Measurement of Sky Brightness and Suppression of Scattering in Sky Brightness Monitor

A modern Sky Brightness Monitor(SBM)was developed for the site survey in West China for the future large solar equipment installation.The performance of this new SBM was tested in the recent preliminary experiments.A lot of sky brightness data had been obtained at a few sites in Yunnan.The blue channel result shows that the sky brightness near the noon time on Jiaozi Snow Mountain is as low as a few millionths of the solar center intensity,indicating the low scattering level inside our SBM instrument.The scattering is mainly from two parts:the diffraction rings from the occulter edges,distributed in the inner field of view but outside the occulter region containing the ND4 filter;the diffraction from the baffle rings,distributed in the extreme edges of the field of view.To suppress the scattering of the latter part,experiments with different aperture sizes of baffle rings are made. The result shows that,by mounting new baffle rings with proper aperture size into the SBM telescope tube,diffraction in the extreme edges of the field of view can be effectively reduced.