale - another example of the work of this illustrious maker.
The Oxford astrolabe has a very similar compass inserted into the throne which again looks to have been made by the same hand as the one found on our astrolabe. Whilst the compass on our astrolabe is set with an engraved border suggesting that it was part of the original design of the throne, the Oxford one looks to be less in harmony with its decorative surroundings. During the 17th century however there was a fashion in Iran for incorporating foreign instruments into astrolabes. It is therefore possible that the compass in our astrolabe was in fact inserted in Iran at the time of manufacture. Another distinct similarity between our astrolabe and that in Oxford is that both have identical brown ropes attached. It is possible that the ropes were added in the workshop where both astrolabes were produced. It is also possible that the rope was added later when the astrolabes had arrived in Europe, and thus that they were sold to the same original owner after their production. The Oxford astrolabe is recorded as having come from the collection of M. Chadenat, who added it to his collection of scientific instruments sometime before the middle of the 20th century. It then passed into the Billmeir Collection, from whence it was donated to the Museum in Oxford. It is possible that our astrolabe at one stage passed through the hands of one or both of these collectors. Another astrolabe signed by Muhammad Mehdi al-Yazdi which is dated AH 1070/1659-60 AD and also has a compass inserted into the throne, is in the collection of the Royal Museums Greenwich (inv.AST0594, http://collections.rmg.co.uk/collections/objects/10756.html). A further astrolabe signed by the same maker but with a later associated rete was sold at Sotheby’s London, 6 October 2010, lot 150.