Fringe-tracking performance: the figures show the fringe-tracking residuals as a function of the reference stars’ K-band correlated magnitude for UTs (top) and ATs (bottom), respectively. The horizontal lines indicate residuals of 300 nm rms, for which the fringe contrast in long exposures is reduced by ~30% in the K-band. For the ATs, we plot the fringe-tracking residuals separately for good seeing with long atmospheric coherence times τ₀> 4 ms (red), and for short coherence times τ₀< 4 ms (blue). We do not have enough statistics to make this distinction for the UTs. The OPD residuals for long coherence times are typically 200 nm and 300 nm rms, the limiting magnitudes around m_K ≈ 7 mag and m_K ≈ 10 mag for the ATs and UTs, respectively.