10.7.3 Photometry: accuracy at the faint end

Stars fainter than $G$ $\sim 18$ 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 $G\sim 18-19$, depending on the local density.