The present paper introduces, investigates, analyses, and comments on an anonymous treatise in Persian named al-Risāla al-Ghāzāniyya fi ‘l-ālāt alra ½$times$diyya,
Ghāzān's (or Ghāzānid) treatise on the observational instruments'', which describes the structure, construction, and functions of twelve new’' observational instruments in the medieval period that appear to have been proposed and invented during the reign of Ghāzān Khān, the seventh Ilkhan of the Ilkhanid dynasty of Iran (21 October 1295–17 May 1304). In the sections below we consider the treatise in the light of two issues: (1) the assumption that the primary historical sources may contain interesting notes and claims concerning Ghāzān Khān’s astronomical activities and especially the new observatory that he founded in Tabriz, and (2) the fact that at present there are hardly any sound and historically reliable accounts of the activities of the Maragha Observatory from around 1280 onwards. It is thus essential to explore the issues that constitute the principal historical features of the research, i.e., Ghāzān, the Maragha Observatory, and astronomical activities, and to clarify the contextual relations between them. In what follows we present the key historical facts (derived from the primary sources) regarding Ghāzān and his connection to both astronomy and the Maragha Observatory. Second, we describe the Maragha Observatory in the period in question, giving further details about the observational programs conducted there and noting the substantial differences between them. These data cast new light on the activities of the observatory and, as we shall see below, may challenge the established history. We then examine the treatise, its contents, the manuscripts available, and the original approach applied to the design and construction of the instruments. In the final section, we examine the notes (and the possible misunderstandings as well) deduced from the treatise as regards the instruments, their physical construction, and their relation to Ghāzān and the Maragha Observatory. The section also contains two open discussions on the only possible archaeological evidence for the instruments and the authorship of the treatise. The most important evidence is, of course, provided by the instruments themselves and the new approach applied to their design and construction, which we discuss in the second part of the paper along with a classification of the different types of the instruments. We describe the configuration and functions of each instrument separately. These two sections are based on the text; a few changes in the order and arrangement of the materials are introduced to give a fuller account of each instrument in relation to the original text. These are followed by a separate section containing critical comments on the instrument with regard to either technical or historical considerations, including critical remarks such as probable mistakes or omissions in the treatise and some suggestions for corrections and completions, an analysis of our author’s claim concerning the superiority of a new instrument over its precursors, the applicability of each instrument, the comparison of a new instrument with similar historical counterparts, and so on.