Astrometric analysis of digitized historical plates: the Ulugh Beg Astronomical Institute archive

ABSTRACT Over more than a century, the Ulugh Beg Astronomical Institute (UBAI) has accumulated a valuable archive of more than 15 000 glass photographic plates obtained mainly with the Tashkent Normal Astrograph (TNA) (1895–1990s) and the Double Astrograph of Zeiss (1975–2005). Since 2014, systematic efforts have been made to classify, preserve, and digitize the archive. This paper presents a detailed overview of the UBAI archive data set, including the classification scheme for the glass plates, the astronomical observations carried out with these plates, and the data processing pipeline applied to the digitized images. The astrometric reduction of digitized single-exposure plates is improved over previous methods by including Point Spread Function (PSF) fitting (using SExtractor and PSFEx) and an improved secondary fitting with the Gaia Data Release 3 catalogue. This process involved determining the optimal third-order plate model, correcting the magnitude equation, and applying a new local correction algorithm to remove residual distortions. Analysis of the first batch of processed plates shows high-astrometric accuracy for the TNA plates. The results confirm the effectiveness of the improved digitization system and the improved processing workflow in obtaining accurate positional information from historical astronomical plates.