Correction of solar dopplergrams' large-scale flatfield

The authors have proposed and compared three different methods of large-scale flatfield correction (time averaging, Fourier filtering, and wavelet-based filtering). As an example, the real dopplergram of the velocity field in a solar active region obtained at the Huairou Solar Observing Station of National Astronomical Observatories of China has been processed. It is shown that any of the applied methods can improve the visual quality of the image. A quantitative comparison of the proposed methods was performed by evaluating the heterogeneity of the images.