Principle of Measurement with an Evans Visual Sky Photometer

An Evans Visual Sky Photometer (EVSP) is important for a site survey for coronagraph(s). It has been applied since 1940s to judge and estimate sky brightness levels for observatories. Long-term site data have been obtained and recorded for some observatories. These data provide the calibration references for sky brightness monitors (SBMs) currently developed for site survey projects for large-aperture solar instruments, especially high-sensitivity coronagraphs. New SBMs have been developed in 2010 for the solar site survey in the west China. Like other observatories in the world, we need to use an EVSP for the calibration in the next measurement stage of work. In this paper, we study the theoretical basis and calibration method of an EVSP in the Yunnan Observatory. We compare the calibration curves for a few EVSPs. Because of the well known parameter variance of the optical elements inside an EVSP, including that of the density of wedge, it is efficient to make the I_sky-Scale_wedge calibration with our method introduced to obtain the calibration curve. With good maintenance of the optical system this EVSP can function properly.