Left-hand panel: histogram of the completeness the OPTICS extracted cluster (C_O). It is clear that a large fraction of our detected clusters have low completeness values, as explain in the caption of Fig. 7 and in the text. Middle panel: contamination histogram, where it is obvious that the vast majority of our detected clusters have contamination rates of less than 10%. Right-hand panel: histogram of what fraction of true clusters show up in any cluster extracted by OPTICS. A dichotomy is can be seen in this panel, where clusters are either very well recovered or only a small fraction are detected. The low total completeness values come from clusters that are so sparse compared to the background that they are difficult to distinguish.