Effect of terrestrial stray light on observed solar magnetic signal

Terrestrial stray light is a troublesome problem for its serious effects on the ground-based measurement of solar photosphere magnetic field. In theory, it could be divided into large-spread-angle stray light (>10") and small-spread-angle stray light (~1"). In this paper we emphasize on the former and study its variation during a day of observation at Huairou Solar Observing Station of Nation Astronomical Observatories of Chinese Academy of Sciences. We find a close correlation between the measured magnetic field and variation of local stray light.