On the error analyses of polarization measurements of the white-light coronagraph aboard ASO-S

The Advanced Space-based Solar Observatory (ASO-S) mission aims to explore two most spectacular eruptions in the Sun: solar flares and coronal mass ejections (CMEs), and their magnetism. For the studies of CMEs, the payload Lyman-alpha Solar Telescope (LST) has been proposed. It includes a traditional white-light coronagraph and a Lyman-alpha coronagraph which opens a new window to CME observations. Polarization measurements taken by white-light coronagraphs are crucial to derive fundamental physical parameters of CMEs. To make such measurements, there are two options of Stokes polarimeter which have been used by existing white-light coronagraphs for space missions. One uses a single or triple linear polarizers, the other involves both a half-wave plate and a linear polarizer. We find that the former option subjects to less uncertainty in the derived Stokes vector propagated from detector noise. The latter option involves two plates which are prone to internal reflections and may have a reduced transmission factor. Therefore, the former option is adopted as our Stokes polarimeter scheme for LST. Based on the parameters of the intended linear polarizer(s) colorPol provided by CODIXX and the half-wave plate 2-APW-L2-012C by Altechna, it is further shown that the imperfect maximum transmittance of the polarizer significantly increases the variance amplification of Stokes vector by at least about 50% when compared with the ideal case. The relative errors of Stokes vector caused by the imperfection of colorPol polarizer and the uncertainty due to the polarizer assembling in the telescope are estimated to be about 5%. Among the considered parameters, we find that the dominant error comes from the uncertainty in the maximum transmittance of the polarizer.
16 pages, 5 figure, Research in Astronomy and Astrophysics, accepted