Colour magnitude diagram for the globular cluster M 13

We measured 3532 stars in two overlapping regions of the galactic globular cluster M 13, and obtained the Colour-Magnitude Diagram, down to the main sequence, as a preliminary search for blue Horizontal Branch stars in this cluster to later observe with a multifilter spectrograph. The results are consistent with the only complete CMD available in literature, Sandage (1970). More reductions are in progress to extend and improve the set of data