Core offset position fit and spectral index maps at 15−43 GHz and 43−86 GHz of 3C 84. Left: core shift of 3C 84 at frequencies of 15, 43, and 86 GHz. The red line shows a fit of Eq. (2) to the core offset positions (for the numbers, see Table A.3). The shaded grey line denotes the distance from the 86 GHz total intensity peak to the true location of the jet apex. The insets show 2D cross-correlation coefficients of the images at two different frequency pairs. Each axis is in units of mas. The contours start at 1% and increase in steps of 10% relative to the maximum cross-correlation coefficient (0.9975 and 0.9995, respectively). The perpendicular dashed white lines intersect at the maximum of the colour map, which corresponds to the maximum value of ρ(i, j). Right: spectral index maps after image alignment. Left panel: 15 GHz image in contours and the 15−43 GHz spectral index in colours. The contours start at 4.36 mJy beam⁻¹ and increase by a factor of two. The maps are convolved with a beam of 0.45 × 0.67 mas, oriented at a PA = −9.44°. Right panel: 43−86 GHz spectral index in colours and the contours of the 43 GHz map, which start at 4.26 mJy beam⁻¹ and increase in steps of two. The maps are convolved with a beam of 0.20 × 0.37 mas, oriented at a PA = 14.1°. We only display regions that have (i) a S/N of at least five for the total intensity contours and (ii) a spectral index in the range −2 to 2, within 1 rms error (±0.14 for the 15−43 GHz pair and ±0.21 for the 43−86 GHz pair), gained from the uncertainty from a region the size of each beam, centred around the core shift location of each frequency (see Appendix A for further details). Boxes marked by dashed white lines correspond to the regions used for the 2D cross-correlation. A1, A2, and A3 are the three characteristic regions where the spectral indices were measured (see Table A.4).