Central surface number densities of sources brighter than a given magnitude for the three clusters of our sample, colour-coded as indicated in the upper left corner of the plot. Magnitudes are given relative to the main sequence turn-off (MSTO) in each cluster. The solid horizontal line corresponds to the source density where we expect to hit the confusion limit. Below the density indicated by a dashed line the deblended spectra will likely have a sufficient signal-to-noise ratio for radial velocity (RV) determination.