Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by AURA, Inc., under NASA contract NAS 5-26555.

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The coordinates and the photometry of the objects are available in electronic form at http://astro.uu.nl/~bastian/M51-bik/ and at http://www.edpsciences.org

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...acceptable

We will refer to this method in subsequent papers as the "3(2)DEF- method'', i.e. the three (or two) dimensional energy distribution fitting''.

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... masses

The initial cluster mass that we derived in this way is in fact "the initial mass of the current stellar population'', i.e. it is the "current mass corrected for stellar evolution effects''. This value can be different from the the initial mass of the original cluster if the cluster suffered evaporation or disruption. In that case the derived initial mass is the initial mass of the original cluster minus the fraction that has disappeared by evaporation or disruption.

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