  In optical astronomical telescopes, the primary baffle is a tube-like
structure centering in the hole of the primary mirror and the vanes usually
locate inside the baffle, improving the suppression of stray light. They are
the most common methods of stray light control. To characterize the performance
of primary baffle and vanes, an empirical comparison based on astronomical
observations has been made with Xinglong 50cm telescope. Considering the
convenience of switching, an independent vanes structure is designed, which can
also improve the process of the primary mirror cooling and the air circulation.
The comparison of two cases: (1) primary baffle plus vanes and (2) vanes alone
involves in-dome and on-sky observations. Both the single star and the various
off-axis angles of the stray light source observations are presented. The
photometrical images are recorded by CCD to analyze the magnitude and the
photometric error. The stray light uniformity of the image background derives
from the reduction image which utilizes the MATLAB software to remove the
stars. The in-dome experiments results reveal the effectiveness of primary
baffle and the independent vanes structure. Meanwhile, the on-sky photometric
data indicate there are little differences between them. The stray light
uniformity has no difference when the angle between the star and the moon is
greater than 20 degrees.
