Calibration and Applications of the All-Sky Camera at the Ali Observatory in Tibet

TL;DR

This paper presents a high-precision calibration method for all-sky cameras at the Ali Observatory, enabling accurate atmospheric and sky brightness measurements despite challenges at large zenith angles.

Contribution

The study introduces a novel calibration approach that reduces deviations to less than 0.5 pixels, improving accuracy for all-sky imaging in Tibet.

Findings

Calibration deviation less than 0.25 pixels within 70° zenith angle

Increased star data improves calibration accuracy beyond 70°

Atmospheric extinction coefficient is approximately 0.20

Abstract

A high-precision calibration method for all-sky cameras has been realized using images from the Ali observatory in Tibet, providing application results for atmospheric extinction, night sky brightness, and known variable stars. This method achieves high-precision calibration for individual all-sky images, with the calibration process introducing deviations of less than 0.5pixels. Within a 70-degree zenith angle, the calibration deviation of the images is less than 0.25pixels. Beyond this angle, the calibration deviation increases significantly due to the sparser distribution of stars. Increasing the number of stars with zenith angles greater than 70degrees used for calibration can improve the calibration accuracy for areas beyond the 70-degree zenith angle, reducing the calibration deviation at an 85-degree zenith angle to 0.2pixels. Analysis of the all-sky images indicates that the atmospheric extinction coefficient at the Ali Observatory is approximately 0.20, and the night sky background brightness is about 21 magnitudes per square arcsecond, suggesting the presence of urban light pollution.