The Co-alignment of Winged Ha Data Observed by the New Vacuum Solar Telescope

Abstract The New Vacuum Solar Telescope (NVST) has been publishing its novel winged Ha data (WHD) since April 2021, namely the Ha imaging spectroscopic data. Compared with the prior released version, the new data has been further co-aligned among the off-band images and packaged into a standard solar physics community format. In this study, we apply the optical flow method to co-align spectroscopic data, and assess the alignment accuracy of algorithm under the different similarities between the images of off-band and line center. As a result, the alignment accuracy can reach up to 0.004 ′′ and 0.095 ′′ in the near wing from line center of 0.2 Å and 0.4 Å, which corresponding to the similarity of about 0.8 and 0.4, respectively. Our algorithm can meet the alignment accuracy requirement of within 0.1 arc sec under the similarity ≥0.3, corresponding to the maximum offset wavelength of 0.5 – 0.6 Å. Otherwise, it is necessary to increase more intermediate off-band images for transition alignment. In addition, the article illustrates the final product of the WHD in detail, which will provide convenience for the solar physicists to use high resolution Ha imaging spectroscopic data of NVST.