Comparison of the observed visibilities of Arcturus with those predicted by the MARCS photospheric model. Panel a shows a comparison of the spectrum, while panels b–d show a comparison of the visibilities observed at the 7.3, 14.6, and 21.8 m baselines, respectively. In each panel, the thick solid line represents the observed data, and the thin solid line represents the model. The typical errors are shown in the same manner as in Fig. 1. The parameters of the MARCS model are described in Sect. 4.1.