Left: impact of calibrator diameter uncertainty on the LBTI null uncertainty (computed at a wavelength of 11.1 μm). The 5 solid lines show the results obtained for calibrators of various diameters (1–5 mas). The dashed lines indicate for comparison the null excesses expected for a Sun located at 10 pc and surrounded by a face-on disk with two different zodi levels (i.e., 3 and 6 zodi, defined as in Kennedy et al. 2015). Assuming that the tolerable error due to the calibrator uncertainty has to be kept below 3 zodi, this corresponds to a ∼12% relative error on the diameter of a 3 mas star, but only a ∼4% relative error on the diameter of a 5 mas star. Right: spectral transmission curve of the N′ filter used to obtain the data presented in this paper (solid blue line). The dotted red line represents the infrared transmission spectrum of the atmosphere, computed for a representative observing site (i.e., Mauna Kea) and assuming 3 mm of PWV and an air mass of 1 (Lord 1992).