Observations in the Mg iλ5173 line. The top row correponds to Dopplergrams (calculated from images at ± 42 mÅ). The other panels, in descending order, show red wing images, λ − λ₀ = 42, 238, and 878 mÅ, respectively. The images in the left column were restored with the flat-fielding scheme by Schnerr et al. (2011). Artifacts due to cavity errors appear as small-scale grains, clearly visible in panels a) and b). In contrast, no such errors are visible in the right column, corresponding to images restored with our new method.