An Introduction to the All-Sky Camera at the YNAO Lijiang Astronomical Station

An all-sky camera serves as a rather important tool for monitoring cloud distributions over an astronomical observation site. An all-sky camera can improve weather monitoring for astronomical observations, including outdoor observations in site selection. Major observatories in the world now have all-sky cameras. In order to meet the requirements on real-time monitoring of clouds for Yunnan Observatories, we have installed an all-sky camera system at the Lijiang astronomical station. In this paper we describe details of the design and implementation of the all-sky camera system. These include issues of installing hardware, debugging software, and practical tests. We list solutions of these issues. A particular issue is that sky-brightness values can vary dramatically with levels of cloud coverage. This makes it difficult to manually set proper exposures in using the camera. We have found a method to automatically adjust camera exposures according to sky-brightness values. The method sets camera exposures to decrease with sky-brightness values. With exposures correctly set with this method in either day or night, the camera can obtain high-quality all-sky images. This method is also the most innovative and characteristic part of our work with the all-sky camera.