Abstract
The site quality of astronomical observatory critically depends on cloud coverage, and the measurement of cloudiness is particularly important for site survey. A method to deal with all-sky images in no-moon nights is described. By identifying the positions of bright reference stars and making photometry for a set of all-sky images in clear nights, we can set up a reference image with median smoothing differential magnitude values. The standard image can be taken as the threshold for clear nights, and the detectivity of stars on other images can be utilize to reveal cloud coverage. Four types of all-sky images, clear night, icy lens, part of cloud, and full of cloud, are selected to check up the method. For the moonlight image, the formula of the CIE (Commission International de l'Eclairage ) standard general sky model are used to fit the luminance distribution of the image, then to estimate the cloud cover of the image from the difference between the fitting image and the real image.
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