Background fields in M 31. The top panel shows a grey-scale 24 μm map of M 31 (Multi-Band Imaging Photometer for Spitzer, Spitzer/MIPS) with the marked locations of background fields (B1–B5). The bottom panels show images of the artificial star cluster (log(M/M_⊙) = 3.5, log(t/yr) = 7.0, r_c = 0.2″, and γ = 2.8) placed in each of the background fields. B0: Background-less case. Magenta and cyan circles mark apertures with radii R_ap = 1″ and 2″, respectively. The colour images (15″×15″) were produced by combining F336W, F475W, and F814W data.