Ratio map between the two NIKA bands, . The unmasked region corresponds to a signal-to-noise ratio on the fluxes above 10 at both wavelengths. Black contours represent R_1,2 equal to 3, 4.5, 5.6, 7, 8, and 9. The purple contour corresponds to the part of the field with homogeneous observing time >240 s per pixel (see Fig. A.1). The open circle in the bottom left corner represents the beam size (24″).