Design and Implementation of the All-Sky Cloud Monitoring System at Shenzhen Astronomical Observatory

All-sky cloud monitoring is of key importance for astronomical observation. Most astronomical observatories in the world have all-sky cloud cameras now. But one common concern is to get accurate exposure in a vast range of illumination conditions. To solve this problem, Shenzhen Astronomical Observatory has developed independently an automatic exposure control system. By using the parameters of the all-sky camera, this system calculates an index curve of exposure value, with error of 0.3, to adjust the exposure of each picture that the camera captures. And based on the pictures with accurate exposure, cloud can be effectively detected by the maximum interclass variance adaptive threshold selection method. It has been proved in practice that this system works well in varying illumination changes, obtaining accurate exposure even in low-light circumstances, and therefore providing substantial convenience to observation planning.