Al-Farghani also wrote several documents about astronomical instruments. His most famous is his treatise on the astrolabe, which is the oldest surviving document that details the theoretical construction and use of the tool. Although historical sources indicate that there were probably other documents regarding the theory of astrolabes (including one written by al-Khwarizmi) circulating around the time that al-Farghani wrote his treatise, al-Farghani notes in his treatise that he was not aware of any such documents, suggesting that his treatise was a purely original work. Al-Farghani's treatise on the astrolabe provides the mathematical basis for the construction of the astrolabe, along with tables containing thousands of data points enabling the construction of astrolabes that function at varying lines of longitude. Though a theoretical basis for the construction of an astrolabe is presented in this work, the treatise lacked specific methods for physical construction because the purpose of the treatise was not to give instructions for building an astrolabe, but rather to provide mathematical justification for the functionality of the astrolabe. The work "Kitāb al-Fihrist" by Ibn al-Nadim suggests that al-Farghani was also responsible for writing a book about the use and function of sundials, though no copies exist in the present day.