This paper is the second in a series describing the observations of "young massive star clusters'' (YMCs) in a number of nearby spiral and irregular galaxies. A general outline of the project was given by Larsen & Richtler (1999, Paper I), and some first results were presented there. In this paper the data reductions are discussed in more detail, with special emphasis on two software tools developed as part of the project, which may be of general use to the astronomical community. These tools are mksynth, a programme which generates synthetic images with realistic noise characteristics, and ishape which derives intrinsic shape parameters for compact objects in a digital image by convolving an analytic model profile with the PSF and matching the result to the observed profile.
By YMCs we mean objects of the same type as e.g. the cluster NGC 1866 in the Large Magellanic Cloud (Fischer et al. 1992). They are characterised by being similar (with respect to mass and morphology) to globular clusters seen in the Milky Way, but of much lower ages - in fact, the lower age limit of the clusters in our sample is set only by the fact that we avoid objects located within HII regions.
The paper is organised as follows: The observations are described in Sect. 2, and Sect. 3 is a discussion of the basic data reduction procedure including the standard calibrations. In Sect. 4 the tools mksynth and ishape are introduced, and Sect. 5 gives a discussion of the completeness corrections used in Paper I. Finally, in Sect. 6 ishape is used to derive radii of the clusters, and some implications of the results are discussed.
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