(b − y) −  [Fe/H]  diagram of the photometric sample of Schuster et al. (Salaris, priv. comm.). The red line corresponds to the MSTO determined from the SDSS sample as indicated in Table 1 and transformed to Strömgren photometry using the empirical relations of Ramírez & Meléndez (2005). There is a perfect agreement between the cut-off of SDSS and the uvby − β samples. The blue dashed line corresponds to the MSTO determined applying the Sobel Kernel edge-detector to the stars of the uvby − β sample.