Accurate Measurement of All-sky Camera's Attitude Based on Virtual Planetarium

All-sky cameras have been widely used in meteorology, astronomy and other fields. In the monitoring of cloud cover, night skylight, meteors and other applications, it is necessary to accurately measure cameras' attitude, especially the deviations caused during installation and maintenance. The purpose of this study is to develop a method to measure camera's attitude parameters with high precision, by comparing the polar coordinates of reference stars' image points and the stars' horizon coordinates computed by a virtual planetarium, and generate visualization products with errors corrected. Using the all-sky camera developed by the Yunnan Observatories of the Chinese Academy of Sciences and deployed in Sichuan's Wumingshan observation station, we analyze four sets of all-sky images obtained in October 2016 and June 2017, select star samples with capacities of ~10, ~50, and ~200, obtain their coordinates through the Stellarium planetarium, and accurately acquire the attitude parameters such as the zenith position and rotation angle of the image field. Main results are: (1) We developed a method that is based on virtual planetarium's function to compute stellar coordinates, and improved the accuracy of the measurement with a large capacity of reference star samples. The method does not rely on the camera's specs and therefore is transplantable. (2) According to the camera's attitude on 2016-10-20, its image field's radius and center coordinates are 0.428 9 and (0.759 0, 0.504 8) respectively (in the unit length of the image's Y direction, the same below); the zenith's coordinates are: (0.751 18 ±0.000 20, 0.495 72 ±0.000 22); the angle between the zenith and the optic axis is 2.36°; the rotation angle is 152.71 ±1.58. (3) Maintenance operations have little (≤0.28%) influence upon the geometric parameters of the round-shaped image field, but might cause the optical axis to deviate from the zenith, which will have a non-negligible effect on the axial symmetry of the camera's projection. Each maintenance will also change the camera's rotation angle which needs to be re-determined afterwards. (4) Visualization products conforming to industry norms are produced, which will lay the foundation for further study of the surveillance data.