Left panel: spectral index map obtained from the RadioAstron images of 3C 273 at 1.6 GHz and 4.8 GHz, both restored considering only the common UV-range 2.5−340 Mλ, and convolved with the MOJAVE circular beam (0.83 × 0.83 mas). The map is plotted over the 1.6 GHz image shown in contours. Pixel values below 5σ for both the 1.6 GHz and 4.8 GHz images were blanked. Right panel: spectral index value and its uncertainty along the jet axis, plotted as a function of the radial distance from the core. The adopted convention for the spectral index α is S ∝ ν^α.