Multi-site Cross Calibration of Sky Brightness Monitors for Solar Coronal Observations: Comparative Analysis of CGST and ATST Site Survey Instruments

Accurate calibration of the Sky Brightness Monitors (SBMs) is crucial for reliable astronomical site evaluation and solar coronal observation planning. However, the measurement accuracy and the mutual consistency between SBMs developed by different teams remain unverified, limiting the integration of data sets from multiple instruments. In this study, we present a comprehensive cross calibration of the SBMs respectively developed by the Chinese Giant Solar Telescope (CGST) team and the Advanced Technology Solar Telescope (ATST, previous name for the 4 m DKIST) team, based on three calibration campaigns conducted at Zhaotong, Ali and Daocheng sites. By overcoming the harsh conditions brought by high-altitude field observation, we used the two sets of SBM equipments to observe simultaneously at these sites, and obtained the data profile of each site for a whole day, enabling a robust analysis for the cross-calibration of the SBMs. The results show that the efficacy of the calibration process is highly dependent on site-specific atmospheric stability conditions, with Daocheng providing the most stable and reliable calibration environment. The analysis further distinguishes site-dependent atmospheric effects from instrument-intrinsic calibration offsets, showing that the latter persist under stable observing conditions and can be robustly quantified at Daocheng. Based on the complete measurement records at Daocheng, the final calibration values at 530 nm are determined to be −0.92 × 10⁻⁶I_⊙ (I_⊙: solar disk-center brightness) and −0.515 × 10⁻⁶I_⊙/airmass (airmass: air mass along the line of sight for observation) for the two instruments, i.e., the measurement value of CGST SBM is only 1 × 10⁻⁶I_⊙ less than that of ATST SBM equipment, which is completely within the measurement error range of the equipment itself. The high consistency achieved validates the integration of CGST SBM data sets into ATST-related analyses and confirms both instruments’ suitability for routine sky brightness monitoring at optimal sites.