Table 2: Log of optical observations.
Cluster Survey Obs. date Filter Exp. $m_{\rm comp}$, $m_{\rm lim}$ Airmass
    (UT)   (s) (mag)  
$\sigma $ Orionis Palomar 2002 Feb. 07 i' 46 $\times$ 200 23.2, 24.5 1.38-1.39
  Palomar 2002 Feb. 07 z' 21 $\times$ 200 22.5, 24.0 1.40-1.93
  Calar 2003 Dec. 23 i' 26 $\times$ 500 23.8, 24.5 1.30-1.93
  Calar 2003 Dec. 24 z' 26 $\times$ 600 22.8, 24.0 1.30-1.95
Praesepe Palomar 2002 Feb. 07 i' 28 $\times$ 200 23.8, 24.8 1.03-1.23
  Palomar 2002 Feb. 07 z' 21 $\times$ 200 23.3, 24.3 1.25-1.80
  Calar 2003 Dec. 23 i' 9 $\times$ 500 23.3, 24.5 1.05-1.13
  Calar 2003 Dec. 23-24 z' 18 $\times$ 500 22.8, 24.0 1.05-1.44


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