Model source geometries and their Fourier transforms. In real space (top row), the two black rings stand for the minimum and maximum angular resolution of the interferometer (5 and 50 mas, respectively), the orange and red Gaussians stand for the (over-)resolved and unresolved mid-IR emitters, respectively. In case 1, both the compact unresolved as well as the larger, interferometrically resolved component are constrained by our model fit. Model 2 consists of an essentially unresolved source (red) with over-resolved flux (orange). The size of the over-resolved emitter cannot be determined and could be as large as the single-dish PSF of about 300 mas. Models 3 and 4 contain only one, essentially unresolved, component which in model 3 is marginally resolved and in model 4 entirely unresolved. In the bottom row, the Fourier transforms of these models are shown. Here the two black rings denote the minimum and maximum projected baseline using the 8m Unit Telescopes of the VLTI. Note how the compact source (red) contributes to the correlated flux at all baselines while the extended source (orange) is only observable at short baselines.