It is a discussion of the mathematical principles of astrolabe construction directed toward serious scholars at an “intermediate” level, according to a statement in the introduction.  This treatise also seems to have been somewhat influential, since a “supplement” (Tatmīm `Amal al-Asṭurlāb) was composed by Aḥmad ibn Muḥammad al-Azharī al-Khāniqī (fl. 1350).  An anonymous summary (Tajrīd) is also extant. [BEA, DeYoung]