The basic scheme

The analysis of photometric diagrams, and especially the colour-magnitude diagrams that can be built in several photometric systems, remains the key method to determine the cluster parameters (reddening, distance and age). The study of a cluster necessitates to have also easily available all complementary information, remarks and bibliography. This is why everything is organised in the cluster directories.

If the production of a colour-magnitude diagram seems rather easy, the real situation is unfortunately not simple and the data are not always of sufficient quality. Therefore a large amount of work is necessary to plot a reliable colour-magnitude diagram. The various steps involved are:

1. the census of the stars in the cluster field, to determine the completeness in terms of both limiting magnitude and surface coverage;
2. the comparison of the various sources of data;
3. the selection of cluster members. This is certainly the main problem and no straightforward solution has yet been found. The other data contained in the database, like spectral types, radial velocities, remarks, and so on, may help in assigning cluster membership;
4. with the best selected data, it is eventually possible to plot a nice colour-magnitude diagram and determine more accurate cluster distances and ages.

The database cross-reference tables and the collection of rectangular positions are useful to achieve the first step. Tools to perform the second step and compute mean values are also provided.