Second-order spatial coherence 1⁽²⁾ measured for artificial single stars of different angular sizes. The coherence values come from correlation functions such as in Fig. 4, averaged for delays between 1−10 μs, normalized to baseline zero. Superposed are theoretical dependences for ideal circular apertures (Airy patterns; squared moduli of the Fourier transforms, 1+ (2 J₁(x) /x)²), confirming the expected trends. The precise effectively illuminated source shapes depend on focusing details and are not exactly known; however typical measuring reproducibilities were within ~0.02 of the coherence values.