Illustration of RC star selection. From left to right: panel a shows the original F090W – F200W CMD and panel b shows the same CMD after applying unsharp masking. We used this to obtain initial estimates for the blue and red end of the RC sequence as shown by the triangle symbols. Panel c shows the selection of RC stars using these initial estimates to obtain a selection region. Blue dots show all stars in this CMD falling into the selection region while orange dots show the final sample of stars that fall also in similar selection regions for other filter combinations. Panel d shows the final resulting fitted slope.