Digitization of Astronomical Photographic Plate of China: Photometric Calibration of Single-exposure Plates and Release of Photometric Catalogs
The Chinese Plate-Digitizing Project has digitized a total number of about 30,000 astronomical plates from 11 telescopes of five observatories (SHAO, NAOC, PMO, YNAO, and QDO) in China, spanning nearly 100 years of observations. In this work, we present a photometric calibration method to calibrate about 15,000 single-exposure plates to the JKC photometric system. Using standard stars constructed from the BEST database, we have identified and corrected various systematic effects (including the magnitude term, color term, and flat-field term) to a high precision. The final photometric precision is typically 0.15, 0.23, 0.17, 0.11, 0.19 mag for plates collected in SHAO, NAOC, PMO, YNAO, and QDO, respectively, with best cases reaching 0.07, 0.08, 0.06, 0.05, and 0.11 mag, respectively. Candidates of variable sources are also identified. The catalogs are publicly available at the China National Astronomical Data Center. Such a dataset will provide a valuable resource for conducting long-term, temporal-scale astronomical research. Our calibration method can also be applied to other digitized astronomical plates in general.