The Co-alignment of Winged Hα 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.

GST Data-processing Workflow: Image Registration and Alignment

“…This method is mainly used in object recognition and object tracking aimed at deriving object motion parameters from two neighboring images in the time domain. Cai et al (2022) developed an algorithm to coalign multiwavelength imaging spectroscopic data collected by NVST and evaluated its performance accuracy using raster images acquired by the Fast Imaging Solar Spectrograph (FISS) operating at GST. Liang et al (2022) proposed a two-step imaging registration method based on the regular step gradient descent algorithm and Mattes mutual information similarity metric to register the GST and SDO/HMI data.…”