Illustrative example of an AMBER HR-mode (R = 12 000) observation of η Car (see Sect. 2). The figure shows from top to bottom: example interferogram, spectrum of the Brγ region (heliocentric wavelengths; the telluric lines are not removed), wavelength dependence of the visibilities at three baselines (PAs and projected baseline lengths are indicated in the panels), wavelength-differential phases (the curve of the shortest baseline is shifted up by 90° and the longest down by 90°), and closure phases.