Left: Examples of Planck y-maps of three clusters with different values of the 𝒞 parameter. Top: cluster PSZ2 G075.71+13.51 (PSZ2 index 322). Middle: cluster PSZ2 G340.88−33.36 (PSZ2 index 1591). Bottom: cluster PSZ2 G093.94−38.82 (PSZ2 index 430). Each map is centred on the cluster position reported in the PSZ2 catalogue, with side-length equal to 2θ₅₀₀. The white circle is the aperture defined for the Zernike fit. The markers in green in the top-right corner of each map are used to indicate the respective values of the 𝒞 parameter in Fig. 3. Right: Bar charts of the c_n,±m coefficients (see Eq. (11)) resulting from the Zernike fit applied on each map on the left. The ZPs on the x axis are ordered following the Noll's scheme (Noll 1976). The polynomial order n increases from left to right. Orange and blue bars refer to ZPs with m = 0 and m≠0, respectively.