Differential histograms were plotted class by class and CCD by CCD for each
band. A typical example of the results is shown on Fig. 
.
As expected, stars cover a wide range of magnitudes while galaxies are
generally fainter. However the degree of flatness of the
distribution for stars varies: see Fig. .
There is little variation with band except for the fact that some bands go
deeper as previously mentioned.
No discrepancies were detected for different CCDs.
The faintest part of the distribution is generally constituted of objects classified as noise. Is it worth noticing? Is the excess of bright stars worrying?
For each band, an additional database is generated (named, for example, wfci_diffmag) containing the values of differential and cumulative histograms of magnitude for than band. These are used later in the evaluation of the quality of the XID results to calculate a magnitude--dependent bagkround density of WFC objects.
Figure: i--band differential magnitude histograms for the GRB 001025 field, showing (a) all objects, (b) stars only, (c) galaxies only and (d) objects classified as noise only. Different line styles correspond to different CCDs.
Figure: r--band differential magnitude histograms for the HCG 16 field, showing (a) all objects, (b) stars only, (c) galaxies only and (d) objects classified as noise only. Different line styles correspond to different CCDs.