Example of Sobel Kernel technique used to detect the turn-off. The TO is indicated by the red vertical lines, with the dashed lines corresponding to their errors. Panel A) temperature distribution of the field stars with  [Fe/H]  = −1.95  ±  0.15. Panel B) Sobel Kernel filter response given by Eq. (2). Panel C) histogram of temperature and the probability function of Eq. (1) by the blue dotted line.