10.6.3 Photometry: accuracy at the faint end

Stars fainter than G 18 are typically affected by an imperfect background subtraction (see Evans et al. 2018, for a discussion). An example is shown in Figure 10.49 for Alessi 10, where faint stars are observed as excessively blue. This contamination typically affects stars fainter than G18-19, depending on the local density.

Figure 10.49: Map of Alessi 10 showing star G magnitudes (left upper panel), the analogous showing the number of photometric observations per CCD used in the data analysis, Gobs (right upper panel) and the corresponding colour-magnitude diagrams (lower panels) (from Arenou et al. 2018).