Telescope Array (TA) is an international joint experiment observing ultra-high energy cosmic rays. TA employs the fluorescence detection technique to observe cosmic rays. In this tech- nique, the existence of cloud significantly affects the quality of data. Therefore, cloud monitoring provides important information.
We are developing methods for evaluating night sky weather with pictures taken by a charge- coupled device (CCD) camera. We found that there was a change in the brightness of the whole image due to the weather. We tried to judge the weather only by the brightness of the whole image and gained only limited success. On the other hand, it is expected that there will be a correlation between the amount of clouds and the number of stars appearing in the image. We first devised a method to count the number of stars in the image. The stars are clusters of very bright pixels in the image, the method developed focusing on this has worked well.
Currently, we are developing evaluation methods to compare detected stars and catalogs.