Panel a: S/N of the two-point auto-correlation functions (C(r)∕σ_C) of 48 noise channels, four per molecule, as a function of pixel separation r. All points at pixel separations smaller than the beam are coloured in purple, the other ones in cyan. The vertical dashed line at 2′′ correspondsto the major axis of the beam. The area above and to the right of the magenta line is the area where the correlation function of a channel containing signal indicates a significant correlation. Panel b: distribution of C∕σ_C. The regions of S/N smaller than 3, 4, and 5 are highlighted in shades of grey.