Study of the chromatic scaling of the diffraction images of the host star for the two datasets. a: coloured projection of the HR 8799 reconstructed hyperspectral cube (cubic root stretch of the intensity). b: slice of the HR 8799 reconstructed hyperspectral cube (λ = 1486 nm, cubic root stretch of the intensity, colour bar: arbitrary unit). The five black dots represent the fitted positions of the four images of the star and of the centre of the fitted square (dashed lines). c: linear fits of the square diagonals according to the wavelength for each dataset at two different times in the night with the proposed PIC method and, for the first time in the night, with the SPHERE reduction pipeline (RP, Delorme et al. 2017). d: fitted positions of the squares at each wavelength compared to the estimation of the star position. Only the wavelengths whose position falls inside the grey circle of 0.4 voxels radius are used to fit the linear laws.