Step-by-step of the preprocessing applied to the J-PLUS data cube of M101. The columns show, respectively: the original data, Butterworth filter, PSF homogenization, 2 × 2 binning, and Voronoi binning (defined by the u-band). The blue line represents the contour of the galaxy mask. The first and second rows are the surface brightness maps (in AB mag/arcsec²) in the u and r-band respectively. Pixels with < 0 flux appear in white. The legends f_u⁻ and f_r⁻ show the number of pixels with negative fluxes inside the galaxy mask in the u and r bands, while the f₁₂⁻ numbers under the plots are the sum of the f_λ < 0 pixels in all the 12 bands for each of the preprocessing steps. Masked stars are marked in green.