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Context: A turbulent atmosphere causes atmospheric piston variations leadingto rapid changes in the optical path difference of an interferometer, whichcauses correlated flux losses. This leads to decreased sensitivity and accuracyin the correlated flux measurement. Aims: To stabilize the N bandinterferometric signal in MIDI (MID-infrared Interferometric instrument), weuse an external fringe tracker working in K band, the so-called FSU-A (fringesensor unit) of the PRIMA (Phase-Referenced Imaging and Micro-arcsecondAstrometry) facility at VLTI. We present measurements obtained using the newlycommissioned and publicly offered MIDI+FSU-A mode. A first characterization ofthe fringe-tracking performance and resulting gains in the N band arepresented. In addition, we demonstrate the possibility of using the FSU-A tomeasure visibilities in the K band. Methods: We analyzed FSU-A fringe trackdata of 43 individual observations covering different baselines and object Kband magnitudes with respect to the fringe-tracking performance. The N bandgroup delay and phase delay values could be predicted by computing the relativechange in the differential water vapor column density from FSU-A data.Visibility measurements in the K band were carried out using a scanning mode ofthe FSU-A. Results: Using the FSU-A K band group delay and phase delaymeasurements, we were able to predict the corresponding N band values with highaccuracy with residuals of less than 1 micrometer. This allows the coherentintegration of the MIDI fringes of faint or resolved N band targets,respectively. With that method we could decrease the detection limit ofcorrelated fluxes of MIDI down to 0.5 Jy (vs. 5 Jy without FSU-A) and 0.05 Jy(vs. 0.2 Jy without FSU-A) using the ATs and UTs, respectively. The K bandvisibilities could be measured with a precision down to ~2%.

Mid-infrared interferometry with K band fringe-tracking I. The VLTI MIDI+FSU experiment