Simplified example of the clustering performed by OCA to identify modes of density, shown in the space formed by the first two principal components. The black lines are the contours of the data density. The blue circles and red crosses correspond to the modes of the large (0.025) and small (0.015) kernel sizes respectively. At the lower resolution (blue circles), the three cluster modes obtained correspond to clusters dominated by stars, quasars and galaxies, although with significant contamination from other types.