A Method of Extracting Flat Field from Real Time Solar Observation Data

Abstract

Existing methods for obtaining a flat field rely on observed data collected under specific observation conditions to determine the flat field. However, the telescope pointing and the column-fixed pattern noise of the CMOS detector change during actual observations. This leads to the residual signals in real-time observation data after flat field correction, such as interference fringes and column-fixed pattern noise. In actual observations the wind causes the telescope to wobble slightly, which leads to shifts in the observed data. In this paper, we propose a method of extracting the flat field from the real-time solar observation data. Firstly, the average flat field obtained by multiframe averaging is used as the initial value. A set of real-time observation data is input into the KLL method to calculate the correction amount for the average flat field. Secondly, the average flat field is corrected using the calculated correction amount to obtain the real flat field for the current observation conditions. To overcome the residual solar structures caused by atmospheric turbulence in the correction amount, real-time observation data are grouped to calculate the correction amounts. These residual solar structures are suppressed by averaging multiple groups, improving the accuracy of the correction amount. The test results from diffraction-limited and ground-based simulated data demonstrate that our method can effectively calculate the correction amount for the average flat field. The New Vacuum Solar Telescope (NVST) He I 10830 Å/H\(\alpha \) data were also tested. High-resolution reconstruction confirms that the correction amount effectively corrects the average flat field to obtain the real flat field for the current observation conditions. Our method works for chromosphere and photosphere data.