Filtering of the sample stars based on the CMD. Left: CDF of the CMD parametrised by the offset of the adopted IS slope. The inset shows the derivative of the CDF in the range of interest. The dashed red lines show the limits at which the CDF values start to either increase significantly or level off, i.e. the positions of the edges of the M 51 IS. Right: CMD of the M 51 sample, with the IS edge derived based on the CDF (red line). The grey points show the stars that were flagged as outliers. The grey curves show the theoretical instability strip edges from Anderson et al. (2016) reddened by the Galactic colour excess of 0.03 mag for comparison.