Processing Method of Night-Time Cloudiness for Astronomical Site Selection

The cloudiness is one of the most important factors which affect the quality of an astronomical site, the monitoring and processing of the night- time cloudiness are especially important. The ground-based cloudiness camera is adopted to carry out the monitoring of the all-sky cloudiness, the images taken need to be processed by means of an effective method so as to quantize the cloudiness. The night-time cloudiness images are seriously affected by the moon- light, and therefore, the night-time cloudiness images are processed by dividing them into the moonlight and moonless two sorts. In the light of the condition of moonless night, the processing method of night-time cloudiness is given. The positioning and photometry of the bright stars in the image are conducted to determine their magnitude differences. By referring to the magnitude differences of the bright stars in the clear-night image, the probability of the bright stars of which the magnitude differences are lower than the threshold value are regarded as the probability standard of clear nights. Three sorts of images are selected to test the method. The cloudiness is determined, and the effect of the threshold condition on the result is analyzed. Finally, the applicable range and uncertainty of the method are discussed.